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mariadb/sql/mdl.cc
Marko Mäkelä fa7d85bb87 Merge bb-10.2-ext into 10.3
2018-01-05 22:52:06 +02:00

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/* Copyright (c) 2007, 2012, Oracle and/or its affiliates.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
51 Franklin Street, Suite 500, Boston, MA 02110-1335 USA */
#include "mariadb.h"
#include "sql_class.h"
#include "debug_sync.h"
#include "sql_array.h"
#include "rpl_rli.h"
#include <lf.h>
#include "unireg.h"
#include <mysql/plugin.h>
#include <mysql/service_thd_wait.h>
#include <mysql/psi/mysql_stage.h>
#include "wsrep_mysqld.h"
#include "wsrep_thd.h"
#ifdef HAVE_PSI_INTERFACE
static PSI_mutex_key key_MDL_wait_LOCK_wait_status;
static PSI_mutex_info all_mdl_mutexes[]=
{
{ &key_MDL_wait_LOCK_wait_status, "MDL_wait::LOCK_wait_status", 0}
};
static PSI_rwlock_key key_MDL_lock_rwlock;
static PSI_rwlock_key key_MDL_context_LOCK_waiting_for;
static PSI_rwlock_info all_mdl_rwlocks[]=
{
{ &key_MDL_lock_rwlock, "MDL_lock::rwlock", 0},
{ &key_MDL_context_LOCK_waiting_for, "MDL_context::LOCK_waiting_for", 0}
};
static PSI_cond_key key_MDL_wait_COND_wait_status;
static PSI_cond_info all_mdl_conds[]=
{
{ &key_MDL_wait_COND_wait_status, "MDL_context::COND_wait_status", 0}
};
/**
Initialise all the performance schema instrumentation points
used by the MDL subsystem.
*/
static void init_mdl_psi_keys(void)
{
int count;
count= array_elements(all_mdl_mutexes);
mysql_mutex_register("sql", all_mdl_mutexes, count);
count= array_elements(all_mdl_rwlocks);
mysql_rwlock_register("sql", all_mdl_rwlocks, count);
count= array_elements(all_mdl_conds);
mysql_cond_register("sql", all_mdl_conds, count);
MDL_key::init_psi_keys();
}
#endif /* HAVE_PSI_INTERFACE */
/**
Thread state names to be used in case when we have to wait on resource
belonging to certain namespace.
*/
PSI_stage_info MDL_key::m_namespace_to_wait_state_name[NAMESPACE_END]=
{
{0, "Waiting for global read lock", 0},
{0, "Waiting for schema metadata lock", 0},
{0, "Waiting for table metadata lock", 0},
{0, "Waiting for stored function metadata lock", 0},
{0, "Waiting for stored procedure metadata lock", 0},
{0, "Waiting for trigger metadata lock", 0},
{0, "Waiting for event metadata lock", 0},
{0, "Waiting for commit lock", 0},
{0, "User lock", 0} /* Be compatible with old status. */
};
#ifdef HAVE_PSI_INTERFACE
void MDL_key::init_psi_keys()
{
int i;
int count;
PSI_stage_info *info __attribute__((unused));
count= array_elements(MDL_key::m_namespace_to_wait_state_name);
for (i= 0; i<count; i++)
{
/* mysql_stage_register wants an array of pointers, registering 1 by 1. */
info= & MDL_key::m_namespace_to_wait_state_name[i];
mysql_stage_register("sql", &info, 1);
}
}
#endif
static bool mdl_initialized= 0;
/**
A collection of all MDL locks. A singleton,
there is only one instance of the map in the server.
*/
class MDL_map
{
public:
void init();
void destroy();
MDL_lock *find_or_insert(LF_PINS *pins, const MDL_key *key);
unsigned long get_lock_owner(LF_PINS *pins, const MDL_key *key);
void remove(LF_PINS *pins, MDL_lock *lock);
LF_PINS *get_pins() { return lf_hash_get_pins(&m_locks); }
private:
LF_HASH m_locks; /**< All acquired locks in the server. */
/** Pre-allocated MDL_lock object for GLOBAL namespace. */
MDL_lock *m_global_lock;
/** Pre-allocated MDL_lock object for COMMIT namespace. */
MDL_lock *m_commit_lock;
friend int mdl_iterate(int (*)(MDL_ticket *, void *), void *);
};
/**
A context of the recursive traversal through all contexts
in all sessions in search for deadlock.
*/
class Deadlock_detection_visitor: public MDL_wait_for_graph_visitor
{
public:
Deadlock_detection_visitor(MDL_context *start_node_arg)
: m_start_node(start_node_arg),
m_victim(NULL),
m_current_search_depth(0),
m_found_deadlock(FALSE)
{}
virtual bool enter_node(MDL_context *node);
virtual void leave_node(MDL_context *node);
virtual bool inspect_edge(MDL_context *dest);
MDL_context *get_victim() const { return m_victim; }
private:
/**
Change the deadlock victim to a new one if it has lower deadlock
weight.
*/
void opt_change_victim_to(MDL_context *new_victim);
private:
/**
The context which has initiated the search. There
can be multiple searches happening in parallel at the same time.
*/
MDL_context *m_start_node;
/** If a deadlock is found, the context that identifies the victim. */
MDL_context *m_victim;
/** Set to the 0 at start. Increased whenever
we descend into another MDL context (aka traverse to the next
wait-for graph node). When MAX_SEARCH_DEPTH is reached, we
assume that a deadlock is found, even if we have not found a
loop.
*/
uint m_current_search_depth;
/** TRUE if we found a deadlock. */
bool m_found_deadlock;
/**
Maximum depth for deadlock searches. After this depth is
achieved we will unconditionally declare that there is a
deadlock.
@note This depth should be small enough to avoid stack
being exhausted by recursive search algorithm.
TODO: Find out what is the optimal value for this parameter.
Current value is safe, but probably sub-optimal,
as there is an anecdotal evidence that real-life
deadlocks are even shorter typically.
*/
static const uint MAX_SEARCH_DEPTH= 32;
};
/**
Enter a node of a wait-for graph. After
a node is entered, inspect_edge() will be called
for all wait-for destinations of this node. Then
leave_node() will be called.
We call "enter_node()" for all nodes we inspect,
including the starting node.
@retval TRUE Maximum search depth exceeded.
@retval FALSE OK.
*/
bool Deadlock_detection_visitor::enter_node(MDL_context *node)
{
m_found_deadlock= ++m_current_search_depth >= MAX_SEARCH_DEPTH;
if (m_found_deadlock)
{
DBUG_ASSERT(! m_victim);
opt_change_victim_to(node);
}
return m_found_deadlock;
}
/**
Done inspecting this node. Decrease the search
depth. If a deadlock is found, and we are
backtracking to the start node, optionally
change the deadlock victim to one with lower
deadlock weight.
*/
void Deadlock_detection_visitor::leave_node(MDL_context *node)
{
--m_current_search_depth;
if (m_found_deadlock)
opt_change_victim_to(node);
}
/**
Inspect a wait-for graph edge from one MDL context to another.
@retval TRUE A loop is found.
@retval FALSE No loop is found.
*/
bool Deadlock_detection_visitor::inspect_edge(MDL_context *node)
{
m_found_deadlock= node == m_start_node;
return m_found_deadlock;
}
/**
Change the deadlock victim to a new one if it has lower deadlock
weight.
@retval new_victim Victim is not changed.
@retval !new_victim New victim became the current.
*/
void
Deadlock_detection_visitor::opt_change_victim_to(MDL_context *new_victim)
{
if (m_victim == NULL ||
m_victim->get_deadlock_weight() >= new_victim->get_deadlock_weight())
{
/* Swap victims, unlock the old one. */
MDL_context *tmp= m_victim;
m_victim= new_victim;
m_victim->lock_deadlock_victim();
if (tmp)
tmp->unlock_deadlock_victim();
}
}
/**
Get a bit corresponding to enum_mdl_type value in a granted/waiting bitmaps
and compatibility matrices.
*/
#define MDL_BIT(A) static_cast<MDL_lock::bitmap_t>(1U << A)
/**
The lock context. Created internally for an acquired lock.
For a given name, there exists only one MDL_lock instance,
and it exists only when the lock has been granted.
Can be seen as an MDL subsystem's version of TABLE_SHARE.
This is an abstract class which lacks information about
compatibility rules for lock types. They should be specified
in its descendants.
*/
class MDL_lock
{
public:
typedef unsigned short bitmap_t;
class Ticket_list
{
public:
typedef I_P_List<MDL_ticket,
I_P_List_adapter<MDL_ticket,
&MDL_ticket::next_in_lock,
&MDL_ticket::prev_in_lock>,
I_P_List_null_counter,
I_P_List_fast_push_back<MDL_ticket> >
List;
operator const List &() const { return m_list; }
Ticket_list() :m_bitmap(0) {}
void add_ticket(MDL_ticket *ticket);
void remove_ticket(MDL_ticket *ticket);
bool is_empty() const { return m_list.is_empty(); }
bitmap_t bitmap() const { return m_bitmap; }
private:
void clear_bit_if_not_in_list(enum_mdl_type type);
private:
/** List of tickets. */
List m_list;
/** Bitmap of types of tickets in this list. */
bitmap_t m_bitmap;
};
typedef Ticket_list::List::Iterator Ticket_iterator;
/**
Helper struct which defines how different types of locks are handled
for a specific MDL_lock. In practice we use only two strategies: "scoped"
lock strategy for locks in GLOBAL, COMMIT and SCHEMA namespaces and
"object" lock strategy for all other namespaces.
*/
struct MDL_lock_strategy
{
virtual const bitmap_t *incompatible_granted_types_bitmap() const = 0;
virtual const bitmap_t *incompatible_waiting_types_bitmap() const = 0;
virtual bool needs_notification(const MDL_ticket *ticket) const = 0;
virtual bool conflicting_locks(const MDL_ticket *ticket) const = 0;
virtual bitmap_t hog_lock_types_bitmap() const = 0;
virtual ~MDL_lock_strategy() {}
};
/**
An implementation of the scoped metadata lock. The only locking modes
which are supported at the moment are SHARED and INTENTION EXCLUSIVE
and EXCLUSIVE
*/
struct MDL_scoped_lock : public MDL_lock_strategy
{
MDL_scoped_lock() {}
virtual const bitmap_t *incompatible_granted_types_bitmap() const
{ return m_granted_incompatible; }
virtual const bitmap_t *incompatible_waiting_types_bitmap() const
{ return m_waiting_incompatible; }
virtual bool needs_notification(const MDL_ticket *ticket) const
{ return (ticket->get_type() == MDL_SHARED); }
/**
Notify threads holding scoped IX locks which conflict with a pending
S lock.
Thread which holds global IX lock can be a handler thread for
insert delayed. We need to kill such threads in order to get
global shared lock. We do this my calling code outside of MDL.
*/
virtual bool conflicting_locks(const MDL_ticket *ticket) const
{ return ticket->get_type() == MDL_INTENTION_EXCLUSIVE; }
/*
In scoped locks, only IX lock request would starve because of X/S. But that
is practically very rare case. So just return 0 from this function.
*/
virtual bitmap_t hog_lock_types_bitmap() const
{ return 0; }
private:
static const bitmap_t m_granted_incompatible[MDL_TYPE_END];
static const bitmap_t m_waiting_incompatible[MDL_TYPE_END];
};
/**
An implementation of a per-object lock. Supports SHARED, SHARED_UPGRADABLE,
SHARED HIGH PRIORITY and EXCLUSIVE locks.
*/
struct MDL_object_lock : public MDL_lock_strategy
{
MDL_object_lock() {}
virtual const bitmap_t *incompatible_granted_types_bitmap() const
{ return m_granted_incompatible; }
virtual const bitmap_t *incompatible_waiting_types_bitmap() const
{ return m_waiting_incompatible; }
virtual bool needs_notification(const MDL_ticket *ticket) const
{
return ticket->get_type() == MDL_SHARED_NO_WRITE ||
ticket->get_type() == MDL_SHARED_NO_READ_WRITE ||
ticket->get_type() == MDL_EXCLUSIVE;
}
/**
Notify threads holding a shared metadata locks on object which
conflict with a pending X, SNW or SNRW lock.
If thread which holds conflicting lock is waiting on table-level
lock or some other non-MDL resource we might need to wake it up
by calling code outside of MDL.
*/
virtual bool conflicting_locks(const MDL_ticket *ticket) const
{ return ticket->get_type() < MDL_SHARED_UPGRADABLE; }
/*
To prevent starvation, these lock types that are only granted
max_write_lock_count times in a row while other lock types are
waiting.
*/
virtual bitmap_t hog_lock_types_bitmap() const
{
return (MDL_BIT(MDL_SHARED_NO_WRITE) |
MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_EXCLUSIVE));
}
private:
static const bitmap_t m_granted_incompatible[MDL_TYPE_END];
static const bitmap_t m_waiting_incompatible[MDL_TYPE_END];
};
public:
/** The key of the object (data) being protected. */
MDL_key key;
/**
Read-write lock protecting this lock context.
@note The fact that we use read-write lock prefers readers here is
important as deadlock detector won't work correctly otherwise.
For example, imagine that we have following waiters graph:
ctxA -> obj1 -> ctxB -> obj1 -|
^ |
|----------------------------|
and both ctxA and ctxB start deadlock detection process:
ctxA read-locks obj1 ctxB read-locks obj2
ctxA goes deeper ctxB goes deeper
Now ctxC comes in who wants to start waiting on obj1, also
ctxD comes in who wants to start waiting on obj2.
ctxC tries to write-lock obj1 ctxD tries to write-lock obj2
ctxC is blocked ctxD is blocked
Now ctxA and ctxB resume their search:
ctxA tries to read-lock obj2 ctxB tries to read-lock obj1
If m_rwlock prefers writes (or fair) both ctxA and ctxB would be
blocked because of pending write locks from ctxD and ctxC
correspondingly. Thus we will get a deadlock in deadlock detector.
If m_wrlock prefers readers (actually ignoring pending writers is
enough) ctxA and ctxB will continue and no deadlock will occur.
*/
mysql_prlock_t m_rwlock;
bool is_empty() const
{
return (m_granted.is_empty() && m_waiting.is_empty());
}
const bitmap_t *incompatible_granted_types_bitmap() const
{ return m_strategy->incompatible_granted_types_bitmap(); }
const bitmap_t *incompatible_waiting_types_bitmap() const
{ return m_strategy->incompatible_waiting_types_bitmap(); }
bool has_pending_conflicting_lock(enum_mdl_type type);
bool can_grant_lock(enum_mdl_type type, MDL_context *requstor_ctx,
bool ignore_lock_priority) const;
inline unsigned long get_lock_owner() const;
void reschedule_waiters();
void remove_ticket(LF_PINS *pins, Ticket_list MDL_lock::*queue,
MDL_ticket *ticket);
bool visit_subgraph(MDL_ticket *waiting_ticket,
MDL_wait_for_graph_visitor *gvisitor);
bool needs_notification(const MDL_ticket *ticket) const
{ return m_strategy->needs_notification(ticket); }
void notify_conflicting_locks(MDL_context *ctx)
{
Ticket_iterator it(m_granted);
MDL_ticket *conflicting_ticket;
while ((conflicting_ticket= it++))
{
if (conflicting_ticket->get_ctx() != ctx &&
m_strategy->conflicting_locks(conflicting_ticket))
{
MDL_context *conflicting_ctx= conflicting_ticket->get_ctx();
ctx->get_owner()->
notify_shared_lock(conflicting_ctx->get_owner(),
conflicting_ctx->get_needs_thr_lock_abort());
}
}
}
bitmap_t hog_lock_types_bitmap() const
{ return m_strategy->hog_lock_types_bitmap(); }
#ifndef DBUG_OFF
bool check_if_conflicting_replication_locks(MDL_context *ctx);
#endif
/** List of granted tickets for this lock. */
Ticket_list m_granted;
/** Tickets for contexts waiting to acquire a lock. */
Ticket_list m_waiting;
/**
Number of times high priority lock requests have been granted while
low priority lock requests were waiting.
*/
ulong m_hog_lock_count;
public:
MDL_lock()
: m_hog_lock_count(0),
m_strategy(0)
{ mysql_prlock_init(key_MDL_lock_rwlock, &m_rwlock); }
MDL_lock(const MDL_key *key_arg)
: key(key_arg),
m_hog_lock_count(0),
m_strategy(&m_scoped_lock_strategy)
{
DBUG_ASSERT(key_arg->mdl_namespace() == MDL_key::GLOBAL ||
key_arg->mdl_namespace() == MDL_key::COMMIT);
mysql_prlock_init(key_MDL_lock_rwlock, &m_rwlock);
}
~MDL_lock()
{ mysql_prlock_destroy(&m_rwlock); }
static void lf_alloc_constructor(uchar *arg)
{ new (arg + LF_HASH_OVERHEAD) MDL_lock(); }
static void lf_alloc_destructor(uchar *arg)
{ ((MDL_lock*)(arg + LF_HASH_OVERHEAD))->~MDL_lock(); }
static void lf_hash_initializer(LF_HASH *hash __attribute__((unused)),
MDL_lock *lock, MDL_key *key_arg)
{
DBUG_ASSERT(key_arg->mdl_namespace() != MDL_key::GLOBAL &&
key_arg->mdl_namespace() != MDL_key::COMMIT);
new (&lock->key) MDL_key(key_arg);
if (key_arg->mdl_namespace() == MDL_key::SCHEMA)
lock->m_strategy= &m_scoped_lock_strategy;
else
lock->m_strategy= &m_object_lock_strategy;
}
const MDL_lock_strategy *m_strategy;
private:
static const MDL_scoped_lock m_scoped_lock_strategy;
static const MDL_object_lock m_object_lock_strategy;
};
const MDL_lock::MDL_scoped_lock MDL_lock::m_scoped_lock_strategy;
const MDL_lock::MDL_object_lock MDL_lock::m_object_lock_strategy;
static MDL_map mdl_locks;
extern "C"
{
static uchar *
mdl_locks_key(const uchar *record, size_t *length,
my_bool not_used __attribute__((unused)))
{
MDL_lock *lock=(MDL_lock*) record;
*length= lock->key.length();
return (uchar*) lock->key.ptr();
}
} /* extern "C" */
/**
Initialize the metadata locking subsystem.
This function is called at server startup.
In particular, initializes the new global mutex and
the associated condition variable: LOCK_mdl and COND_mdl.
These locking primitives are implementation details of the MDL
subsystem and are private to it.
*/
void mdl_init()
{
DBUG_ASSERT(! mdl_initialized);
mdl_initialized= TRUE;
#ifdef HAVE_PSI_INTERFACE
init_mdl_psi_keys();
#endif
mdl_locks.init();
}
/**
Release resources of metadata locking subsystem.
Destroys the global mutex and the condition variable.
Called at server shutdown.
*/
void mdl_destroy()
{
if (mdl_initialized)
{
mdl_initialized= FALSE;
mdl_locks.destroy();
}
}
struct mdl_iterate_arg
{
int (*callback)(MDL_ticket *ticket, void *arg);
void *argument;
};
static my_bool mdl_iterate_lock(MDL_lock *lock, mdl_iterate_arg *arg)
{
int res= FALSE;
/*
We can skip check for m_strategy here, becase m_granted
must be empty for such locks anyway.
*/
mysql_prlock_rdlock(&lock->m_rwlock);
MDL_lock::Ticket_iterator ticket_it(lock->m_granted);
MDL_ticket *ticket;
while ((ticket= ticket_it++) && !(res= arg->callback(ticket, arg->argument)))
/* no-op */;
mysql_prlock_unlock(&lock->m_rwlock);
return MY_TEST(res);
}
int mdl_iterate(int (*callback)(MDL_ticket *ticket, void *arg), void *arg)
{
DBUG_ENTER("mdl_iterate");
mdl_iterate_arg argument= { callback, arg };
LF_PINS *pins= mdl_locks.get_pins();
int res= 1;
if (pins)
{
res= mdl_iterate_lock(mdl_locks.m_global_lock, &argument) ||
mdl_iterate_lock(mdl_locks.m_commit_lock, &argument) ||
lf_hash_iterate(&mdl_locks.m_locks, pins,
(my_hash_walk_action) mdl_iterate_lock, &argument);
lf_hash_put_pins(pins);
}
DBUG_RETURN(res);
}
my_hash_value_type mdl_hash_function(CHARSET_INFO *cs,
const uchar *key, size_t length)
{
MDL_key *mdl_key= (MDL_key*) (key - my_offsetof(MDL_key, m_ptr));
return mdl_key->hash_value();
}
/** Initialize the container for all MDL locks. */
void MDL_map::init()
{
MDL_key global_lock_key(MDL_key::GLOBAL, "", "");
MDL_key commit_lock_key(MDL_key::COMMIT, "", "");
m_global_lock= new (std::nothrow) MDL_lock(&global_lock_key);
m_commit_lock= new (std::nothrow) MDL_lock(&commit_lock_key);
lf_hash_init(&m_locks, sizeof(MDL_lock), LF_HASH_UNIQUE, 0, 0,
mdl_locks_key, &my_charset_bin);
m_locks.alloc.constructor= MDL_lock::lf_alloc_constructor;
m_locks.alloc.destructor= MDL_lock::lf_alloc_destructor;
m_locks.initializer= (lf_hash_initializer) MDL_lock::lf_hash_initializer;
m_locks.hash_function= mdl_hash_function;
}
/**
Destroy the container for all MDL locks.
@pre It must be empty.
*/
void MDL_map::destroy()
{
delete m_global_lock;
delete m_commit_lock;
DBUG_ASSERT(!my_atomic_load32(&m_locks.count));
lf_hash_destroy(&m_locks);
}
/**
Find MDL_lock object corresponding to the key, create it
if it does not exist.
@retval non-NULL - Success. MDL_lock instance for the key with
locked MDL_lock::m_rwlock.
@retval NULL - Failure (OOM).
*/
MDL_lock* MDL_map::find_or_insert(LF_PINS *pins, const MDL_key *mdl_key)
{
MDL_lock *lock;
if (mdl_key->mdl_namespace() == MDL_key::GLOBAL ||
mdl_key->mdl_namespace() == MDL_key::COMMIT)
{
/*
Avoid locking any m_mutex when lock for GLOBAL or COMMIT namespace is
requested. Return pointer to pre-allocated MDL_lock instance instead.
Such an optimization allows to save one mutex lock/unlock for any
statement changing data.
It works since these namespaces contain only one element so keys
for them look like '<namespace-id>\0\0'.
*/
DBUG_ASSERT(mdl_key->length() == 3);
lock= (mdl_key->mdl_namespace() == MDL_key::GLOBAL) ? m_global_lock :
m_commit_lock;
mysql_prlock_wrlock(&lock->m_rwlock);
return lock;
}
retry:
while (!(lock= (MDL_lock*) lf_hash_search(&m_locks, pins, mdl_key->ptr(),
mdl_key->length())))
if (lf_hash_insert(&m_locks, pins, (uchar*) mdl_key) == -1)
return NULL;
mysql_prlock_wrlock(&lock->m_rwlock);
if (unlikely(!lock->m_strategy))
{
mysql_prlock_unlock(&lock->m_rwlock);
lf_hash_search_unpin(pins);
goto retry;
}
lf_hash_search_unpin(pins);
return lock;
}
/**
* Return thread id of the owner of the lock, if it is owned.
*/
unsigned long
MDL_map::get_lock_owner(LF_PINS *pins, const MDL_key *mdl_key)
{
MDL_lock *lock;
unsigned long res= 0;
if (mdl_key->mdl_namespace() == MDL_key::GLOBAL ||
mdl_key->mdl_namespace() == MDL_key::COMMIT)
{
lock= (mdl_key->mdl_namespace() == MDL_key::GLOBAL) ? m_global_lock :
m_commit_lock;
mysql_prlock_rdlock(&lock->m_rwlock);
res= lock->get_lock_owner();
mysql_prlock_unlock(&lock->m_rwlock);
}
else
{
lock= (MDL_lock*) lf_hash_search(&m_locks, pins, mdl_key->ptr(),
mdl_key->length());
if (lock)
{
/*
We can skip check for m_strategy here, becase m_granted
must be empty for such locks anyway.
*/
mysql_prlock_rdlock(&lock->m_rwlock);
res= lock->get_lock_owner();
mysql_prlock_unlock(&lock->m_rwlock);
lf_hash_search_unpin(pins);
}
}
return res;
}
/**
Destroy MDL_lock object or delegate this responsibility to
whatever thread that holds the last outstanding reference to
it.
*/
void MDL_map::remove(LF_PINS *pins, MDL_lock *lock)
{
if (lock->key.mdl_namespace() == MDL_key::GLOBAL ||
lock->key.mdl_namespace() == MDL_key::COMMIT)
{
/*
Never destroy pre-allocated MDL_lock objects for GLOBAL and
COMMIT namespaces.
*/
mysql_prlock_unlock(&lock->m_rwlock);
return;
}
lock->m_strategy= 0;
mysql_prlock_unlock(&lock->m_rwlock);
lf_hash_delete(&m_locks, pins, lock->key.ptr(), lock->key.length());
}
/**
Initialize a metadata locking context.
This is to be called when a new server connection is created.
*/
MDL_context::MDL_context()
:
m_owner(NULL),
m_needs_thr_lock_abort(FALSE),
m_waiting_for(NULL),
m_pins(NULL)
{
mysql_prlock_init(key_MDL_context_LOCK_waiting_for, &m_LOCK_waiting_for);
}
/**
Destroy metadata locking context.
Assumes and asserts that there are no active or pending locks
associated with this context at the time of the destruction.
Currently does nothing. Asserts that there are no pending
or satisfied lock requests. The pending locks must be released
prior to destruction. This is a new way to express the assertion
that all tables are closed before a connection is destroyed.
*/
void MDL_context::destroy()
{
DBUG_ASSERT(m_tickets[MDL_STATEMENT].is_empty());
DBUG_ASSERT(m_tickets[MDL_TRANSACTION].is_empty());
DBUG_ASSERT(m_tickets[MDL_EXPLICIT].is_empty());
mysql_prlock_destroy(&m_LOCK_waiting_for);
if (m_pins)
lf_hash_put_pins(m_pins);
}
bool MDL_context::fix_pins()
{
return m_pins ? false : (m_pins= mdl_locks.get_pins()) == 0;
}
/**
Initialize a lock request.
This is to be used for every lock request.
Note that initialization and allocation are split into two
calls. This is to allow flexible memory management of lock
requests. Normally a lock request is stored in statement memory
(e.g. is a member of struct TABLE_LIST), but we would also like
to allow allocation of lock requests in other memory roots,
for example in the grant subsystem, to lock privilege tables.
The MDL subsystem does not own or manage memory of lock requests.
@param mdl_namespace Id of namespace of object to be locked
@param db Name of database to which the object belongs
@param name Name of of the object
@param mdl_type The MDL lock type for the request.
*/
void MDL_request::init(MDL_key::enum_mdl_namespace mdl_namespace,
const char *db_arg,
const char *name_arg,
enum_mdl_type mdl_type_arg,
enum_mdl_duration mdl_duration_arg)
{
key.mdl_key_init(mdl_namespace, db_arg, name_arg);
type= mdl_type_arg;
duration= mdl_duration_arg;
ticket= NULL;
}
/**
Initialize a lock request using pre-built MDL_key.
@sa MDL_request::init(namespace, db, name, type).
@param key_arg The pre-built MDL key for the request.
@param mdl_type_arg The MDL lock type for the request.
*/
void MDL_request::init(const MDL_key *key_arg,
enum_mdl_type mdl_type_arg,
enum_mdl_duration mdl_duration_arg)
{
key.mdl_key_init(key_arg);
type= mdl_type_arg;
duration= mdl_duration_arg;
ticket= NULL;
}
/**
Auxiliary functions needed for creation/destruction of MDL_ticket
objects.
@todo This naive implementation should be replaced with one that saves
on memory allocation by reusing released objects.
*/
MDL_ticket *MDL_ticket::create(MDL_context *ctx_arg, enum_mdl_type type_arg
#ifndef DBUG_OFF
, enum_mdl_duration duration_arg
#endif
)
{
return new (std::nothrow)
MDL_ticket(ctx_arg, type_arg
#ifndef DBUG_OFF
, duration_arg
#endif
);
}
void MDL_ticket::destroy(MDL_ticket *ticket)
{
delete ticket;
}
/**
Return the 'weight' of this ticket for the
victim selection algorithm. Requests with
lower weight are preferred to requests
with higher weight when choosing a victim.
*/
uint MDL_ticket::get_deadlock_weight() const
{
return (m_lock->key.mdl_namespace() == MDL_key::GLOBAL ||
m_type >= MDL_SHARED_UPGRADABLE ?
DEADLOCK_WEIGHT_DDL : DEADLOCK_WEIGHT_DML);
}
/** Construct an empty wait slot. */
MDL_wait::MDL_wait()
:m_wait_status(EMPTY)
{
mysql_mutex_init(key_MDL_wait_LOCK_wait_status, &m_LOCK_wait_status, NULL);
mysql_cond_init(key_MDL_wait_COND_wait_status, &m_COND_wait_status, NULL);
}
/** Destroy system resources. */
MDL_wait::~MDL_wait()
{
mysql_mutex_destroy(&m_LOCK_wait_status);
mysql_cond_destroy(&m_COND_wait_status);
}
/**
Set the status unless it's already set. Return FALSE if set,
TRUE otherwise.
*/
bool MDL_wait::set_status(enum_wait_status status_arg)
{
bool was_occupied= TRUE;
mysql_mutex_lock(&m_LOCK_wait_status);
if (m_wait_status == EMPTY)
{
was_occupied= FALSE;
m_wait_status= status_arg;
mysql_cond_signal(&m_COND_wait_status);
}
mysql_mutex_unlock(&m_LOCK_wait_status);
return was_occupied;
}
/** Query the current value of the wait slot. */
MDL_wait::enum_wait_status MDL_wait::get_status()
{
enum_wait_status result;
mysql_mutex_lock(&m_LOCK_wait_status);
result= m_wait_status;
mysql_mutex_unlock(&m_LOCK_wait_status);
return result;
}
/** Clear the current value of the wait slot. */
void MDL_wait::reset_status()
{
mysql_mutex_lock(&m_LOCK_wait_status);
m_wait_status= EMPTY;
mysql_mutex_unlock(&m_LOCK_wait_status);
}
/**
Wait for the status to be assigned to this wait slot.
@param owner MDL context owner.
@param abs_timeout Absolute time after which waiting should stop.
@param set_status_on_timeout TRUE - If in case of timeout waiting
context should close the wait slot by
sending TIMEOUT to itself.
FALSE - Otherwise.
@param wait_state_name Thread state name to be set for duration of wait.
@returns Signal posted.
*/
MDL_wait::enum_wait_status
MDL_wait::timed_wait(MDL_context_owner *owner, struct timespec *abs_timeout,
bool set_status_on_timeout,
const PSI_stage_info *wait_state_name)
{
PSI_stage_info old_stage;
enum_wait_status result;
int wait_result= 0;
DBUG_ENTER("MDL_wait::timed_wait");
mysql_mutex_lock(&m_LOCK_wait_status);
owner->ENTER_COND(&m_COND_wait_status, &m_LOCK_wait_status,
wait_state_name, & old_stage);
thd_wait_begin(NULL, THD_WAIT_META_DATA_LOCK);
while (!m_wait_status && !owner->is_killed() &&
wait_result != ETIMEDOUT && wait_result != ETIME)
{
#ifdef WITH_WSREP
// Allow tests to block the applier thread using the DBUG facilities
DBUG_EXECUTE_IF("sync.wsrep_before_mdl_wait",
{
const char act[]=
"now "
"wait_for signal.wsrep_before_mdl_wait";
DBUG_ASSERT(!debug_sync_set_action((owner->get_thd()),
STRING_WITH_LEN(act)));
};);
if (wsrep_thd_is_BF(owner->get_thd(), false))
{
wait_result= mysql_cond_wait(&m_COND_wait_status, &m_LOCK_wait_status);
}
else
#endif /* WITH_WSREP */
wait_result= mysql_cond_timedwait(&m_COND_wait_status, &m_LOCK_wait_status,
abs_timeout);
}
thd_wait_end(NULL);
if (m_wait_status == EMPTY)
{
/*
Wait has ended not due to a status being set from another
thread but due to this connection/statement being killed or a
time out.
To avoid races, which may occur if another thread sets
GRANTED status before the code which calls this method
processes the abort/timeout, we assign the status under
protection of the m_LOCK_wait_status, within the critical
section. An exception is when set_status_on_timeout is
false, which means that the caller intends to restart the
wait.
*/
if (owner->is_killed())
m_wait_status= KILLED;
else if (set_status_on_timeout)
m_wait_status= TIMEOUT;
}
result= m_wait_status;
owner->EXIT_COND(& old_stage);
DBUG_RETURN(result);
}
/**
Clear bit corresponding to the type of metadata lock in bitmap representing
set of such types if list of tickets does not contain ticket with such type.
@param[in,out] bitmap Bitmap representing set of types of locks.
@param[in] list List to inspect.
@param[in] type Type of metadata lock to look up in the list.
*/
void MDL_lock::Ticket_list::clear_bit_if_not_in_list(enum_mdl_type type)
{
MDL_lock::Ticket_iterator it(m_list);
const MDL_ticket *ticket;
while ((ticket= it++))
if (ticket->get_type() == type)
return;
m_bitmap&= ~ MDL_BIT(type);
}
/**
Add ticket to MDL_lock's list of waiting requests and
update corresponding bitmap of lock types.
*/
void MDL_lock::Ticket_list::add_ticket(MDL_ticket *ticket)
{
/*
Ticket being added to the list must have MDL_ticket::m_lock set,
since for such tickets methods accessing this member might be
called by other threads.
*/
DBUG_ASSERT(ticket->get_lock());
#ifdef WITH_WSREP
if ((this == &(ticket->get_lock()->m_waiting)) &&
wsrep_thd_is_BF(ticket->get_ctx()->get_thd(), false))
{
Ticket_iterator itw(ticket->get_lock()->m_waiting);
Ticket_iterator itg(ticket->get_lock()->m_granted);
DBUG_ASSERT(WSREP_ON);
MDL_ticket *waiting, *granted;
MDL_ticket *prev=NULL;
bool added= false;
while ((waiting= itw++) && !added)
{
if (!wsrep_thd_is_BF(waiting->get_ctx()->get_thd(), true))
{
WSREP_DEBUG("MDL add_ticket inserted before: %lu %s",
thd_get_thread_id(waiting->get_ctx()->get_thd()),
wsrep_thd_query(waiting->get_ctx()->get_thd()));
/* Insert the ticket before the first non-BF waiting thd. */
m_list.insert_after(prev, ticket);
added= true;
}
prev= waiting;
}
/* Otherwise, insert the ticket at the back of the waiting list. */
if (!added) m_list.push_back(ticket);
while ((granted= itg++))
{
if (granted->get_ctx() != ticket->get_ctx() &&
granted->is_incompatible_when_granted(ticket->get_type()))
{
if (!wsrep_grant_mdl_exception(ticket->get_ctx(), granted,
&ticket->get_lock()->key))
{
WSREP_DEBUG("MDL victim killed at add_ticket");
}
}
}
}
else
#endif /* WITH_WSREP */
{
/*
Add ticket to the *back* of the queue to ensure fairness
among requests with the same priority.
*/
m_list.push_back(ticket);
}
m_bitmap|= MDL_BIT(ticket->get_type());
}
/**
Remove ticket from MDL_lock's list of requests and
update corresponding bitmap of lock types.
*/
void MDL_lock::Ticket_list::remove_ticket(MDL_ticket *ticket)
{
m_list.remove(ticket);
/*
Check if waiting queue has another ticket with the same type as
one which was removed. If there is no such ticket, i.e. we have
removed last ticket of particular type, then we need to update
bitmap of waiting ticket's types.
Note that in most common case, i.e. when shared lock is removed
from waiting queue, we are likely to find ticket of the same
type early without performing full iteration through the list.
So this method should not be too expensive.
*/
clear_bit_if_not_in_list(ticket->get_type());
}
/**
Determine waiting contexts which requests for the lock can be
satisfied, grant lock to them and wake them up.
@note Together with MDL_lock::add_ticket() this method implements
fair scheduling among requests with the same priority.
It tries to grant lock from the head of waiters list, while
add_ticket() adds new requests to the back of this list.
*/
void MDL_lock::reschedule_waiters()
{
MDL_lock::Ticket_iterator it(m_waiting);
MDL_ticket *ticket;
bool skip_high_priority= false;
bitmap_t hog_lock_types= hog_lock_types_bitmap();
if (m_hog_lock_count >= max_write_lock_count)
{
/*
If number of successively granted high-prio, strong locks has exceeded
max_write_lock_count give a way to low-prio, weak locks to avoid their
starvation.
*/
if ((m_waiting.bitmap() & ~hog_lock_types) != 0)
{
/*
Even though normally when m_hog_lock_count is non-0 there is
some pending low-prio lock, we still can encounter situation
when m_hog_lock_count is non-0 and there are no pending low-prio
locks. This, for example, can happen when a ticket for pending
low-prio lock was removed from waiters list due to timeout,
and reschedule_waiters() is called after that to update the
waiters queue. m_hog_lock_count will be reset to 0 at the
end of this call in such case.
Note that it is not an issue if we fail to wake up any pending
waiters for weak locks in the loop below. This would mean that
all of them are either killed, timed out or chosen as a victim
by deadlock resolver, but have not managed to remove ticket
from the waiters list yet. After tickets will be removed from
the waiters queue there will be another call to
reschedule_waiters() with pending bitmap updated to reflect new
state of waiters queue.
*/
skip_high_priority= true;
}
}
/*
Find the first (and hence the oldest) waiting request which
can be satisfied (taking into account priority). Grant lock to it.
Repeat the process for the remainder of waiters.
Note we don't need to re-start iteration from the head of the
list after satisfying the first suitable request as in our case
all compatible types of requests have the same priority.
TODO/FIXME: We should:
- Either switch to scheduling without priorities
which will allow to stop iteration through the
list of waiters once we found the first ticket
which can't be satisfied
- Or implement some check using bitmaps which will
allow to stop iteration in cases when, e.g., we
grant SNRW lock and there are no pending S or
SH locks.
*/
while ((ticket= it++))
{
/*
Skip high-prio, strong locks if earlier we have decided to give way to
low-prio, weaker locks.
*/
if (skip_high_priority &&
((MDL_BIT(ticket->get_type()) & hog_lock_types) != 0))
continue;
if (can_grant_lock(ticket->get_type(), ticket->get_ctx(),
skip_high_priority))
{
if (! ticket->get_ctx()->m_wait.set_status(MDL_wait::GRANTED))
{
/*
Satisfy the found request by updating lock structures.
It is OK to do so even after waking up the waiter since any
session which tries to get any information about the state of
this lock has to acquire MDL_lock::m_rwlock first and thus,
when manages to do so, already sees an updated state of the
MDL_lock object.
*/
m_waiting.remove_ticket(ticket);
m_granted.add_ticket(ticket);
/*
Increase counter of successively granted high-priority strong locks,
if we have granted one.
*/
if ((MDL_BIT(ticket->get_type()) & hog_lock_types) != 0)
m_hog_lock_count++;
}
/*
If we could not update the wait slot of the waiter,
it can be due to fact that its connection/statement was
killed or it has timed out (i.e. the slot is not empty).
Since in all such cases the waiter assumes that the lock was
not been granted, we should keep the request in the waiting
queue and look for another request to reschedule.
*/
}
}
if ((m_waiting.bitmap() & ~hog_lock_types) == 0)
{
/*
Reset number of successively granted high-prio, strong locks
if there are no pending low-prio, weak locks.
This ensures:
- That m_hog_lock_count is correctly reset after strong lock
is released and weak locks are granted (or there are no
other lock requests).
- That situation when SNW lock is granted along with some SR
locks, but SW locks are still blocked are handled correctly.
- That m_hog_lock_count is zero in most cases when there are no pending
weak locks (see comment at the start of this method for example of
exception). This allows to save on checks at the start of this method.
*/
m_hog_lock_count= 0;
}
}
/**
Compatibility (or rather "incompatibility") matrices for scoped metadata
lock. Arrays of bitmaps which elements specify which granted/waiting locks
are incompatible with type of lock being requested.
The first array specifies if particular type of request can be satisfied
if there is granted scoped lock of certain type.
| Type of active |
Request | scoped lock |
type | IS(*) IX S X |
---------+------------------+
IS | + + + + |
IX | + + - - |
S | + - + - |
X | + - - - |
The second array specifies if particular type of request can be satisfied
if there is already waiting request for the scoped lock of certain type.
I.e. it specifies what is the priority of different lock types.
| Pending |
Request | scoped lock |
type | IS(*) IX S X |
---------+-----------------+
IS | + + + + |
IX | + + - - |
S | + + + - |
X | + + + + |
Here: "+" -- means that request can be satisfied
"-" -- means that request can't be satisfied and should wait
(*) Since intention shared scoped locks are compatible with all other
type of locks we don't even have any accounting for them.
Note that relation between scoped locks and objects locks requested
by statement is not straightforward and is therefore fully defined
by SQL-layer.
For example, in order to support global read lock implementation
SQL-layer acquires IX lock in GLOBAL namespace for each statement
that can modify metadata or data (i.e. for each statement that
needs SW, SU, SNW, SNRW or X object locks). OTOH, to ensure that
DROP DATABASE works correctly with concurrent DDL, IX metadata locks
in SCHEMA namespace are acquired for DDL statements which can update
metadata in the schema (i.e. which acquire SU, SNW, SNRW and X locks
on schema objects) and aren't acquired for DML.
*/
const MDL_lock::bitmap_t
MDL_lock::MDL_scoped_lock::m_granted_incompatible[MDL_TYPE_END]=
{
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_INTENTION_EXCLUSIVE),
0, 0, 0, 0, 0, 0, 0,
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED) | MDL_BIT(MDL_INTENTION_EXCLUSIVE)
};
const MDL_lock::bitmap_t
MDL_lock::MDL_scoped_lock::m_waiting_incompatible[MDL_TYPE_END]=
{
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED),
MDL_BIT(MDL_EXCLUSIVE), 0, 0, 0, 0, 0, 0, 0, 0
};
/**
Compatibility (or rather "incompatibility") matrices for per-object
metadata lock. Arrays of bitmaps which elements specify which granted/
waiting locks are incompatible with type of lock being requested.
The first array specifies if particular type of request can be satisfied
if there is granted lock of certain type.
Request | Granted requests for lock |
type | S SH SR SW SU SRO SNW SNRW X |
----------+---------------------------------------+
S | + + + + + + + + - |
SH | + + + + + + + + - |
SR | + + + + + + + - - |
SW | + + + + + - - - - |
SU | + + + + - + - - - |
SRO | + + + - + + + - - |
SNW | + + + - - + - - - |
SNRW | + + - - - - - - - |
X | - - - - - - - - - |
SU -> X | - - - - 0 - 0 0 0 |
SNW -> X | - - - 0 0 - 0 0 0 |
SNRW -> X | - - 0 0 0 0 0 0 0 |
The second array specifies if particular type of request can be satisfied
if there is waiting request for the same lock of certain type. In other
words it specifies what is the priority of different lock types.
Request | Pending requests for lock |
type | S SH SR SW SU SRO SNW SNRW X |
----------+--------------------------------------+
S | + + + + + + + + - |
SH | + + + + + + + + + |
SR | + + + + + + + - - |
SW | + + + + + + - - - |
SU | + + + + + + + + - |
SRO | + + + - + + + - - |
SNW | + + + + + + + + - |
SNRW | + + + + + + + + - |
X | + + + + + + + + + |
SU -> X | + + + + + + + + + |
SNW -> X | + + + + + + + + + |
SNRW -> X | + + + + + + + + + |
Here: "+" -- means that request can be satisfied
"-" -- means that request can't be satisfied and should wait
"0" -- means impossible situation which will trigger assert
@note In cases then current context already has "stronger" type
of lock on the object it will be automatically granted
thanks to usage of the MDL_context::find_ticket() method.
@note IX locks are excluded since they are not used for per-object
metadata locks.
*/
const MDL_lock::bitmap_t
MDL_lock::MDL_object_lock::m_granted_incompatible[MDL_TYPE_END]=
{
0,
MDL_BIT(MDL_EXCLUSIVE),
MDL_BIT(MDL_EXCLUSIVE),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_NO_WRITE) | MDL_BIT(MDL_SHARED_READ_ONLY),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_NO_WRITE) | MDL_BIT(MDL_SHARED_UPGRADABLE),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_WRITE),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_NO_WRITE) | MDL_BIT(MDL_SHARED_UPGRADABLE) |
MDL_BIT(MDL_SHARED_WRITE),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_NO_WRITE) | MDL_BIT(MDL_SHARED_READ_ONLY) |
MDL_BIT(MDL_SHARED_UPGRADABLE) | MDL_BIT(MDL_SHARED_WRITE) |
MDL_BIT(MDL_SHARED_READ),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_NO_WRITE) | MDL_BIT(MDL_SHARED_READ_ONLY) |
MDL_BIT(MDL_SHARED_UPGRADABLE) | MDL_BIT(MDL_SHARED_WRITE) |
MDL_BIT(MDL_SHARED_READ) | MDL_BIT(MDL_SHARED_HIGH_PRIO) |
MDL_BIT(MDL_SHARED)
};
const MDL_lock::bitmap_t
MDL_lock::MDL_object_lock::m_waiting_incompatible[MDL_TYPE_END]=
{
0,
MDL_BIT(MDL_EXCLUSIVE),
0,
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_NO_WRITE),
MDL_BIT(MDL_EXCLUSIVE),
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_WRITE),
MDL_BIT(MDL_EXCLUSIVE),
MDL_BIT(MDL_EXCLUSIVE),
0
};
/**
Check if request for the metadata lock can be satisfied given its
current state.
@param type_arg The requested lock type.
@param requestor_ctx The MDL context of the requestor.
@param ignore_lock_priority Ignore lock priority.
@retval TRUE Lock request can be satisfied
@retval FALSE There is some conflicting lock.
@note In cases then current context already has "stronger" type
of lock on the object it will be automatically granted
thanks to usage of the MDL_context::find_ticket() method.
*/
bool
MDL_lock::can_grant_lock(enum_mdl_type type_arg,
MDL_context *requestor_ctx,
bool ignore_lock_priority) const
{
bool can_grant= FALSE;
bitmap_t waiting_incompat_map= incompatible_waiting_types_bitmap()[type_arg];
bitmap_t granted_incompat_map= incompatible_granted_types_bitmap()[type_arg];
bool wsrep_can_grant= TRUE;
/*
New lock request can be satisfied iff:
- There are no incompatible types of satisfied requests
in other contexts
- There are no waiting requests which have higher priority
than this request when priority was not ignored.
*/
if (ignore_lock_priority || !(m_waiting.bitmap() & waiting_incompat_map))
{
if (! (m_granted.bitmap() & granted_incompat_map))
can_grant= TRUE;
else
{
Ticket_iterator it(m_granted);
MDL_ticket *ticket;
/* Check that the incompatible lock belongs to some other context. */
while ((ticket= it++))
{
if (ticket->get_ctx() != requestor_ctx &&
ticket->is_incompatible_when_granted(type_arg))
{
#ifdef WITH_WSREP
if (wsrep_thd_is_BF(requestor_ctx->get_thd(),false) &&
key.mdl_namespace() == MDL_key::GLOBAL)
{
WSREP_DEBUG("global lock granted for BF: %lu %s",
thd_get_thread_id(requestor_ctx->get_thd()),
wsrep_thd_query(requestor_ctx->get_thd()));
can_grant = true;
}
else if (!wsrep_grant_mdl_exception(requestor_ctx, ticket, &key))
{
wsrep_can_grant= FALSE;
if (wsrep_log_conflicts)
{
MDL_lock * lock = ticket->get_lock();
WSREP_INFO(
"MDL conflict db=%s table=%s ticket=%d solved by %s",
lock->key.db_name(), lock->key.name(), ticket->get_type(),
"abort" );
}
}
else
can_grant= TRUE;
/* Continue loop */
#else
break;
#endif /* WITH_WSREP */
}
}
if ((ticket == NULL) && wsrep_can_grant)
can_grant= TRUE; /* Incompatible locks are our own. */
}
}
else
{
if (wsrep_thd_is_BF(requestor_ctx->get_thd(), false) &&
key.mdl_namespace() == MDL_key::GLOBAL)
{
WSREP_DEBUG("global lock granted for BF (waiting queue): %lu %s",
thd_get_thread_id(requestor_ctx->get_thd()),
wsrep_thd_query(requestor_ctx->get_thd()));
can_grant = true;
}
}
return can_grant;
}
/**
Return thread id of the thread to which the first ticket was
granted.
*/
inline unsigned long
MDL_lock::get_lock_owner() const
{
Ticket_iterator it(m_granted);
MDL_ticket *ticket;
if ((ticket= it++))
return ticket->get_ctx()->get_thread_id();
return 0;
}
/** Remove a ticket from waiting or pending queue and wakeup up waiters. */
void MDL_lock::remove_ticket(LF_PINS *pins, Ticket_list MDL_lock::*list,
MDL_ticket *ticket)
{
mysql_prlock_wrlock(&m_rwlock);
(this->*list).remove_ticket(ticket);
if (is_empty())
mdl_locks.remove(pins, this);
else
{
/*
There can be some contexts waiting to acquire a lock
which now might be able to do it. Grant the lock to
them and wake them up!
We always try to reschedule locks, since there is no easy way
(i.e. by looking at the bitmaps) to find out whether it is
required or not.
In a general case, even when the queue's bitmap is not changed
after removal of the ticket, there is a chance that some request
can be satisfied (due to the fact that a granted request
reflected in the bitmap might belong to the same context as a
pending request).
*/
reschedule_waiters();
mysql_prlock_unlock(&m_rwlock);
}
}
/**
Check if we have any pending locks which conflict with existing
shared lock.
@pre The ticket must match an acquired lock.
@return TRUE if there is a conflicting lock request, FALSE otherwise.
*/
bool MDL_lock::has_pending_conflicting_lock(enum_mdl_type type)
{
bool result;
mysql_prlock_rdlock(&m_rwlock);
result= (m_waiting.bitmap() & incompatible_granted_types_bitmap()[type]);
mysql_prlock_unlock(&m_rwlock);
return result;
}
MDL_wait_for_graph_visitor::~MDL_wait_for_graph_visitor()
{
}
MDL_wait_for_subgraph::~MDL_wait_for_subgraph()
{
}
/**
Check if ticket represents metadata lock of "stronger" or equal type
than specified one. I.e. if metadata lock represented by ticket won't
allow any of locks which are not allowed by specified type of lock.
@return TRUE if ticket has stronger or equal type
FALSE otherwise.
*/
bool MDL_ticket::has_stronger_or_equal_type(enum_mdl_type type) const
{
const MDL_lock::bitmap_t *
granted_incompat_map= m_lock->incompatible_granted_types_bitmap();
return ! (granted_incompat_map[type] & ~(granted_incompat_map[m_type]));
}
bool MDL_ticket::is_incompatible_when_granted(enum_mdl_type type) const
{
return (MDL_BIT(m_type) &
m_lock->incompatible_granted_types_bitmap()[type]);
}
bool MDL_ticket::is_incompatible_when_waiting(enum_mdl_type type) const
{
return (MDL_BIT(m_type) &
m_lock->incompatible_waiting_types_bitmap()[type]);
}
/**
Check whether the context already holds a compatible lock ticket
on an object.
Start searching from list of locks for the same duration as lock
being requested. If not look at lists for other durations.
@param mdl_request Lock request object for lock to be acquired
@param[out] result_duration Duration of lock which was found.
@note Tickets which correspond to lock types "stronger" than one
being requested are also considered compatible.
@return A pointer to the lock ticket for the object or NULL otherwise.
*/
MDL_ticket *
MDL_context::find_ticket(MDL_request *mdl_request,
enum_mdl_duration *result_duration)
{
MDL_ticket *ticket;
int i;
for (i= 0; i < MDL_DURATION_END; i++)
{
enum_mdl_duration duration= (enum_mdl_duration)((mdl_request->duration+i) %
MDL_DURATION_END);
Ticket_iterator it(m_tickets[duration]);
while ((ticket= it++))
{
if (mdl_request->key.is_equal(&ticket->m_lock->key) &&
ticket->has_stronger_or_equal_type(mdl_request->type))
{
DBUG_PRINT("info", ("Adding mdl lock %d to %d",
mdl_request->type, ticket->m_type));
*result_duration= duration;
return ticket;
}
}
}
return NULL;
}
/**
Try to acquire one lock.
Unlike exclusive locks, shared locks are acquired one by
one. This is interface is chosen to simplify introduction of
the new locking API to the system. MDL_context::try_acquire_lock()
is currently used from open_table(), and there we have only one
table to work with.
This function may also be used to try to acquire an exclusive
lock on a destination table, by ALTER TABLE ... RENAME.
Returns immediately without any side effect if encounters a lock
conflict. Otherwise takes the lock.
FIXME: Compared to lock_table_name_if_not_cached() (from 5.1)
it gives slightly more false negatives.
@param mdl_request [in/out] Lock request object for lock to be acquired
@retval FALSE Success. The lock may have not been acquired.
Check the ticket, if it's NULL, a conflicting lock
exists.
@retval TRUE Out of resources, an error has been reported.
*/
bool
MDL_context::try_acquire_lock(MDL_request *mdl_request)
{
MDL_ticket *ticket;
if (try_acquire_lock_impl(mdl_request, &ticket))
return TRUE;
if (! mdl_request->ticket)
{
/*
Our attempt to acquire lock without waiting has failed.
Let us release resources which were acquired in the process.
We can't get here if we allocated a new lock object so there
is no need to release it.
*/
DBUG_ASSERT(! ticket->m_lock->is_empty());
mysql_prlock_unlock(&ticket->m_lock->m_rwlock);
MDL_ticket::destroy(ticket);
}
return FALSE;
}
/**
Auxiliary method for acquiring lock without waiting.
@param mdl_request [in/out] Lock request object for lock to be acquired
@param out_ticket [out] Ticket for the request in case when lock
has not been acquired.
@retval FALSE Success. The lock may have not been acquired.
Check MDL_request::ticket, if it's NULL, a conflicting
lock exists. In this case "out_ticket" out parameter
points to ticket which was constructed for the request.
MDL_ticket::m_lock points to the corresponding MDL_lock
object and MDL_lock::m_rwlock write-locked.
@retval TRUE Out of resources, an error has been reported.
*/
bool
MDL_context::try_acquire_lock_impl(MDL_request *mdl_request,
MDL_ticket **out_ticket)
{
MDL_lock *lock;
MDL_key *key= &mdl_request->key;
MDL_ticket *ticket;
enum_mdl_duration found_duration;
DBUG_ASSERT(mdl_request->type != MDL_EXCLUSIVE ||
is_lock_owner(MDL_key::GLOBAL, "", "", MDL_INTENTION_EXCLUSIVE));
DBUG_ASSERT(mdl_request->ticket == NULL);
/* Don't take chances in production. */
mdl_request->ticket= NULL;
/*
Check whether the context already holds a shared lock on the object,
and if so, grant the request.
*/
if ((ticket= find_ticket(mdl_request, &found_duration)))
{
DBUG_ASSERT(ticket->m_lock);
DBUG_ASSERT(ticket->has_stronger_or_equal_type(mdl_request->type));
/*
If the request is for a transactional lock, and we found
a transactional lock, just reuse the found ticket.
It's possible that we found a transactional lock,
but the request is for a HANDLER lock. In that case HANDLER
code will clone the ticket (see below why it's needed).
If the request is for a transactional lock, and we found
a HANDLER lock, create a copy, to make sure that when user
does HANDLER CLOSE, the transactional lock is not released.
If the request is for a handler lock, and we found a
HANDLER lock, also do the clone. HANDLER CLOSE for one alias
should not release the lock on the table HANDLER opened through
a different alias.
*/
mdl_request->ticket= ticket;
if ((found_duration != mdl_request->duration ||
mdl_request->duration == MDL_EXPLICIT) &&
clone_ticket(mdl_request))
{
/* Clone failed. */
mdl_request->ticket= NULL;
return TRUE;
}
return FALSE;
}
if (fix_pins())
return TRUE;
if (!(ticket= MDL_ticket::create(this, mdl_request->type
#ifndef DBUG_OFF
, mdl_request->duration
#endif
)))
return TRUE;
/* The below call implicitly locks MDL_lock::m_rwlock on success. */
if (!(lock= mdl_locks.find_or_insert(m_pins, key)))
{
MDL_ticket::destroy(ticket);
return TRUE;
}
ticket->m_lock= lock;
if (lock->can_grant_lock(mdl_request->type, this, false))
{
lock->m_granted.add_ticket(ticket);
mysql_prlock_unlock(&lock->m_rwlock);
m_tickets[mdl_request->duration].push_front(ticket);
mdl_request->ticket= ticket;
}
else
*out_ticket= ticket;
return FALSE;
}
/**
Create a copy of a granted ticket.
This is used to make sure that HANDLER ticket
is never shared with a ticket that belongs to
a transaction, so that when we HANDLER CLOSE,
we don't release a transactional ticket, and
vice versa -- when we COMMIT, we don't mistakenly
release a ticket for an open HANDLER.
@retval TRUE Out of memory.
@retval FALSE Success.
*/
bool
MDL_context::clone_ticket(MDL_request *mdl_request)
{
MDL_ticket *ticket;
/*
Since in theory we can clone ticket belonging to a different context
we need to prepare target context for possible attempts to release
lock and thus possible removal of MDL_lock from MDL_map container.
So we allocate pins to be able to work with this container if they
are not allocated already.
*/
if (fix_pins())
return TRUE;
/*
By submitting mdl_request->type to MDL_ticket::create()
we effectively downgrade the cloned lock to the level of
the request.
*/
if (!(ticket= MDL_ticket::create(this, mdl_request->type
#ifndef DBUG_OFF
, mdl_request->duration
#endif
)))
return TRUE;
/* clone() is not supposed to be used to get a stronger lock. */
DBUG_ASSERT(mdl_request->ticket->has_stronger_or_equal_type(ticket->m_type));
ticket->m_lock= mdl_request->ticket->m_lock;
mdl_request->ticket= ticket;
mysql_prlock_wrlock(&ticket->m_lock->m_rwlock);
ticket->m_lock->m_granted.add_ticket(ticket);
mysql_prlock_unlock(&ticket->m_lock->m_rwlock);
m_tickets[mdl_request->duration].push_front(ticket);
return FALSE;
}
/**
Check if there is any conflicting lock that could cause this thread
to wait for another thread which is not ready to commit.
This is always an error, as the upper level of parallel replication
should not allow a scheduling of a conflicting DDL until all earlier
transactions has commited.
This function is only called for a slave using parallel replication
and trying to get an exclusive lock for the table.
*/
#ifndef DBUG_OFF
bool MDL_lock::check_if_conflicting_replication_locks(MDL_context *ctx)
{
Ticket_iterator it(m_granted);
MDL_ticket *conflicting_ticket;
rpl_group_info *rgi_slave= ctx->get_thd()->rgi_slave;
if (!rgi_slave->gtid_sub_id)
return 0;
while ((conflicting_ticket= it++))
{
if (conflicting_ticket->get_ctx() != ctx)
{
MDL_context *conflicting_ctx= conflicting_ticket->get_ctx();
rpl_group_info *conflicting_rgi_slave;
conflicting_rgi_slave= conflicting_ctx->get_thd()->rgi_slave;
/*
If the conflicting thread is another parallel replication
thread for the same master and it's not in commit stage, then
the current transaction has started too early and something is
seriously wrong.
*/
if (conflicting_rgi_slave &&
conflicting_rgi_slave->gtid_sub_id &&
conflicting_rgi_slave->rli == rgi_slave->rli &&
conflicting_rgi_slave->current_gtid.domain_id ==
rgi_slave->current_gtid.domain_id &&
!conflicting_rgi_slave->did_mark_start_commit)
return 1; // Fatal error
}
}
return 0;
}
#endif
/**
Acquire one lock with waiting for conflicting locks to go away if needed.
@param mdl_request [in/out] Lock request object for lock to be acquired
@param lock_wait_timeout [in] Seconds to wait before timeout.
@retval FALSE Success. MDL_request::ticket points to the ticket
for the lock.
@retval TRUE Failure (Out of resources or waiting is aborted),
*/
bool
MDL_context::acquire_lock(MDL_request *mdl_request, double lock_wait_timeout)
{
MDL_lock *lock;
MDL_ticket *ticket;
MDL_wait::enum_wait_status wait_status;
DBUG_ENTER("MDL_context::acquire_lock");
DBUG_PRINT("enter", ("lock_type: %d", mdl_request->type));
if (try_acquire_lock_impl(mdl_request, &ticket))
DBUG_RETURN(TRUE);
if (mdl_request->ticket)
{
/*
We have managed to acquire lock without waiting.
MDL_lock, MDL_context and MDL_request were updated
accordingly, so we can simply return success.
*/
DBUG_PRINT("info", ("Got lock without waiting"));
DBUG_RETURN(FALSE);
}
/*
Our attempt to acquire lock without waiting has failed.
As a result of this attempt we got MDL_ticket with m_lock
member pointing to the corresponding MDL_lock object which
has MDL_lock::m_rwlock write-locked.
*/
lock= ticket->m_lock;
if (lock_wait_timeout == 0)
{
mysql_prlock_unlock(&lock->m_rwlock);
MDL_ticket::destroy(ticket);
my_error(ER_LOCK_WAIT_TIMEOUT, MYF(0));
DBUG_RETURN(TRUE);
}
lock->m_waiting.add_ticket(ticket);
/*
Once we added a pending ticket to the waiting queue,
we must ensure that our wait slot is empty, so
that our lock request can be scheduled. Do that in the
critical section formed by the acquired write lock on MDL_lock.
*/
m_wait.reset_status();
/*
Don't break conflicting locks if timeout is 0 as 0 is used
To check if there is any conflicting locks...
*/
if (lock->needs_notification(ticket) && lock_wait_timeout)
lock->notify_conflicting_locks(this);
/*
Ensure that if we are trying to get an exclusive lock for a slave
running parallel replication, then we are not blocked by another
parallel slave thread that is not committed. This should never happen as
the parallel replication scheduler should never schedule a DDL while
DML's are still running.
*/
DBUG_SLOW_ASSERT((mdl_request->type != MDL_INTENTION_EXCLUSIVE &&
mdl_request->type != MDL_EXCLUSIVE) ||
!(get_thd()->rgi_slave &&
get_thd()->rgi_slave->is_parallel_exec &&
lock->check_if_conflicting_replication_locks(this)));
mysql_prlock_unlock(&lock->m_rwlock);
will_wait_for(ticket);
/* There is a shared or exclusive lock on the object. */
DEBUG_SYNC(get_thd(), "mdl_acquire_lock_wait");
find_deadlock();
struct timespec abs_timeout, abs_shortwait;
set_timespec_nsec(abs_timeout,
(ulonglong)(lock_wait_timeout * 1000000000ULL));
set_timespec(abs_shortwait, 1);
wait_status= MDL_wait::EMPTY;
while (cmp_timespec(abs_shortwait, abs_timeout) <= 0)
{
/* abs_timeout is far away. Wait a short while and notify locks. */
wait_status= m_wait.timed_wait(m_owner, &abs_shortwait, FALSE,
mdl_request->key.get_wait_state_name());
if (wait_status != MDL_wait::EMPTY)
break;
/* Check if the client is gone while we were waiting. */
if (! thd_is_connected(m_owner->get_thd()))
{
/*
* The client is disconnected. Don't wait forever:
* assume it's the same as a wait timeout, this
* ensures all error handling is correct.
*/
wait_status= MDL_wait::TIMEOUT;
break;
}
mysql_prlock_wrlock(&lock->m_rwlock);
if (lock->needs_notification(ticket))
lock->notify_conflicting_locks(this);
mysql_prlock_unlock(&lock->m_rwlock);
set_timespec(abs_shortwait, 1);
}
if (wait_status == MDL_wait::EMPTY)
wait_status= m_wait.timed_wait(m_owner, &abs_timeout, TRUE,
mdl_request->key.get_wait_state_name());
done_waiting_for();
if (wait_status != MDL_wait::GRANTED)
{
lock->remove_ticket(m_pins, &MDL_lock::m_waiting, ticket);
MDL_ticket::destroy(ticket);
switch (wait_status)
{
case MDL_wait::VICTIM:
my_error(ER_LOCK_DEADLOCK, MYF(0));
break;
case MDL_wait::TIMEOUT:
my_error(ER_LOCK_WAIT_TIMEOUT, MYF(0));
break;
case MDL_wait::KILLED:
get_thd()->send_kill_message();
break;
default:
DBUG_ASSERT(0);
break;
}
DBUG_RETURN(TRUE);
}
/*
We have been granted our request.
State of MDL_lock object is already being appropriately updated by a
concurrent thread (@sa MDL_lock:reschedule_waiters()).
So all we need to do is to update MDL_context and MDL_request objects.
*/
DBUG_ASSERT(wait_status == MDL_wait::GRANTED);
m_tickets[mdl_request->duration].push_front(ticket);
mdl_request->ticket= ticket;
DBUG_RETURN(FALSE);
}
extern "C" int mdl_request_ptr_cmp(const void* ptr1, const void* ptr2)
{
MDL_request *req1= *(MDL_request**)ptr1;
MDL_request *req2= *(MDL_request**)ptr2;
return req1->key.cmp(&req2->key);
}
/**
Acquire exclusive locks. There must be no granted locks in the
context.
This is a replacement of lock_table_names(). It is used in
RENAME, DROP and other DDL SQL statements.
@param mdl_requests List of requests for locks to be acquired.
@param lock_wait_timeout Seconds to wait before timeout.
@note The list of requests should not contain non-exclusive lock requests.
There should not be any acquired locks in the context.
@note Assumes that one already owns scoped intention exclusive lock.
@retval FALSE Success
@retval TRUE Failure
*/
bool MDL_context::acquire_locks(MDL_request_list *mdl_requests,
double lock_wait_timeout)
{
MDL_request_list::Iterator it(*mdl_requests);
MDL_request **sort_buf, **p_req;
MDL_savepoint mdl_svp= mdl_savepoint();
ssize_t req_count= static_cast<ssize_t>(mdl_requests->elements());
DBUG_ENTER("MDL_context::acquire_locks");
if (req_count == 0)
DBUG_RETURN(FALSE);
/* Sort requests according to MDL_key. */
if (! (sort_buf= (MDL_request **)my_malloc(req_count *
sizeof(MDL_request*),
MYF(MY_WME))))
DBUG_RETURN(TRUE);
for (p_req= sort_buf; p_req < sort_buf + req_count; p_req++)
*p_req= it++;
my_qsort(sort_buf, req_count, sizeof(MDL_request*),
mdl_request_ptr_cmp);
for (p_req= sort_buf; p_req < sort_buf + req_count; p_req++)
{
if (acquire_lock(*p_req, lock_wait_timeout))
goto err;
}
my_free(sort_buf);
DBUG_RETURN(FALSE);
err:
/*
Release locks we have managed to acquire so far.
Use rollback_to_savepoint() since there may be duplicate
requests that got assigned the same ticket.
*/
rollback_to_savepoint(mdl_svp);
/* Reset lock requests back to its initial state. */
for (req_count= p_req - sort_buf, p_req= sort_buf;
p_req < sort_buf + req_count; p_req++)
{
(*p_req)->ticket= NULL;
}
my_free(sort_buf);
DBUG_RETURN(TRUE);
}
/**
Upgrade a shared metadata lock.
Used in ALTER TABLE.
@param mdl_ticket Lock to upgrade.
@param new_type Lock type to upgrade to.
@param lock_wait_timeout Seconds to wait before timeout.
@note In case of failure to upgrade lock (e.g. because upgrader
was killed) leaves lock in its original state (locked in
shared mode).
@note There can be only one upgrader for a lock or we will have deadlock.
This invariant is ensured by the fact that upgradeable locks SU, SNW
and SNRW are not compatible with each other and themselves.
@retval FALSE Success
@retval TRUE Failure (thread was killed)
*/
bool
MDL_context::upgrade_shared_lock(MDL_ticket *mdl_ticket,
enum_mdl_type new_type,
double lock_wait_timeout)
{
MDL_request mdl_xlock_request;
MDL_savepoint mdl_svp= mdl_savepoint();
bool is_new_ticket;
DBUG_ENTER("MDL_context::upgrade_shared_lock");
DBUG_PRINT("enter",("new_type: %d lock_wait_timeout: %f", new_type,
lock_wait_timeout));
DEBUG_SYNC(get_thd(), "mdl_upgrade_lock");
/*
Do nothing if already upgraded. Used when we FLUSH TABLE under
LOCK TABLES and a table is listed twice in LOCK TABLES list.
*/
if (mdl_ticket->has_stronger_or_equal_type(new_type))
DBUG_RETURN(FALSE);
mdl_xlock_request.init(&mdl_ticket->m_lock->key, new_type,
MDL_TRANSACTION);
if (acquire_lock(&mdl_xlock_request, lock_wait_timeout))
DBUG_RETURN(TRUE);
is_new_ticket= ! has_lock(mdl_svp, mdl_xlock_request.ticket);
/* Merge the acquired and the original lock. @todo: move to a method. */
mysql_prlock_wrlock(&mdl_ticket->m_lock->m_rwlock);
if (is_new_ticket)
mdl_ticket->m_lock->m_granted.remove_ticket(mdl_xlock_request.ticket);
/*
Set the new type of lock in the ticket. To update state of
MDL_lock object correctly we need to temporarily exclude
ticket from the granted queue and then include it back.
*/
mdl_ticket->m_lock->m_granted.remove_ticket(mdl_ticket);
mdl_ticket->m_type= new_type;
mdl_ticket->m_lock->m_granted.add_ticket(mdl_ticket);
mysql_prlock_unlock(&mdl_ticket->m_lock->m_rwlock);
if (is_new_ticket)
{
m_tickets[MDL_TRANSACTION].remove(mdl_xlock_request.ticket);
MDL_ticket::destroy(mdl_xlock_request.ticket);
}
DBUG_RETURN(FALSE);
}
/**
A fragment of recursive traversal of the wait-for graph
in search for deadlocks. Direct the deadlock visitor to all
contexts that own the lock the current node in the wait-for
graph is waiting for.
As long as the initial node is remembered in the visitor,
a deadlock is found when the same node is seen twice.
*/
bool MDL_lock::visit_subgraph(MDL_ticket *waiting_ticket,
MDL_wait_for_graph_visitor *gvisitor)
{
MDL_ticket *ticket;
MDL_context *src_ctx= waiting_ticket->get_ctx();
bool result= TRUE;
mysql_prlock_rdlock(&m_rwlock);
/* Must be initialized after taking a read lock. */
Ticket_iterator granted_it(m_granted);
Ticket_iterator waiting_it(m_waiting);
/*
MDL_lock's waiting and granted queues and MDL_context::m_waiting_for
member are updated by different threads when the lock is granted
(see MDL_context::acquire_lock() and MDL_lock::reschedule_waiters()).
As a result, here we may encounter a situation when MDL_lock data
already reflects the fact that the lock was granted but
m_waiting_for member has not been updated yet.
For example, imagine that:
thread1: Owns SNW lock on table t1.
thread2: Attempts to acquire SW lock on t1,
but sees an active SNW lock.
Thus adds the ticket to the waiting queue and
sets m_waiting_for to point to the ticket.
thread1: Releases SNW lock, updates MDL_lock object to
grant SW lock to thread2 (moves the ticket for
SW from waiting to the active queue).
Attempts to acquire a new SNW lock on t1,
sees an active SW lock (since it is present in the
active queue), adds ticket for SNW lock to the waiting
queue, sets m_waiting_for to point to this ticket.
At this point deadlock detection algorithm run by thread1 will see that:
- Thread1 waits for SNW lock on t1 (since m_waiting_for is set).
- SNW lock is not granted, because it conflicts with active SW lock
owned by thread 2 (since ticket for SW is present in granted queue).
- Thread2 waits for SW lock (since its m_waiting_for has not been
updated yet!).
- SW lock is not granted because there is pending SNW lock from thread1.
Therefore deadlock should exist [sic!].
To avoid detection of such false deadlocks we need to check the "actual"
status of the ticket being waited for, before analyzing its blockers.
We do this by checking the wait status of the context which is waiting
for it. To avoid races this has to be done under protection of
MDL_lock::m_rwlock lock.
*/
if (src_ctx->m_wait.get_status() != MDL_wait::EMPTY)
{
result= FALSE;
goto end;
}
/*
To avoid visiting nodes which were already marked as victims of
deadlock detection (or whose requests were already satisfied) we
enter the node only after peeking at its wait status.
This is necessary to avoid active waiting in a situation
when previous searches for a deadlock already selected the
node we're about to enter as a victim (see the comment
in MDL_context::find_deadlock() for explanation why several searches
can be performed for the same wait).
There is no guarantee that the node isn't chosen a victim while we
are visiting it but this is OK: in the worst case we might do some
extra work and one more context might be chosen as a victim.
*/
if (gvisitor->enter_node(src_ctx))
goto end;
/*
We do a breadth-first search first -- that is, inspect all
edges of the current node, and only then follow up to the next
node. In workloads that involve wait-for graph loops this
has proven to be a more efficient strategy [citation missing].
*/
while ((ticket= granted_it++))
{
/* Filter out edges that point to the same node. */
if (ticket->get_ctx() != src_ctx &&
ticket->is_incompatible_when_granted(waiting_ticket->get_type()) &&
gvisitor->inspect_edge(ticket->get_ctx()))
{
goto end_leave_node;
}
}
while ((ticket= waiting_it++))
{
/* Filter out edges that point to the same node. */
if (ticket->get_ctx() != src_ctx &&
ticket->is_incompatible_when_waiting(waiting_ticket->get_type()) &&
gvisitor->inspect_edge(ticket->get_ctx()))
{
goto end_leave_node;
}
}
/* Recurse and inspect all adjacent nodes. */
granted_it.rewind();
while ((ticket= granted_it++))
{
if (ticket->get_ctx() != src_ctx &&
ticket->is_incompatible_when_granted(waiting_ticket->get_type()) &&
ticket->get_ctx()->visit_subgraph(gvisitor))
{
goto end_leave_node;
}
}
waiting_it.rewind();
while ((ticket= waiting_it++))
{
if (ticket->get_ctx() != src_ctx &&
ticket->is_incompatible_when_waiting(waiting_ticket->get_type()) &&
ticket->get_ctx()->visit_subgraph(gvisitor))
{
goto end_leave_node;
}
}
result= FALSE;
end_leave_node:
gvisitor->leave_node(src_ctx);
end:
mysql_prlock_unlock(&m_rwlock);
return result;
}
/**
Traverse a portion of wait-for graph which is reachable
through the edge represented by this ticket and search
for deadlocks.
@retval TRUE A deadlock is found. A pointer to deadlock
victim is saved in the visitor.
@retval FALSE
*/
bool MDL_ticket::accept_visitor(MDL_wait_for_graph_visitor *gvisitor)
{
return m_lock->visit_subgraph(this, gvisitor);
}
/**
A fragment of recursive traversal of the wait-for graph of
MDL contexts in the server in search for deadlocks.
Assume this MDL context is a node in the wait-for graph,
and direct the visitor to all adjacent nodes. As long
as the starting node is remembered in the visitor, a
deadlock is found when the same node is visited twice.
One MDL context is connected to another in the wait-for
graph if it waits on a resource that is held by the other
context.
@retval TRUE A deadlock is found. A pointer to deadlock
victim is saved in the visitor.
@retval FALSE
*/
bool MDL_context::visit_subgraph(MDL_wait_for_graph_visitor *gvisitor)
{
bool result= FALSE;
mysql_prlock_rdlock(&m_LOCK_waiting_for);
if (m_waiting_for)
result= m_waiting_for->accept_visitor(gvisitor);
mysql_prlock_unlock(&m_LOCK_waiting_for);
return result;
}
/**
Try to find a deadlock. This function produces no errors.
@note If during deadlock resolution context which performs deadlock
detection is chosen as a victim it will be informed about the
fact by setting VICTIM status to its wait slot.
*/
void MDL_context::find_deadlock()
{
while (1)
{
/*
The fact that we use fresh instance of gvisitor for each
search performed by find_deadlock() below is important,
the code responsible for victim selection relies on this.
*/
Deadlock_detection_visitor dvisitor(this);
MDL_context *victim;
if (! visit_subgraph(&dvisitor))
{
/* No deadlocks are found! */
break;
}
victim= dvisitor.get_victim();
/*
Failure to change status of the victim is OK as it means
that the victim has received some other message and is
about to stop its waiting/to break deadlock loop.
Even when the initiator of the deadlock search is
chosen the victim, we need to set the respective wait
result in order to "close" it for any attempt to
schedule the request.
This is needed to avoid a possible race during
cleanup in case when the lock request on which the
context was waiting is concurrently satisfied.
*/
(void) victim->m_wait.set_status(MDL_wait::VICTIM);
victim->unlock_deadlock_victim();
if (victim == this)
break;
/*
After adding a new edge to the waiting graph we found that it
creates a loop (i.e. there is a deadlock). We decided to destroy
this loop by removing an edge, but not the one that we added.
Since this doesn't guarantee that all loops created by addition
of the new edge are destroyed, we have to repeat the search.
*/
}
}
/**
Release lock.
@param duration Lock duration.
@param ticket Ticket for lock to be released.
*/
void MDL_context::release_lock(enum_mdl_duration duration, MDL_ticket *ticket)
{
MDL_lock *lock= ticket->m_lock;
DBUG_ENTER("MDL_context::release_lock");
DBUG_PRINT("enter", ("db: '%s' name: '%s'",
lock->key.db_name(), lock->key.name()));
DBUG_ASSERT(this == ticket->get_ctx());
lock->remove_ticket(m_pins, &MDL_lock::m_granted, ticket);
m_tickets[duration].remove(ticket);
MDL_ticket::destroy(ticket);
DBUG_VOID_RETURN;
}
/**
Release lock with explicit duration.
@param ticket Ticket for lock to be released.
*/
void MDL_context::release_lock(MDL_ticket *ticket)
{
DBUG_SLOW_ASSERT(ticket->m_duration == MDL_EXPLICIT);
release_lock(MDL_EXPLICIT, ticket);
}
/**
Release all locks associated with the context. If the sentinel
is not NULL, do not release locks stored in the list after and
including the sentinel.
Statement and transactional locks are added to the beginning of
the corresponding lists, i.e. stored in reverse temporal order.
This allows to employ this function to:
- back off in case of a lock conflict.
- release all locks in the end of a statement or transaction
- rollback to a savepoint.
*/
void MDL_context::release_locks_stored_before(enum_mdl_duration duration,
MDL_ticket *sentinel)
{
MDL_ticket *ticket;
Ticket_iterator it(m_tickets[duration]);
DBUG_ENTER("MDL_context::release_locks_stored_before");
if (m_tickets[duration].is_empty())
DBUG_VOID_RETURN;
while ((ticket= it++) && ticket != sentinel)
{
DBUG_PRINT("info", ("found lock to release ticket=%p", ticket));
release_lock(duration, ticket);
}
DBUG_VOID_RETURN;
}
/**
Release all explicit locks in the context which correspond to the
same name/object as this lock request.
@param ticket One of the locks for the name/object for which all
locks should be released.
*/
void MDL_context::release_all_locks_for_name(MDL_ticket *name)
{
/* Use MDL_ticket::m_lock to identify other locks for the same object. */
MDL_lock *lock= name->m_lock;
/* Remove matching lock tickets from the context. */
MDL_ticket *ticket;
Ticket_iterator it_ticket(m_tickets[MDL_EXPLICIT]);
while ((ticket= it_ticket++))
{
DBUG_ASSERT(ticket->m_lock);
if (ticket->m_lock == lock)
release_lock(MDL_EXPLICIT, ticket);
}
}
/**
Downgrade an EXCLUSIVE or SHARED_NO_WRITE lock to shared metadata lock.
@param type Type of lock to which exclusive lock should be downgraded.
*/
void MDL_ticket::downgrade_lock(enum_mdl_type type)
{
/*
Do nothing if already downgraded. Used when we FLUSH TABLE under
LOCK TABLES and a table is listed twice in LOCK TABLES list.
Note that this code might even try to "downgrade" a weak lock
(e.g. SW) to a stronger one (e.g SNRW). So we can't even assert
here that target lock is weaker than existing lock.
*/
if (m_type == type || !has_stronger_or_equal_type(type))
return;
/* Only allow downgrade from EXCLUSIVE and SHARED_NO_WRITE. */
DBUG_ASSERT(m_type == MDL_EXCLUSIVE ||
m_type == MDL_SHARED_NO_WRITE);
mysql_prlock_wrlock(&m_lock->m_rwlock);
/*
To update state of MDL_lock object correctly we need to temporarily
exclude ticket from the granted queue and then include it back.
*/
m_lock->m_granted.remove_ticket(this);
m_type= type;
m_lock->m_granted.add_ticket(this);
m_lock->reschedule_waiters();
mysql_prlock_unlock(&m_lock->m_rwlock);
}
/**
Auxiliary function which allows to check if we have some kind of lock on
a object. Returns TRUE if we have a lock of a given or stronger type.
@param mdl_namespace Id of object namespace
@param db Name of the database
@param name Name of the object
@param mdl_type Lock type. Pass in the weakest type to find
out if there is at least some lock.
@return TRUE if current context contains satisfied lock for the object,
FALSE otherwise.
*/
bool
MDL_context::is_lock_owner(MDL_key::enum_mdl_namespace mdl_namespace,
const char *db, const char *name,
enum_mdl_type mdl_type)
{
MDL_request mdl_request;
enum_mdl_duration not_unused;
/* We don't care about exact duration of lock here. */
mdl_request.init(mdl_namespace, db, name, mdl_type, MDL_TRANSACTION);
MDL_ticket *ticket= find_ticket(&mdl_request, &not_unused);
DBUG_ASSERT(ticket == NULL || ticket->m_lock);
return ticket;
}
/**
Return thread id of the owner of the lock or 0 if
there is no owner.
@note: Lock type is not considered at all, the function
simply checks that there is some lock for the given key.
@return thread id of the owner of the lock or 0
*/
unsigned long
MDL_context::get_lock_owner(MDL_key *key)
{
fix_pins();
return mdl_locks.get_lock_owner(m_pins, key);
}
/**
Check if we have any pending locks which conflict with existing shared lock.
@pre The ticket must match an acquired lock.
@return TRUE if there is a conflicting lock request, FALSE otherwise.
*/
bool MDL_ticket::has_pending_conflicting_lock() const
{
return m_lock->has_pending_conflicting_lock(m_type);
}
/** Return a key identifying this lock. */
MDL_key *MDL_ticket::get_key() const
{
return &m_lock->key;
}
/**
Releases metadata locks that were acquired after a specific savepoint.
@note Used to release tickets acquired during a savepoint unit.
@note It's safe to iterate and unlock any locks after taken after this
savepoint because other statements that take other special locks
cause a implicit commit (ie LOCK TABLES).
*/
void MDL_context::rollback_to_savepoint(const MDL_savepoint &mdl_savepoint)
{
DBUG_ENTER("MDL_context::rollback_to_savepoint");
/* If savepoint is NULL, it is from the start of the transaction. */
release_locks_stored_before(MDL_STATEMENT, mdl_savepoint.m_stmt_ticket);
release_locks_stored_before(MDL_TRANSACTION, mdl_savepoint.m_trans_ticket);
DBUG_VOID_RETURN;
}
/**
Release locks acquired by normal statements (SELECT, UPDATE,
DELETE, etc) in the course of a transaction. Do not release
HANDLER locks, if there are any.
This method is used at the end of a transaction, in
implementation of COMMIT (implicit or explicit) and ROLLBACK.
*/
void MDL_context::release_transactional_locks()
{
DBUG_ENTER("MDL_context::release_transactional_locks");
release_locks_stored_before(MDL_STATEMENT, NULL);
release_locks_stored_before(MDL_TRANSACTION, NULL);
DBUG_VOID_RETURN;
}
void MDL_context::release_statement_locks()
{
DBUG_ENTER("MDL_context::release_transactional_locks");
release_locks_stored_before(MDL_STATEMENT, NULL);
DBUG_VOID_RETURN;
}
/**
Does this savepoint have this lock?
@retval TRUE The ticket is older than the savepoint or
is an LT, HA or GLR ticket. Thus it belongs
to the savepoint or has explicit duration.
@retval FALSE The ticket is newer than the savepoint.
and is not an LT, HA or GLR ticket.
*/
bool MDL_context::has_lock(const MDL_savepoint &mdl_savepoint,
MDL_ticket *mdl_ticket)
{
MDL_ticket *ticket;
/* Start from the beginning, most likely mdl_ticket's been just acquired. */
MDL_context::Ticket_iterator s_it(m_tickets[MDL_STATEMENT]);
MDL_context::Ticket_iterator t_it(m_tickets[MDL_TRANSACTION]);
while ((ticket= s_it++) && ticket != mdl_savepoint.m_stmt_ticket)
{
if (ticket == mdl_ticket)
return FALSE;
}
while ((ticket= t_it++) && ticket != mdl_savepoint.m_trans_ticket)
{
if (ticket == mdl_ticket)
return FALSE;
}
return TRUE;
}
/**
Change lock duration for transactional lock.
@param ticket Ticket representing lock.
@param duration Lock duration to be set.
@note This method only supports changing duration of
transactional lock to some other duration.
*/
void MDL_context::set_lock_duration(MDL_ticket *mdl_ticket,
enum_mdl_duration duration)
{
DBUG_SLOW_ASSERT(mdl_ticket->m_duration == MDL_TRANSACTION &&
duration != MDL_TRANSACTION);
m_tickets[MDL_TRANSACTION].remove(mdl_ticket);
m_tickets[duration].push_front(mdl_ticket);
#ifndef DBUG_OFF
mdl_ticket->m_duration= duration;
#endif
}
/**
Set explicit duration for all locks in the context.
*/
void MDL_context::set_explicit_duration_for_all_locks()
{
int i;
MDL_ticket *ticket;
/*
In the most common case when this function is called list
of transactional locks is bigger than list of locks with
explicit duration. So we start by swapping these two lists
and then move elements from new list of transactional
locks and list of statement locks to list of locks with
explicit duration.
*/
m_tickets[MDL_EXPLICIT].swap(m_tickets[MDL_TRANSACTION]);
for (i= 0; i < MDL_EXPLICIT; i++)
{
Ticket_iterator it_ticket(m_tickets[i]);
while ((ticket= it_ticket++))
{
m_tickets[i].remove(ticket);
m_tickets[MDL_EXPLICIT].push_front(ticket);
}
}
#ifndef DBUG_OFF
Ticket_iterator exp_it(m_tickets[MDL_EXPLICIT]);
while ((ticket= exp_it++))
ticket->m_duration= MDL_EXPLICIT;
#endif
}
/**
Set transactional duration for all locks in the context.
*/
void MDL_context::set_transaction_duration_for_all_locks()
{
MDL_ticket *ticket;
/*
In the most common case when this function is called list
of explicit locks is bigger than two other lists (in fact,
list of statement locks is always empty). So we start by
swapping list of explicit and transactional locks and then
move contents of new list of explicit locks to list of
locks with transactional duration.
*/
DBUG_ASSERT(m_tickets[MDL_STATEMENT].is_empty());
m_tickets[MDL_TRANSACTION].swap(m_tickets[MDL_EXPLICIT]);
Ticket_iterator it_ticket(m_tickets[MDL_EXPLICIT]);
while ((ticket= it_ticket++))
{
m_tickets[MDL_EXPLICIT].remove(ticket);
m_tickets[MDL_TRANSACTION].push_front(ticket);
}
#ifndef DBUG_OFF
Ticket_iterator trans_it(m_tickets[MDL_TRANSACTION]);
while ((ticket= trans_it++))
ticket->m_duration= MDL_TRANSACTION;
#endif
}
void MDL_context::release_explicit_locks()
{
release_locks_stored_before(MDL_EXPLICIT, NULL);
}
bool MDL_context::has_explicit_locks()
{
MDL_ticket *ticket = NULL;
Ticket_iterator it(m_tickets[MDL_EXPLICIT]);
while ((ticket = it++))
{
return true;
}
return false;
}
#ifdef WITH_WSREP
static
const char *wsrep_get_mdl_type_name(enum_mdl_type type)
{
switch (type)
{
case MDL_INTENTION_EXCLUSIVE : return "intention exclusive";
case MDL_SHARED : return "shared";
case MDL_SHARED_HIGH_PRIO : return "shared high prio";
case MDL_SHARED_READ : return "shared read";
case MDL_SHARED_WRITE : return "shared write";
case MDL_SHARED_UPGRADABLE : return "shared upgradable";
case MDL_SHARED_NO_WRITE : return "shared no write";
case MDL_SHARED_NO_READ_WRITE : return "shared no read write";
case MDL_EXCLUSIVE : return "exclusive";
default: break;
}
return "UNKNOWN";
}
static
const char *wsrep_get_mdl_namespace_name(MDL_key::enum_mdl_namespace ns)
{
switch (ns)
{
case MDL_key::GLOBAL : return "GLOBAL";
case MDL_key::SCHEMA : return "SCHEMA";
case MDL_key::TABLE : return "TABLE";
case MDL_key::FUNCTION : return "FUNCTION";
case MDL_key::PROCEDURE : return "PROCEDURE";
case MDL_key::TRIGGER : return "TRIGGER";
case MDL_key::EVENT : return "EVENT";
case MDL_key::COMMIT : return "COMMIT";
case MDL_key::USER_LOCK : return "USER_LOCK";
default: break;
}
return "UNKNOWN";
}
void MDL_ticket::wsrep_report(bool debug)
{
if (!debug) return;
const PSI_stage_info *psi_stage= m_lock->key.get_wait_state_name();
WSREP_DEBUG("MDL ticket: type: %s space: %s db: %s name: %s (%s)",
wsrep_get_mdl_type_name(get_type()),
wsrep_get_mdl_namespace_name(m_lock->key.mdl_namespace()),
m_lock->key.db_name(),
m_lock->key.name(),
psi_stage->m_name);
}
#endif /* WITH_WSREP */
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