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mariadb/sql/lock.cc
Dmitry Lenev 378cdc58c1 Patch that refactors global read lock implementation and fixes 
bug #57006 "Deadlock between HANDLER and FLUSH TABLES WITH READ
LOCK" and bug #54673 "It takes too long to get readlock for
'FLUSH TABLES WITH READ LOCK'".

The first bug manifested itself as a deadlock which occurred
when a connection, which had some table open through HANDLER
statement, tried to update some data through DML statement
while another connection tried to execute FLUSH TABLES WITH
READ LOCK concurrently.

What happened was that FTWRL in the second connection managed
to perform first step of GRL acquisition and thus blocked all
upcoming DML. After that it started to wait for table open
through HANDLER statement to be flushed. When the first connection
tried to execute DML it has started to wait for GRL/the second
connection creating deadlock.

The second bug manifested itself as starvation of FLUSH TABLES
WITH READ LOCK statements in cases when there was a constant
stream of concurrent DML statements (in two or more
connections).

This has happened because requests for protection against GRL
which were acquired by DML statements were ignoring presence of
pending GRL and thus the latter was starved.

This patch solves both these problems by re-implementing GRL
using metadata locks.

Similar to the old implementation acquisition of GRL in new
implementation is two-step. During the first step we block
all concurrent DML and DDL statements by acquiring global S
metadata lock (each DML and DDL statement acquires global IX
lock for its duration). During the second step we block commits
by acquiring global S lock in COMMIT namespace (commit code
acquires global IX lock in this namespace).

Note that unlike in old implementation acquisition of
protection against GRL in DML and DDL is semi-automatic.
We assume that any statement which should be blocked by GRL
will either open and acquires write-lock on tables or acquires
metadata locks on objects it is going to modify. For any such
statement global IX metadata lock is automatically acquired
for its duration.

The first problem is solved because waits for GRL become
visible to deadlock detector in metadata locking subsystem
and thus deadlocks like one in the first bug become impossible.

The second problem is solved because global S locks which
are used for GRL implementation are given preference over
IX locks which are acquired by concurrent DML (and we can
switch to fair scheduling in future if needed).

Important change:
FTWRL/GRL no longer blocks DML and DDL on temporary tables.
Before this patch behavior was not consistent in this respect:
in some cases DML/DDL statements on temporary tables were
blocked while in others they were not. Since the main use cases
for FTWRL are various forms of backups and temporary tables are
not preserved during backups we have opted for consistently
allowing DML/DDL on temporary tables during FTWRL/GRL.

Important change:
This patch changes thread state names which are used when
DML/DDL of FTWRL is waiting for global read lock. It is now
either "Waiting for global read lock" or "Waiting for commit
lock" depending on the stage on which FTWRL is.

Incompatible change:
To solve deadlock in events code which was exposed by this
patch we have to replace LOCK_event_metadata mutex with
metadata locks on events. As result we have to prohibit
DDL on events under LOCK TABLES.

This patch also adds extensive test coverage for interaction
of DML/DDL and FTWRL.

Performance of new and old global read lock implementations
in sysbench tests were compared. There were no significant
difference between new and old implementations.
2010-11-11 20:11:05 +03:00

1074 lines
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/* Copyright 2000-2008 MySQL AB, 2008-2009 Sun Microsystems, Inc.
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 */
/**
@file
Locking functions for mysql.
Because of the new concurrent inserts, we must first get external locks
before getting internal locks. If we do it in the other order, the status
information is not up to date when called from the lock handler.
GENERAL DESCRIPTION OF LOCKING
When not using LOCK TABLES:
- For each SQL statement mysql_lock_tables() is called for all involved
tables.
- mysql_lock_tables() will call
table_handler->external_lock(thd,locktype) for each table.
This is followed by a call to thr_multi_lock() for all tables.
- When statement is done, we call mysql_unlock_tables().
This will call thr_multi_unlock() followed by
table_handler->external_lock(thd, F_UNLCK) for each table.
- Note that mysql_unlock_tables() may be called several times as
MySQL in some cases can free some tables earlier than others.
- The above is true both for normal and temporary tables.
- Temporary non transactional tables are never passed to thr_multi_lock()
and we never call external_lock(thd, F_UNLOCK) on these.
When using LOCK TABLES:
- LOCK TABLE will call mysql_lock_tables() for all tables.
mysql_lock_tables() will call
table_handler->external_lock(thd,locktype) for each table.
This is followed by a call to thr_multi_lock() for all tables.
- For each statement, we will call table_handler->start_stmt(THD)
to inform the table handler that we are using the table.
The tables used can only be tables used in LOCK TABLES or a
temporary table.
- When statement is done, we will call ha_commit_stmt(thd);
- When calling UNLOCK TABLES we call mysql_unlock_tables() for all
tables used in LOCK TABLES
If table_handler->external_lock(thd, locktype) fails, we call
table_handler->external_lock(thd, F_UNLCK) for each table that was locked,
excluding one that caused failure. That means handler must cleanup itself
in case external_lock() fails.
@todo
Change to use my_malloc() ONLY when using LOCK TABLES command or when
we are forced to use mysql_lock_merge.
*/
#include "sql_priv.h"
#include "debug_sync.h"
#include "unireg.h" // REQUIRED: for other includes
#include "lock.h"
#include "sql_base.h" // close_tables_for_reopen
#include "sql_parse.h" // is_log_table_write_query
#include "sql_acl.h" // SUPER_ACL
#include <hash.h>
#include <assert.h>
/**
@defgroup Locking Locking
@{
*/
extern HASH open_cache;
/* flags for get_lock_data */
#define GET_LOCK_UNLOCK 1
#define GET_LOCK_STORE_LOCKS 2
static MYSQL_LOCK *get_lock_data(THD *thd, TABLE **table_ptr, uint count,
uint flags);
static int lock_external(THD *thd, TABLE **table,uint count);
static int unlock_external(THD *thd, TABLE **table,uint count);
static void print_lock_error(int error, const char *);
/* Map the return value of thr_lock to an error from errmsg.txt */
static int thr_lock_errno_to_mysql[]=
{ 0, ER_LOCK_ABORTED, ER_LOCK_WAIT_TIMEOUT, ER_LOCK_DEADLOCK };
/**
Perform semantic checks for mysql_lock_tables.
@param thd The current thread
@param tables The tables to lock
@param count The number of tables to lock
@param flags Lock flags
@return 0 if all the check passed, non zero if a check failed.
*/
static int
lock_tables_check(THD *thd, TABLE **tables, uint count, uint flags)
{
uint system_count, i;
bool is_superuser, log_table_write_query;
DBUG_ENTER("lock_tables_check");
system_count= 0;
is_superuser= thd->security_ctx->master_access & SUPER_ACL;
log_table_write_query= (is_log_table_write_query(thd->lex->sql_command)
|| ((flags & MYSQL_LOCK_LOG_TABLE) != 0));
for (i=0 ; i<count; i++)
{
TABLE *t= tables[i];
/* Protect against 'fake' partially initialized TABLE_SHARE */
DBUG_ASSERT(t->s->table_category != TABLE_UNKNOWN_CATEGORY);
/*
Table I/O to performance schema tables is performed
only internally by the server implementation.
When a user is requesting a lock, the following
constraints are enforced:
*/
if (t->s->require_write_privileges() &&
! log_table_write_query)
{
/*
A user should not be able to prevent writes,
or hold any type of lock in a session,
since this would be a DOS attack.
*/
if ((t->reginfo.lock_type >= TL_READ_NO_INSERT)
|| (thd->lex->sql_command == SQLCOM_LOCK_TABLES))
{
my_error(ER_CANT_LOCK_LOG_TABLE, MYF(0));
DBUG_RETURN(1);
}
}
if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE)
{
if (t->s->table_category == TABLE_CATEGORY_SYSTEM)
system_count++;
if (t->db_stat & HA_READ_ONLY)
{
my_error(ER_OPEN_AS_READONLY, MYF(0), t->alias);
DBUG_RETURN(1);
}
}
/*
If we are going to lock a non-temporary table we must own metadata
lock of appropriate type on it (I.e. for table to be locked for
write we must own metadata lock of MDL_SHARED_WRITE or stronger
type. For table to be locked for read we must own metadata lock
of MDL_SHARED_READ or stronger type).
The only exception are HANDLER statements which are allowed to
lock table for read while having only MDL_SHARED lock on it.
*/
DBUG_ASSERT(t->s->tmp_table ||
thd->mdl_context.is_lock_owner(MDL_key::TABLE,
t->s->db.str, t->s->table_name.str,
t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE ?
MDL_SHARED_WRITE : MDL_SHARED_READ) ||
(t->open_by_handler &&
thd->mdl_context.is_lock_owner(MDL_key::TABLE,
t->s->db.str, t->s->table_name.str,
MDL_SHARED)));
/*
Prevent modifications to base tables if READ_ONLY is activated.
In any case, read only does not apply to temporary tables.
*/
if (!(flags & MYSQL_LOCK_IGNORE_GLOBAL_READ_ONLY) && !t->s->tmp_table)
{
if (t->reginfo.lock_type >= TL_WRITE_ALLOW_WRITE &&
!is_superuser && opt_readonly && !thd->slave_thread)
{
my_error(ER_OPTION_PREVENTS_STATEMENT, MYF(0), "--read-only");
DBUG_RETURN(1);
}
}
}
/*
Locking of system tables is restricted:
locking a mix of system and non-system tables in the same lock
is prohibited, to prevent contention.
*/
if ((system_count > 0) && (system_count < count))
{
my_error(ER_WRONG_LOCK_OF_SYSTEM_TABLE, MYF(0));
DBUG_RETURN(1);
}
DBUG_RETURN(0);
}
/**
Reset lock type in lock data
@param mysql_lock Lock structures to reset.
@note After a locking error we want to quit the locking of the table(s).
The test case in the bug report for Bug #18544 has the following
cases: 1. Locking error in lock_external() due to InnoDB timeout.
2. Locking error in get_lock_data() due to missing write permission.
3. Locking error in wait_if_global_read_lock() due to lock conflict.
@note In all these cases we have already set the lock type into the lock
data of the open table(s). If the table(s) are in the open table
cache, they could be reused with the non-zero lock type set. This
could lead to ignoring a different lock type with the next lock.
@note Clear the lock type of all lock data. This ensures that the next
lock request will set its lock type properly.
*/
static void reset_lock_data(MYSQL_LOCK *sql_lock)
{
THR_LOCK_DATA **ldata, **ldata_end;
DBUG_ENTER("reset_lock_data");
/* Clear the lock type of all lock data to avoid reusage. */
for (ldata= sql_lock->locks, ldata_end= ldata + sql_lock->lock_count;
ldata < ldata_end;
ldata++)
{
/* Reset lock type. */
(*ldata)->type= TL_UNLOCK;
}
DBUG_VOID_RETURN;
}
/**
Reset lock type in lock data and free.
@param mysql_lock Lock structures to reset.
*/
static void reset_lock_data_and_free(MYSQL_LOCK **mysql_lock)
{
reset_lock_data(*mysql_lock);
my_free(*mysql_lock);
*mysql_lock= 0;
}
/**
Lock tables.
@param thd The current thread.
@param tables An array of pointers to the tables to lock.
@param count The number of tables to lock.
@param flags Options:
MYSQL_LOCK_IGNORE_GLOBAL_READ_ONLY Ignore SET GLOBAL READ_ONLY
MYSQL_LOCK_IGNORE_TIMEOUT Use maximum timeout value.
@retval A lock structure pointer on success.
@retval NULL if an error or if wait on a lock was killed.
*/
MYSQL_LOCK *mysql_lock_tables(THD *thd, TABLE **tables, uint count, uint flags)
{
int rc;
MYSQL_LOCK *sql_lock;
ulong timeout= (flags & MYSQL_LOCK_IGNORE_TIMEOUT) ?
LONG_TIMEOUT : thd->variables.lock_wait_timeout;
DBUG_ENTER("mysql_lock_tables");
if (lock_tables_check(thd, tables, count, flags))
DBUG_RETURN(NULL);
if (! (sql_lock= get_lock_data(thd, tables, count, GET_LOCK_STORE_LOCKS)))
DBUG_RETURN(NULL);
thd_proc_info(thd, "System lock");
DBUG_PRINT("info", ("thd->proc_info %s", thd->proc_info));
if (sql_lock->table_count && lock_external(thd, sql_lock->table,
sql_lock->table_count))
{
/* Clear the lock type of all lock data to avoid reusage. */
reset_lock_data_and_free(&sql_lock);
goto end;
}
/* Copy the lock data array. thr_multi_lock() reorders its contents. */
memcpy(sql_lock->locks + sql_lock->lock_count, sql_lock->locks,
sql_lock->lock_count * sizeof(*sql_lock->locks));
/* Lock on the copied half of the lock data array. */
rc= thr_lock_errno_to_mysql[(int) thr_multi_lock(sql_lock->locks +
sql_lock->lock_count,
sql_lock->lock_count,
&thd->lock_info, timeout)];
if (rc)
{
if (sql_lock->table_count)
(void) unlock_external(thd, sql_lock->table, sql_lock->table_count);
reset_lock_data_and_free(&sql_lock);
if (! thd->killed)
my_error(rc, MYF(0));
}
end:
thd_proc_info(thd, 0);
if (thd->killed)
{
thd->send_kill_message();
if (sql_lock)
{
mysql_unlock_tables(thd, sql_lock);
sql_lock= 0;
}
}
thd->set_time_after_lock();
DBUG_RETURN(sql_lock);
}
static int lock_external(THD *thd, TABLE **tables, uint count)
{
reg1 uint i;
int lock_type,error;
DBUG_ENTER("lock_external");
DBUG_PRINT("info", ("count %d", count));
for (i=1 ; i <= count ; i++, tables++)
{
DBUG_ASSERT((*tables)->reginfo.lock_type >= TL_READ);
lock_type=F_WRLCK; /* Lock exclusive */
if ((*tables)->db_stat & HA_READ_ONLY ||
((*tables)->reginfo.lock_type >= TL_READ &&
(*tables)->reginfo.lock_type <= TL_READ_NO_INSERT))
lock_type=F_RDLCK;
if ((error=(*tables)->file->ha_external_lock(thd,lock_type)))
{
print_lock_error(error, (*tables)->file->table_type());
while (--i)
{
tables--;
(*tables)->file->ha_external_lock(thd, F_UNLCK);
(*tables)->current_lock=F_UNLCK;
}
DBUG_RETURN(error);
}
else
{
(*tables)->db_stat &= ~ HA_BLOCK_LOCK;
(*tables)->current_lock= lock_type;
}
}
DBUG_RETURN(0);
}
void mysql_unlock_tables(THD *thd, MYSQL_LOCK *sql_lock)
{
DBUG_ENTER("mysql_unlock_tables");
if (sql_lock->lock_count)
thr_multi_unlock(sql_lock->locks,sql_lock->lock_count);
if (sql_lock->table_count)
(void) unlock_external(thd,sql_lock->table,sql_lock->table_count);
my_free(sql_lock);
DBUG_VOID_RETURN;
}
/**
Unlock some of the tables locked by mysql_lock_tables.
This will work even if get_lock_data fails (next unlock will free all)
*/
void mysql_unlock_some_tables(THD *thd, TABLE **table,uint count)
{
MYSQL_LOCK *sql_lock;
if ((sql_lock= get_lock_data(thd, table, count, GET_LOCK_UNLOCK)))
mysql_unlock_tables(thd, sql_lock);
}
/**
unlock all tables locked for read.
*/
void mysql_unlock_read_tables(THD *thd, MYSQL_LOCK *sql_lock)
{
uint i,found;
DBUG_ENTER("mysql_unlock_read_tables");
/* Move all write locks first */
THR_LOCK_DATA **lock=sql_lock->locks;
for (i=found=0 ; i < sql_lock->lock_count ; i++)
{
if (sql_lock->locks[i]->type > TL_WRITE_ALLOW_WRITE)
{
swap_variables(THR_LOCK_DATA *, *lock, sql_lock->locks[i]);
lock++;
found++;
}
}
/* unlock the read locked tables */
if (i != found)
{
thr_multi_unlock(lock,i-found);
sql_lock->lock_count= found;
}
/* Then do the same for the external locks */
/* Move all write locked tables first */
TABLE **table=sql_lock->table;
for (i=found=0 ; i < sql_lock->table_count ; i++)
{
DBUG_ASSERT(sql_lock->table[i]->lock_position == i);
if ((uint) sql_lock->table[i]->reginfo.lock_type > TL_WRITE_ALLOW_WRITE)
{
swap_variables(TABLE *, *table, sql_lock->table[i]);
table++;
found++;
}
}
/* Unlock all read locked tables */
if (i != found)
{
(void) unlock_external(thd,table,i-found);
sql_lock->table_count=found;
}
/* Fix the lock positions in TABLE */
table= sql_lock->table;
found= 0;
for (i= 0; i < sql_lock->table_count; i++)
{
TABLE *tbl= *table;
tbl->lock_position= (uint) (table - sql_lock->table);
tbl->lock_data_start= found;
found+= tbl->lock_count;
table++;
}
DBUG_VOID_RETURN;
}
/**
Try to find the table in the list of locked tables.
In case of success, unlock the table and remove it from this list.
If a table has more than one lock instance, removes them all.
@param thd thread context
@param locked list of locked tables
@param table the table to unlock
*/
void mysql_lock_remove(THD *thd, MYSQL_LOCK *locked,TABLE *table)
{
if (locked)
{
reg1 uint i;
for (i=0; i < locked->table_count; i++)
{
if (locked->table[i] == table)
{
uint j, removed_locks, old_tables;
TABLE *tbl;
uint lock_data_end;
DBUG_ASSERT(table->lock_position == i);
/* Unlock the table. */
mysql_unlock_some_tables(thd, &table, /* table count */ 1);
/* Decrement table_count in advance, making below expressions easier */
old_tables= --locked->table_count;
/* The table has 'removed_locks' lock data elements in locked->locks */
removed_locks= table->lock_count;
/* Move down all table pointers above 'i'. */
bmove((char*) (locked->table+i),
(char*) (locked->table+i+1),
(old_tables - i) * sizeof(TABLE*));
lock_data_end= table->lock_data_start + table->lock_count;
/* Move down all lock data pointers above 'table->lock_data_end-1' */
bmove((char*) (locked->locks + table->lock_data_start),
(char*) (locked->locks + lock_data_end),
(locked->lock_count - lock_data_end) *
sizeof(THR_LOCK_DATA*));
/*
Fix moved table elements.
lock_position is the index in the 'locked->table' array,
it must be fixed by one.
table->lock_data_start is pointer to the lock data for this table
in the 'locked->locks' array, they must be fixed by 'removed_locks',
the lock data count of the removed table.
*/
for (j= i ; j < old_tables; j++)
{
tbl= locked->table[j];
tbl->lock_position--;
DBUG_ASSERT(tbl->lock_position == j);
tbl->lock_data_start-= removed_locks;
}
/* Finally adjust lock_count. */
locked->lock_count-= removed_locks;
break;
}
}
}
}
/** Abort all other threads waiting to get lock in table. */
void mysql_lock_abort(THD *thd, TABLE *table, bool upgrade_lock)
{
MYSQL_LOCK *locked;
DBUG_ENTER("mysql_lock_abort");
if ((locked= get_lock_data(thd, &table, 1, GET_LOCK_UNLOCK)))
{
for (uint i=0; i < locked->lock_count; i++)
thr_abort_locks(locked->locks[i]->lock, upgrade_lock);
my_free(locked);
}
DBUG_VOID_RETURN;
}
/**
Abort one thread / table combination.
@param thd Thread handler
@param table Table that should be removed from lock queue
@retval
0 Table was not locked by another thread
@retval
1 Table was locked by at least one other thread
*/
bool mysql_lock_abort_for_thread(THD *thd, TABLE *table)
{
MYSQL_LOCK *locked;
bool result= FALSE;
DBUG_ENTER("mysql_lock_abort_for_thread");
if ((locked= get_lock_data(thd, &table, 1, GET_LOCK_UNLOCK)))
{
for (uint i=0; i < locked->lock_count; i++)
{
if (thr_abort_locks_for_thread(locked->locks[i]->lock,
table->in_use->thread_id))
result= TRUE;
}
my_free(locked);
}
DBUG_RETURN(result);
}
MYSQL_LOCK *mysql_lock_merge(MYSQL_LOCK *a,MYSQL_LOCK *b)
{
MYSQL_LOCK *sql_lock;
TABLE **table, **end_table;
DBUG_ENTER("mysql_lock_merge");
if (!(sql_lock= (MYSQL_LOCK*)
my_malloc(sizeof(*sql_lock)+
sizeof(THR_LOCK_DATA*)*(a->lock_count+b->lock_count)+
sizeof(TABLE*)*(a->table_count+b->table_count),MYF(MY_WME))))
DBUG_RETURN(0); // Fatal error
sql_lock->lock_count=a->lock_count+b->lock_count;
sql_lock->table_count=a->table_count+b->table_count;
sql_lock->locks=(THR_LOCK_DATA**) (sql_lock+1);
sql_lock->table=(TABLE**) (sql_lock->locks+sql_lock->lock_count);
memcpy(sql_lock->locks,a->locks,a->lock_count*sizeof(*a->locks));
memcpy(sql_lock->locks+a->lock_count,b->locks,
b->lock_count*sizeof(*b->locks));
memcpy(sql_lock->table,a->table,a->table_count*sizeof(*a->table));
memcpy(sql_lock->table+a->table_count,b->table,
b->table_count*sizeof(*b->table));
/*
Now adjust lock_position and lock_data_start for all objects that was
moved in 'b' (as there is now all objects in 'a' before these).
*/
for (table= sql_lock->table + a->table_count,
end_table= table + b->table_count;
table < end_table;
table++)
{
(*table)->lock_position+= a->table_count;
(*table)->lock_data_start+= a->lock_count;
}
/* Delete old, not needed locks */
my_free(a);
my_free(b);
thr_lock_merge_status(sql_lock->locks, sql_lock->lock_count);
DBUG_RETURN(sql_lock);
}
/** Unlock a set of external. */
static int unlock_external(THD *thd, TABLE **table,uint count)
{
int error,error_code;
DBUG_ENTER("unlock_external");
error_code=0;
do
{
if ((*table)->current_lock != F_UNLCK)
{
(*table)->current_lock = F_UNLCK;
if ((error=(*table)->file->ha_external_lock(thd, F_UNLCK)))
{
error_code=error;
print_lock_error(error_code, (*table)->file->table_type());
}
}
table++;
} while (--count);
DBUG_RETURN(error_code);
}
/**
Get lock structures from table structs and initialize locks.
@param thd Thread handler
@param table_ptr Pointer to tables that should be locks
@param flags One of:
- GET_LOCK_UNLOCK : If we should send TL_IGNORE to store lock
- GET_LOCK_STORE_LOCKS : Store lock info in TABLE
*/
static MYSQL_LOCK *get_lock_data(THD *thd, TABLE **table_ptr, uint count,
uint flags)
{
uint i,tables,lock_count;
MYSQL_LOCK *sql_lock;
THR_LOCK_DATA **locks, **locks_buf, **locks_start;
TABLE **to, **table_buf;
DBUG_ENTER("get_lock_data");
DBUG_ASSERT((flags == GET_LOCK_UNLOCK) || (flags == GET_LOCK_STORE_LOCKS));
DBUG_PRINT("info", ("count %d", count));
for (i=tables=lock_count=0 ; i < count ; i++)
{
TABLE *t= table_ptr[i];
if (t->s->tmp_table != NON_TRANSACTIONAL_TMP_TABLE)
{
tables+= t->file->lock_count();
lock_count++;
}
}
/*
Allocating twice the number of pointers for lock data for use in
thr_mulit_lock(). This function reorders the lock data, but cannot
update the table values. So the second part of the array is copied
from the first part immediately before calling thr_multi_lock().
*/
if (!(sql_lock= (MYSQL_LOCK*)
my_malloc(sizeof(*sql_lock) +
sizeof(THR_LOCK_DATA*) * tables * 2 +
sizeof(table_ptr) * lock_count,
MYF(0))))
DBUG_RETURN(0);
locks= locks_buf= sql_lock->locks= (THR_LOCK_DATA**) (sql_lock + 1);
to= table_buf= sql_lock->table= (TABLE**) (locks + tables * 2);
sql_lock->table_count=lock_count;
for (i=0 ; i < count ; i++)
{
TABLE *table;
enum thr_lock_type lock_type;
THR_LOCK_DATA **org_locks = locks;
if ((table=table_ptr[i])->s->tmp_table == NON_TRANSACTIONAL_TMP_TABLE)
continue;
lock_type= table->reginfo.lock_type;
DBUG_ASSERT(lock_type != TL_WRITE_DEFAULT && lock_type != TL_READ_DEFAULT);
locks_start= locks;
locks= table->file->store_lock(thd, locks,
(flags & GET_LOCK_UNLOCK) ? TL_IGNORE :
lock_type);
if (flags & GET_LOCK_STORE_LOCKS)
{
table->lock_position= (uint) (to - table_buf);
table->lock_data_start= (uint) (locks_start - locks_buf);
table->lock_count= (uint) (locks - locks_start);
}
*to++= table;
if (locks)
for ( ; org_locks != locks ; org_locks++)
(*org_locks)->debug_print_param= (void *) table;
}
/*
We do not use 'tables', because there are cases where store_lock()
returns less locks than lock_count() claimed. This can happen when
a FLUSH TABLES tries to abort locks from a MERGE table of another
thread. When that thread has just opened the table, but not yet
attached its children, it cannot return the locks. lock_count()
always returns the number of locks that an attached table has.
This is done to avoid the reverse situation: If lock_count() would
return 0 for a non-attached MERGE table, and that table becomes
attached between the calls to lock_count() and store_lock(), then
we would have allocated too little memory for the lock data. Now
we may allocate too much, but better safe than memory overrun.
And in the FLUSH case, the memory is released quickly anyway.
*/
sql_lock->lock_count= locks - locks_buf;
DBUG_PRINT("info", ("sql_lock->table_count %d sql_lock->lock_count %d",
sql_lock->table_count, sql_lock->lock_count));
DBUG_RETURN(sql_lock);
}
/**
Obtain an exclusive metadata lock on a schema name.
@param thd Thread handle.
@param db The database name.
This function cannot be called while holding LOCK_open mutex.
To avoid deadlocks, we do not try to obtain exclusive metadata
locks in LOCK TABLES mode, since in this mode there may be
other metadata locks already taken by the current connection,
and we must not wait for MDL locks while holding locks.
@retval FALSE Success.
@retval TRUE Failure: we're in LOCK TABLES mode, or out of memory,
or this connection was killed.
*/
bool lock_schema_name(THD *thd, const char *db)
{
MDL_request_list mdl_requests;
MDL_request global_request;
MDL_request mdl_request;
if (thd->locked_tables_mode)
{
my_message(ER_LOCK_OR_ACTIVE_TRANSACTION,
ER(ER_LOCK_OR_ACTIVE_TRANSACTION), MYF(0));
return TRUE;
}
if (thd->global_read_lock.can_acquire_protection())
return TRUE;
global_request.init(MDL_key::GLOBAL, "", "", MDL_INTENTION_EXCLUSIVE,
MDL_STATEMENT);
mdl_request.init(MDL_key::SCHEMA, db, "", MDL_EXCLUSIVE, MDL_TRANSACTION);
mdl_requests.push_front(&mdl_request);
mdl_requests.push_front(&global_request);
if (thd->mdl_context.acquire_locks(&mdl_requests,
thd->variables.lock_wait_timeout))
return TRUE;
DEBUG_SYNC(thd, "after_wait_locked_schema_name");
return FALSE;
}
/**
Obtain an exclusive metadata lock on an object name.
@param thd Thread handle.
@param mdl_type Object type (currently functions, procedures
and events can be name-locked).
@param db The schema the object belongs to.
@param name Object name in the schema.
This function assumes that no metadata locks were acquired
before calling it. Additionally, it cannot be called while
holding LOCK_open mutex. Both these invariants are enforced by
asserts in MDL_context::acquire_locks().
To avoid deadlocks, we do not try to obtain exclusive metadata
locks in LOCK TABLES mode, since in this mode there may be
other metadata locks already taken by the current connection,
and we must not wait for MDL locks while holding locks.
@retval FALSE Success.
@retval TRUE Failure: we're in LOCK TABLES mode, or out of memory,
or this connection was killed.
*/
bool lock_object_name(THD *thd, MDL_key::enum_mdl_namespace mdl_type,
const char *db, const char *name)
{
MDL_request_list mdl_requests;
MDL_request global_request;
MDL_request schema_request;
MDL_request mdl_request;
if (thd->locked_tables_mode)
{
my_message(ER_LOCK_OR_ACTIVE_TRANSACTION,
ER(ER_LOCK_OR_ACTIVE_TRANSACTION), MYF(0));
return TRUE;
}
DBUG_ASSERT(name);
DEBUG_SYNC(thd, "before_wait_locked_pname");
if (thd->global_read_lock.can_acquire_protection())
return TRUE;
global_request.init(MDL_key::GLOBAL, "", "", MDL_INTENTION_EXCLUSIVE,
MDL_STATEMENT);
schema_request.init(MDL_key::SCHEMA, db, "", MDL_INTENTION_EXCLUSIVE,
MDL_TRANSACTION);
mdl_request.init(mdl_type, db, name, MDL_EXCLUSIVE, MDL_TRANSACTION);
mdl_requests.push_front(&mdl_request);
mdl_requests.push_front(&schema_request);
mdl_requests.push_front(&global_request);
if (thd->mdl_context.acquire_locks(&mdl_requests,
thd->variables.lock_wait_timeout))
return TRUE;
DEBUG_SYNC(thd, "after_wait_locked_pname");
return FALSE;
}
static void print_lock_error(int error, const char *table)
{
int textno;
DBUG_ENTER("print_lock_error");
switch (error) {
case HA_ERR_LOCK_WAIT_TIMEOUT:
textno=ER_LOCK_WAIT_TIMEOUT;
break;
case HA_ERR_READ_ONLY_TRANSACTION:
textno=ER_READ_ONLY_TRANSACTION;
break;
case HA_ERR_LOCK_DEADLOCK:
textno=ER_LOCK_DEADLOCK;
break;
case HA_ERR_WRONG_COMMAND:
textno=ER_ILLEGAL_HA;
break;
default:
textno=ER_CANT_LOCK;
break;
}
if ( textno == ER_ILLEGAL_HA )
my_error(textno, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG), table);
else
my_error(textno, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG), error);
DBUG_VOID_RETURN;
}
/****************************************************************************
Handling of global read locks
Global read lock is implemented using metadata lock infrastructure.
Taking the global read lock is TWO steps (2nd step is optional; without
it, COMMIT of existing transactions will be allowed):
lock_global_read_lock() THEN make_global_read_lock_block_commit().
How blocking of threads by global read lock is achieved: that's
semi-automatic. We assume that any statement which should be blocked
by global read lock will either open and acquires write-lock on tables
or acquires metadata locks on objects it is going to modify. For any
such statement global IX metadata lock is automatically acquired for
its duration (in case of LOCK TABLES until end of LOCK TABLES mode).
And lock_global_read_lock() simply acquires global S metadata lock
and thus prohibits execution of statements which modify data (unless
they modify only temporary tables). If deadlock happens it is detected
by MDL subsystem and resolved in the standard fashion (by backing-off
metadata locks acquired so far and restarting open tables process
if possible).
Why does FLUSH TABLES WITH READ LOCK need to block COMMIT: because it's used
to read a non-moving SHOW MASTER STATUS, and a COMMIT writes to the binary
log.
Why getting the global read lock is two steps and not one. Because FLUSH
TABLES WITH READ LOCK needs to insert one other step between the two:
flushing tables. So the order is
1) lock_global_read_lock() (prevents any new table write locks, i.e. stalls
all new updates)
2) close_cached_tables() (the FLUSH TABLES), which will wait for tables
currently opened and being updated to close (so it's possible that there is
a moment where all new updates of server are stalled *and* FLUSH TABLES WITH
READ LOCK is, too).
3) make_global_read_lock_block_commit().
If we have merged 1) and 3) into 1), we would have had this deadlock:
imagine thread 1 and 2, in non-autocommit mode, thread 3, and an InnoDB
table t.
thd1: SELECT * FROM t FOR UPDATE;
thd2: UPDATE t SET a=1; # blocked by row-level locks of thd1
thd3: FLUSH TABLES WITH READ LOCK; # blocked in close_cached_tables() by the
table instance of thd2
thd1: COMMIT; # blocked by thd3.
thd1 blocks thd2 which blocks thd3 which blocks thd1: deadlock.
Note that we need to support that one thread does
FLUSH TABLES WITH READ LOCK; and then COMMIT;
(that's what innobackup does, for some good reason).
So in this exceptional case the COMMIT should not be blocked by the FLUSH
TABLES WITH READ LOCK.
****************************************************************************/
/**
Take global read lock, wait if there is protection against lock.
If the global read lock is already taken by this thread, then nothing is done.
See also "Handling of global read locks" above.
@param thd Reference to thread.
@retval False Success, global read lock set, commits are NOT blocked.
@retval True Failure, thread was killed.
*/
bool Global_read_lock::lock_global_read_lock(THD *thd)
{
DBUG_ENTER("lock_global_read_lock");
if (!m_state)
{
MDL_request mdl_request;
DBUG_ASSERT(! thd->mdl_context.is_lock_owner(MDL_key::GLOBAL, "", "",
MDL_SHARED));
mdl_request.init(MDL_key::GLOBAL, "", "", MDL_SHARED, MDL_EXPLICIT);
if (thd->mdl_context.acquire_lock(&mdl_request,
thd->variables.lock_wait_timeout))
DBUG_RETURN(1);
m_mdl_global_shared_lock= mdl_request.ticket;
m_state= GRL_ACQUIRED;
}
/*
We DON'T set global_read_lock_blocks_commit now, it will be set after
tables are flushed (as the present function serves for FLUSH TABLES WITH
READ LOCK only). Doing things in this order is necessary to avoid
deadlocks (we must allow COMMIT until all tables are closed; we should not
forbid it before, or we can have a 3-thread deadlock if 2 do SELECT FOR
UPDATE and one does FLUSH TABLES WITH READ LOCK).
*/
DBUG_RETURN(0);
}
/**
Unlock global read lock.
Commits may or may not be blocked when this function is called.
See also "Handling of global read locks" above.
@param thd Reference to thread.
*/
void Global_read_lock::unlock_global_read_lock(THD *thd)
{
DBUG_ENTER("unlock_global_read_lock");
DBUG_ASSERT(m_mdl_global_shared_lock && m_state);
if (m_mdl_blocks_commits_lock)
{
thd->mdl_context.release_lock(m_mdl_blocks_commits_lock);
m_mdl_blocks_commits_lock= NULL;
}
thd->mdl_context.release_lock(m_mdl_global_shared_lock);
m_mdl_global_shared_lock= NULL;
m_state= GRL_NONE;
DBUG_VOID_RETURN;
}
/**
Make global read lock also block commits.
The scenario is:
- This thread has the global read lock.
- Global read lock blocking of commits is not set.
See also "Handling of global read locks" above.
@param thd Reference to thread.
@retval False Success, global read lock set, commits are blocked.
@retval True Failure, thread was killed.
*/
bool Global_read_lock::make_global_read_lock_block_commit(THD *thd)
{
MDL_request mdl_request;
DBUG_ENTER("make_global_read_lock_block_commit");
/*
If we didn't succeed lock_global_read_lock(), or if we already suceeded
make_global_read_lock_block_commit(), do nothing.
*/
if (m_state != GRL_ACQUIRED)
DBUG_RETURN(0);
mdl_request.init(MDL_key::COMMIT, "", "", MDL_SHARED, MDL_EXPLICIT);
if (thd->mdl_context.acquire_lock(&mdl_request,
thd->variables.lock_wait_timeout))
DBUG_RETURN(TRUE);
m_mdl_blocks_commits_lock= mdl_request.ticket;
m_state= GRL_ACQUIRED_AND_BLOCKS_COMMIT;
DBUG_RETURN(FALSE);
}
/**
Set explicit duration for metadata locks which are used to implement GRL.
@param thd Reference to thread.
*/
void Global_read_lock::set_explicit_lock_duration(THD *thd)
{
if (m_mdl_global_shared_lock)
thd->mdl_context.set_lock_duration(m_mdl_global_shared_lock, MDL_EXPLICIT);
if (m_mdl_blocks_commits_lock)
thd->mdl_context.set_lock_duration(m_mdl_blocks_commits_lock, MDL_EXPLICIT);
}
/**
@} (end of group Locking)
*/
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