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mariadb/sql/mdl.cc
Konstantin Osipov 29c4873ad5 Commit on behalf of Dmitry Lenev. 
Merge his patch for Bug#52044 into 5.5, and apply 
review comments.
2010-08-12 17:50:23 +04:00

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/* Copyright (C) 2007-2008 MySQL AB
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, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
#include "mdl.h"
#include "debug_sync.h"
#include <hash.h>
#include <mysqld_error.h>
#ifdef HAVE_PSI_INTERFACE
static PSI_mutex_key key_MDL_map_mutex;
static PSI_mutex_key key_MDL_wait_LOCK_wait_status;
static PSI_mutex_info all_mdl_mutexes[]=
{
{ &key_MDL_map_mutex, "MDL_map::mutex", PSI_FLAG_GLOBAL},
{ &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)
{
const char *category= "sql";
int count;
if (PSI_server == NULL)
return;
count= array_elements(all_mdl_mutexes);
PSI_server->register_mutex(category, all_mdl_mutexes, count);
count= array_elements(all_mdl_rwlocks);
PSI_server->register_rwlock(category, all_mdl_rwlocks, count);
count= array_elements(all_mdl_conds);
PSI_server->register_cond(category, all_mdl_conds, count);
}
#endif /* HAVE_PSI_INTERFACE */
void notify_shared_lock(THD *thd, MDL_ticket *conflicting_ticket);
/**
Thread state names to be used in case when we have to wait on resource
belonging to certain namespace.
*/
const char *MDL_key::m_namespace_to_wait_state_name[NAMESPACE_END]=
{
"Waiting for global metadata lock",
"Waiting for schema metadata lock",
"Waiting for table metadata lock",
"Waiting for stored function metadata lock",
"Waiting for stored procedure metadata lock",
NULL
};
static bool mdl_initialized= 0;
/**
A collection of all MDL locks. A singleton,
there is only one instance of the map in the server.
Maps MDL_key to MDL_lock instances.
*/
class MDL_map
{
public:
void init();
void destroy();
MDL_lock *find(const MDL_key *key);
MDL_lock *find_or_insert(const MDL_key *key);
void remove(MDL_lock *lock);
private:
bool move_from_hash_to_lock_mutex(MDL_lock *lock);
private:
/** All acquired locks in the server. */
HASH m_locks;
/* Protects access to m_locks hash. */
mysql_mutex_t m_mutex;
};
/**
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 uchar 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;
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());
}
virtual const bitmap_t *incompatible_granted_types_bitmap() const = 0;
virtual const bitmap_t *incompatible_waiting_types_bitmap() const = 0;
bool has_pending_conflicting_lock(enum_mdl_type type);
bool can_grant_lock(enum_mdl_type type, MDL_context *requstor_ctx) const;
inline static MDL_lock *create(const MDL_key *key);
void notify_shared_locks(MDL_context *ctx)
{
Ticket_iterator it(m_granted);
MDL_ticket *conflicting_ticket;
while ((conflicting_ticket= it++))
{
if (conflicting_ticket->get_ctx() != ctx)
notify_shared_lock(ctx->get_thd(), conflicting_ticket);
}
}
void reschedule_waiters();
void remove_ticket(Ticket_list MDL_lock::*queue, MDL_ticket *ticket);
bool visit_subgraph(MDL_ticket *waiting_ticket,
MDL_wait_for_graph_visitor *gvisitor);
/** List of granted tickets for this lock. */
Ticket_list m_granted;
/** Tickets for contexts waiting to acquire a lock. */
Ticket_list m_waiting;
public:
MDL_lock(const MDL_key *key_arg)
: key(key_arg),
m_ref_usage(0),
m_ref_release(0),
m_is_destroyed(FALSE)
{
mysql_prlock_init(key_MDL_lock_rwlock, &m_rwlock);
}
virtual ~MDL_lock()
{
mysql_prlock_destroy(&m_rwlock);
}
inline static void destroy(MDL_lock *lock);
public:
/**
These three members are used to make it possible to separate
the mdl_locks.m_mutex mutex and MDL_lock::m_rwlock in
MDL_map::find_or_insert() for increased scalability.
The 'm_is_destroyed' member is only set by destroyers that
have both the mdl_locks.m_mutex and MDL_lock::m_rwlock, thus
holding any of the mutexes is sufficient to read it.
The 'm_ref_usage; is incremented under protection by
mdl_locks.m_mutex, but when 'm_is_destroyed' is set to TRUE, this
member is moved to be protected by the MDL_lock::m_rwlock.
This means that the MDL_map::find_or_insert() which only
holds the MDL_lock::m_rwlock can compare it to 'm_ref_release'
without acquiring mdl_locks.m_mutex again and if equal it can also
destroy the lock object safely.
The 'm_ref_release' is incremented under protection by
MDL_lock::m_rwlock.
Note since we are only interested in equality of these two
counters we don't have to worry about overflows as long as
their size is big enough to hold maximum number of concurrent
threads on the system.
*/
uint m_ref_usage;
uint m_ref_release;
bool m_is_destroyed;
};
/**
An implementation of the scoped metadata lock. The only locking modes
which are supported at the moment are SHARED and INTENTION EXCLUSIVE
and EXCLUSIVE
*/
class MDL_scoped_lock : public MDL_lock
{
public:
MDL_scoped_lock(const MDL_key *key_arg)
: MDL_lock(key_arg)
{ }
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;
}
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.
*/
class MDL_object_lock : public MDL_lock
{
public:
MDL_object_lock(const MDL_key *key_arg)
: MDL_lock(key_arg)
{ }
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;
}
private:
static const bitmap_t m_granted_incompatible[MDL_TYPE_END];
static const bitmap_t m_waiting_incompatible[MDL_TYPE_END];
};
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();
}
}
/** Initialize the global hash containing all MDL locks. */
void MDL_map::init()
{
mysql_mutex_init(key_MDL_map_mutex, &m_mutex, NULL);
my_hash_init(&m_locks, &my_charset_bin, 16 /* FIXME */, 0, 0,
mdl_locks_key, 0, 0);
}
/**
Destroy the global hash containing all MDL locks.
@pre It must be empty.
*/
void MDL_map::destroy()
{
DBUG_ASSERT(!m_locks.records);
mysql_mutex_destroy(&m_mutex);
my_hash_free(&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(const MDL_key *mdl_key)
{
MDL_lock *lock;
my_hash_value_type hash_value;
hash_value= my_calc_hash(&m_locks, mdl_key->ptr(), mdl_key->length());
retry:
mysql_mutex_lock(&m_mutex);
if (!(lock= (MDL_lock*) my_hash_search_using_hash_value(&m_locks,
hash_value,
mdl_key->ptr(),
mdl_key->length())))
{
lock= MDL_lock::create(mdl_key);
if (!lock || my_hash_insert(&m_locks, (uchar*)lock))
{
mysql_mutex_unlock(&m_mutex);
MDL_lock::destroy(lock);
return NULL;
}
}
if (move_from_hash_to_lock_mutex(lock))
goto retry;
return lock;
}
/**
Find MDL_lock object corresponding to the key.
@retval non-NULL - MDL_lock instance for the key with locked
MDL_lock::m_rwlock.
@retval NULL - There was no MDL_lock for the key.
*/
MDL_lock* MDL_map::find(const MDL_key *mdl_key)
{
MDL_lock *lock;
my_hash_value_type hash_value;
hash_value= my_calc_hash(&m_locks, mdl_key->ptr(), mdl_key->length());
retry:
mysql_mutex_lock(&m_mutex);
if (!(lock= (MDL_lock*) my_hash_search_using_hash_value(&m_locks,
hash_value,
mdl_key->ptr(),
mdl_key->length())))
{
mysql_mutex_unlock(&m_mutex);
return NULL;
}
if (move_from_hash_to_lock_mutex(lock))
goto retry;
return lock;
}
/**
Release mdl_locks.m_mutex mutex and lock MDL_lock::m_rwlock for lock
object from the hash. Handle situation when object was released
while the held no mutex.
@retval FALSE - Success.
@retval TRUE - Object was released while we held no mutex, caller
should re-try looking up MDL_lock object in the hash.
*/
bool MDL_map::move_from_hash_to_lock_mutex(MDL_lock *lock)
{
DBUG_ASSERT(! lock->m_is_destroyed);
mysql_mutex_assert_owner(&m_mutex);
/*
We increment m_ref_usage which is a reference counter protected by
mdl_locks.m_mutex under the condition it is present in the hash and
m_is_destroyed is FALSE.
*/
lock->m_ref_usage++;
mysql_mutex_unlock(&m_mutex);
mysql_prlock_wrlock(&lock->m_rwlock);
lock->m_ref_release++;
if (unlikely(lock->m_is_destroyed))
{
/*
Object was released while we held no mutex, we need to
release it if no others hold references to it, while our own
reference count ensured that the object as such haven't got
its memory released yet. We can also safely compare
m_ref_usage and m_ref_release since the object is no longer
present in the hash so no one will be able to find it and
increment m_ref_usage anymore.
*/
uint ref_usage= lock->m_ref_usage;
uint ref_release= lock->m_ref_release;
mysql_prlock_unlock(&lock->m_rwlock);
if (ref_usage == ref_release)
MDL_lock::destroy(lock);
return TRUE;
}
return FALSE;
}
/**
Destroy MDL_lock object or delegate this responsibility to
whatever thread that holds the last outstanding reference to
it.
*/
void MDL_map::remove(MDL_lock *lock)
{
uint ref_usage, ref_release;
/*
Destroy the MDL_lock object, but ensure that anyone that is
holding a reference to the object is not remaining, if so he
has the responsibility to release it.
Setting of m_is_destroyed to TRUE while holding _both_
mdl_locks.m_mutex and MDL_lock::m_rwlock mutexes transfers the
protection of m_ref_usage from mdl_locks.m_mutex to
MDL_lock::m_rwlock while removal of object from the hash makes
it read-only. Therefore whoever acquires MDL_lock::m_rwlock next
will see most up to date version of m_ref_usage.
This means that when m_is_destroyed is TRUE and we hold the
MDL_lock::m_rwlock we can safely read the m_ref_usage
member.
*/
mysql_mutex_lock(&m_mutex);
my_hash_delete(&m_locks, (uchar*) lock);
lock->m_is_destroyed= TRUE;
ref_usage= lock->m_ref_usage;
ref_release= lock->m_ref_release;
mysql_prlock_unlock(&lock->m_rwlock);
mysql_mutex_unlock(&m_mutex);
if (ref_usage == ref_release)
MDL_lock::destroy(lock);
}
/**
Initialize a metadata locking context.
This is to be called when a new server connection is created.
*/
MDL_context::MDL_context()
:m_trans_sentinel(NULL),
m_thd(NULL),
m_needs_thr_lock_abort(FALSE),
m_waiting_for(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.is_empty());
mysql_prlock_destroy(&m_LOCK_waiting_for);
}
/**
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 enum_mdl_type mdl_type_arg)
{
key.mdl_key_init(mdl_namespace, db_arg, name_arg);
type= mdl_type_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 enum_mdl_type mdl_type_arg)
{
key.mdl_key_init(key_arg);
type= mdl_type_arg;
ticket= NULL;
}
/**
Allocate and initialize one lock request.
Same as mdl_init_lock(), but allocates the lock and the key buffer
on a memory root. Necessary to lock ad-hoc tables, e.g.
mysql.* tables of grant and data dictionary subsystems.
@param mdl_namespace Id of namespace of object to be locked
@param db Name of database to which object belongs
@param name Name of of object
@param root MEM_ROOT on which object should be allocated
@note The allocated lock request will have MDL_SHARED type.
@retval 0 Error if out of memory
@retval non-0 Pointer to an object representing a lock request
*/
MDL_request *
MDL_request::create(MDL_key::enum_mdl_namespace mdl_namespace, const char *db,
const char *name, enum_mdl_type mdl_type,
MEM_ROOT *root)
{
MDL_request *mdl_request;
if (!(mdl_request= (MDL_request*) alloc_root(root, sizeof(MDL_request))))
return NULL;
mdl_request->init(mdl_namespace, db, name, mdl_type);
return mdl_request;
}
/**
Auxiliary functions needed for creation/destruction of MDL_lock objects.
@note Also chooses an MDL_lock descendant appropriate for object namespace.
@todo This naive implementation should be replaced with one that saves
on memory allocation by reusing released objects.
*/
inline MDL_lock *MDL_lock::create(const MDL_key *mdl_key)
{
switch (mdl_key->mdl_namespace())
{
case MDL_key::GLOBAL:
case MDL_key::SCHEMA:
return new MDL_scoped_lock(mdl_key);
default:
return new MDL_object_lock(mdl_key);
}
}
void MDL_lock::destroy(MDL_lock *lock)
{
delete lock;
}
/**
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)
{
return new MDL_ticket(ctx_arg, type_arg);
}
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_NO_WRITE ?
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 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(THD *thd, struct timespec *abs_timeout,
bool set_status_on_timeout, const char *wait_state_name)
{
const char *old_msg;
enum_wait_status result;
int wait_result= 0;
mysql_mutex_lock(&m_LOCK_wait_status);
old_msg= thd_enter_cond(thd, &m_COND_wait_status, &m_LOCK_wait_status,
wait_state_name);
while (!m_wait_status && !thd_killed(thd) &&
wait_result != ETIMEDOUT && wait_result != ETIME)
wait_result= mysql_cond_timedwait(&m_COND_wait_status, &m_LOCK_wait_status,
abs_timeout);
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 (thd_killed(thd))
m_wait_status= KILLED;
else if (set_status_on_timeout)
m_wait_status= TIMEOUT;
}
result= m_wait_status;
thd_exit_cond(thd, old_msg);
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());
/*
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;
/*
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++))
{
if (can_grant_lock(ticket->get_type(), ticket->get_ctx()))
{
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);
}
/*
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.
*/
}
}
}
/**
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.
Here is how types of individual locks are translated to type of scoped lock:
----------------+-------------+
Type of request | Correspond. |
for indiv. lock | scoped lock |
----------------+-------------+
S, SH, SR, SW | IS |
SNW, SNRW, X | IX |
SNW, SNRW -> X | IX (*) |
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 for upgradable locks we always take intention exclusive scoped
lock at the same time when obtaining the shared lock, there is no
need to obtain such lock during the upgrade itself.
(**) Since intention shared scoped locks are compatible with all other
type of locks we don't even have any accounting for them.
*/
const MDL_lock::bitmap_t 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,
MDL_BIT(MDL_EXCLUSIVE) | MDL_BIT(MDL_SHARED) | MDL_BIT(MDL_INTENTION_EXCLUSIVE)
};
const MDL_lock::bitmap_t 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
};
/**
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 SNW SNRW X |
----------+------------------------------+
S | + + + + + + - |
SH | + + + + + + - |
SR | + + + + + - - |
SW | + + + + - - - |
SNW | + + + - - - - |
SNRW | + + - - - - - |
X | - - - - - - - |
SNW -> X | - - - 0 0 0 0 |
SNRW -> X | - - 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 SNW SNRW X |
----------+-----------------------------+
S | + + + + + + - |
SH | + + + + + + + |
SR | + + + + + - - |
SW | + + + + - - - |
SNW | + + + + + + - |
SNRW | + + + + + + - |
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.
*/
const MDL_lock::bitmap_t
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_EXCLUSIVE) | MDL_BIT(MDL_SHARED_NO_READ_WRITE) |
MDL_BIT(MDL_SHARED_NO_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_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_WRITE) |
MDL_BIT(MDL_SHARED_READ) | MDL_BIT(MDL_SHARED_HIGH_PRIO) |
MDL_BIT(MDL_SHARED)
};
const MDL_lock::bitmap_t
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),
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.
@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) 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];
/*
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.
*/
if (! (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))
break;
}
if (ticket == NULL) /* Incompatible locks are our own. */
can_grant= TRUE;
}
}
return can_grant;
}
/** Remove a ticket from waiting or pending queue and wakeup up waiters. */
void MDL_lock::remove_ticket(Ticket_list MDL_lock::*list, MDL_ticket *ticket)
{
mysql_prlock_wrlock(&m_rwlock);
(this->*list).remove_ticket(ticket);
if (is_empty())
mdl_locks.remove(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_mutex_assert_not_owner(&LOCK_open);
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 the transactional locks. If not
found in the list of transactional locks, look at LOCK TABLES
and HANDLER locks.
@param mdl_request Lock request object for lock to be acquired
@param[out] is_transactional FALSE if we pass beyond m_trans_sentinel
while searching for ticket, otherwise TRUE.
@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,
bool *is_transactional)
{
MDL_ticket *ticket;
Ticket_iterator it(m_tickets);
*is_transactional= TRUE;
while ((ticket= it++))
{
if (ticket == m_trans_sentinel)
*is_transactional= FALSE;
if (mdl_request->key.is_equal(&ticket->m_lock->key) &&
ticket->has_stronger_or_equal_type(mdl_request->type))
break;
}
return ticket;
}
/**
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;
bool is_transactional;
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;
mysql_mutex_assert_not_owner(&LOCK_open);
/*
Check whether the context already holds a shared lock on the object,
and if so, grant the request.
*/
if ((ticket= find_ticket(mdl_request, &is_transactional)))
{
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 (!is_transactional && clone_ticket(mdl_request))
{
/* Clone failed. */
mdl_request->ticket= NULL;
return TRUE;
}
return FALSE;
}
if (!(ticket= MDL_ticket::create(this, mdl_request->type)))
return TRUE;
/* The below call implicitly locks MDL_lock::m_rwlock on success. */
if (!(lock= mdl_locks.find_or_insert(key)))
{
MDL_ticket::destroy(ticket);
return TRUE;
}
ticket->m_lock= lock;
if (lock->can_grant_lock(mdl_request->type, this))
{
lock->m_granted.add_ticket(ticket);
mysql_prlock_unlock(&lock->m_rwlock);
m_tickets.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;
mysql_mutex_assert_not_owner(&LOCK_open);
/*
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)))
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.push_front(ticket);
return FALSE;
}
/**
Notify a thread holding a shared metadata lock which
conflicts with a pending exclusive lock.
@param thd Current thread context
@param conflicting_ticket Conflicting metadata lock
*/
void notify_shared_lock(THD *thd, MDL_ticket *conflicting_ticket)
{
/* Only try to abort locks on which we back off. */
if (conflicting_ticket->get_type() < MDL_SHARED_NO_WRITE)
{
MDL_context *conflicting_ctx= conflicting_ticket->get_ctx();
THD *conflicting_thd= conflicting_ctx->get_thd();
DBUG_ASSERT(thd != conflicting_thd); /* Self-deadlock */
/*
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.
*/
mysql_notify_thread_having_shared_lock(thd, conflicting_thd,
conflicting_ctx->get_needs_thr_lock_abort());
}
}
/**
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, ulong lock_wait_timeout)
{
MDL_lock *lock;
MDL_ticket *ticket;
struct timespec abs_timeout;
MDL_wait::enum_wait_status wait_status;
/* Do some work outside the critical section. */
set_timespec(abs_timeout, lock_wait_timeout);
if (try_acquire_lock_impl(mdl_request, &ticket))
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.
*/
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;
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();
if (ticket->is_upgradable_or_exclusive())
lock->notify_shared_locks(this);
mysql_prlock_unlock(&lock->m_rwlock);
will_wait_for(ticket);
/* There is a shared or exclusive lock on the object. */
DEBUG_SYNC(m_thd, "mdl_acquire_lock_wait");
find_deadlock();
if (ticket->is_upgradable_or_exclusive())
{
struct timespec abs_shortwait;
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_thd, &abs_shortwait, FALSE,
mdl_request->key.get_wait_state_name());
if (wait_status != MDL_wait::EMPTY)
break;
mysql_prlock_wrlock(&lock->m_rwlock);
lock->notify_shared_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_thd, &abs_timeout, TRUE,
mdl_request->key.get_wait_state_name());
}
else
wait_status= m_wait.timed_wait(m_thd, &abs_timeout, TRUE,
mdl_request->key.get_wait_state_name());
done_waiting_for();
if (wait_status != MDL_wait::GRANTED)
{
lock->remove_ticket(&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:
break;
default:
DBUG_ASSERT(0);
break;
}
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.push_front(ticket);
mdl_request->ticket= ticket;
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,
ulong lock_wait_timeout)
{
MDL_request_list::Iterator it(*mdl_requests);
MDL_request **sort_buf, **p_req;
MDL_ticket *mdl_svp= mdl_savepoint();
ssize_t req_count= static_cast<ssize_t>(mdl_requests->elements());
if (req_count == 0)
return FALSE;
/* Sort requests according to MDL_key. */
if (! (sort_buf= (MDL_request **)my_malloc(req_count *
sizeof(MDL_request*),
MYF(MY_WME))))
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);
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);
return TRUE;
}
/**
Upgrade a shared metadata lock to exclusive.
Used in ALTER TABLE, when a copy of the table with the
new definition has been constructed.
@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 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_to_exclusive(MDL_ticket *mdl_ticket,
ulong lock_wait_timeout)
{
MDL_request mdl_xlock_request;
MDL_ticket *mdl_svp= mdl_savepoint();
bool is_new_ticket;
DBUG_ENTER("MDL_ticket::upgrade_shared_lock_to_exclusive");
DEBUG_SYNC(get_thd(), "mdl_upgrade_shared_lock_to_exclusive");
/*
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->m_type == MDL_EXCLUSIVE)
DBUG_RETURN(FALSE);
/* Only allow upgrades from MDL_SHARED_NO_WRITE/NO_READ_WRITE */
DBUG_ASSERT(mdl_ticket->m_type == MDL_SHARED_NO_WRITE ||
mdl_ticket->m_type == MDL_SHARED_NO_READ_WRITE);
mdl_xlock_request.init(&mdl_ticket->m_lock->key, MDL_EXCLUSIVE);
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= MDL_EXCLUSIVE;
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.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.
@retval TRUE A deadlock is found.
@retval FALSE No deadlock found.
*/
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 ticket Ticket for lock to be released.
*/
void MDL_context::release_lock(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());
mysql_mutex_assert_not_owner(&LOCK_open);
if (ticket == m_trans_sentinel)
m_trans_sentinel= ++Ticket_list::Iterator(m_tickets, ticket);
lock->remove_ticket(&MDL_lock::m_granted, ticket);
m_tickets.remove(ticket);
MDL_ticket::destroy(ticket);
DBUG_VOID_RETURN;
}
/**
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.
Transactional locks are added to the beginning of the list, 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 transaction
- rollback to a savepoint.
The sentinel semantics is used to support LOCK TABLES
mode and HANDLER statements: locks taken by these statements
survive COMMIT, ROLLBACK, ROLLBACK TO SAVEPOINT.
*/
void MDL_context::release_locks_stored_before(MDL_ticket *sentinel)
{
MDL_ticket *ticket;
Ticket_iterator it(m_tickets);
DBUG_ENTER("MDL_context::release_locks_stored_before");
if (m_tickets.is_empty())
DBUG_VOID_RETURN;
while ((ticket= it++) && ticket != sentinel)
{
DBUG_PRINT("info", ("found lock to release ticket=%p", ticket));
release_lock(ticket);
}
/*
If all locks were released, then the sentinel was not present
in the list. It must never happen because the sentinel was
bogus, i.e. pointed to a ticket that no longer exists.
*/
DBUG_ASSERT(! m_tickets.is_empty() || sentinel == NULL);
DBUG_VOID_RETURN;
}
/**
Release all 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);
while ((ticket= it_ticket++))
{
DBUG_ASSERT(ticket->m_lock);
/*
We rarely have more than one ticket in this loop,
let's not bother saving on pthread_cond_broadcast().
*/
if (ticket->m_lock == lock)
release_lock(ticket);
}
}
/**
Downgrade an exclusive lock to shared metadata lock.
@param type Type of lock to which exclusive lock should be downgraded.
*/
void MDL_ticket::downgrade_exclusive_lock(enum_mdl_type type)
{
mysql_mutex_assert_not_owner(&LOCK_open);
/*
Do nothing if already downgraded. Used when we FLUSH TABLE under
LOCK TABLES and a table is listed twice in LOCK TABLES list.
*/
if (m_type != MDL_EXCLUSIVE)
return;
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;
bool is_transactional_unused;
mdl_request.init(mdl_namespace, db, name, mdl_type);
MDL_ticket *ticket= find_ticket(&mdl_request, &is_transactional_unused);
DBUG_ASSERT(ticket == NULL || ticket->m_lock);
return ticket;
}
/**
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);
}
/**
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).
@param mdl_savepont The last acquired MDL lock when the
savepoint was set.
*/
void MDL_context::rollback_to_savepoint(MDL_ticket *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_savepoint ?
mdl_savepoint : m_trans_sentinel);
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(m_trans_sentinel);
DBUG_VOID_RETURN;
}
/**
Does this savepoint have this lock?
@retval TRUE The ticket is older than the savepoint and
is not LT, HA or GLR ticket. Thus it belongs
to the savepoint.
@retval FALSE The ticket is newer than the savepoint
or is an LT, HA or GLR ticket.
*/
bool MDL_context::has_lock(MDL_ticket *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 it(m_tickets);
bool found_savepoint= FALSE;
while ((ticket= it++) && ticket != m_trans_sentinel)
{
/*
First met the savepoint. The ticket must be
somewhere after it.
*/
if (ticket == mdl_savepoint)
found_savepoint= TRUE;
/*
Met the ticket. If we haven't yet met the savepoint,
the ticket is newer than the savepoint.
*/
if (ticket == mdl_ticket)
return found_savepoint;
}
/* Reached m_trans_sentinel. The ticket must be LT, HA or GRL ticket. */
return FALSE;
}
/**
Rearrange the ticket to reside in the part of the list that's
beyond m_trans_sentinel. This effectively changes the ticket
life cycle, from automatic to manual: i.e. the ticket is no
longer released by MDL_context::release_transactional_locks() or
MDL_context::rollback_to_savepoint(), it must be released manually.
*/
void MDL_context::move_ticket_after_trans_sentinel(MDL_ticket *mdl_ticket)
{
m_tickets.remove(mdl_ticket);
if (m_trans_sentinel == NULL)
{
m_trans_sentinel= mdl_ticket;
m_tickets.push_back(mdl_ticket);
}
else
m_tickets.insert_after(m_trans_sentinel, mdl_ticket);
}
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