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|
/* Copyright (C) 2000-2006 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., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 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(session,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(session, 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(session, 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(session,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(Session)
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(session);
- When calling UNLOCK TABLES we call mysql_unlock_tables() for all
tables used in LOCK TABLES
If table_handler->external_lock(session, locktype) fails, we call
table_handler->external_lock(session, 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 malloc() ONLY when using LOCK TABLES command or when
we are forced to use mysql_lock_merge.
*/
#include "config.h"
#include <fcntl.h>
#include <drizzled/error.h>
#include <drizzled/my_hash.h>
#include <drizzled/thr_lock.h>
#include <drizzled/session.h>
#include <drizzled/sql_base.h>
#include <drizzled/lock.h>
#include "drizzled/pthread_globals.h"
#include "drizzled/internal/my_sys.h"
#include <set>
#include <vector>
#include <algorithm>
#include <functional>
#include <boost/thread/mutex.hpp>
#include <boost/thread/condition_variable.hpp>
using namespace std;
namespace drizzled
{
static boost::mutex LOCK_global_read_lock;
static boost::condition_variable COND_global_read_lock;
/**
@defgroup Locking Locking
@{
*/
static DrizzleLock *get_lock_data(Session *session, Table **table,
uint32_t count,
bool should_lock, Table **write_locked);
static int lock_external(Session *session, Table **table,uint32_t count);
static int unlock_external(Session *session, Table **table,uint32_t count);
static void print_lock_error(int error, const char *);
/*
Lock tables.
SYNOPSIS
mysql_lock_tables()
session The current thread.
tables An array of pointers to the tables to lock.
count The number of tables to lock.
flags Options:
DRIZZLE_LOCK_IGNORE_GLOBAL_READ_LOCK Ignore a global read lock
DRIZZLE_LOCK_IGNORE_FLUSH Ignore a flush tables.
DRIZZLE_LOCK_NOTIFY_IF_NEED_REOPEN Instead of reopening altered
or dropped tables by itself,
mysql_lock_tables() should
notify upper level and rely
on caller doing this.
need_reopen Out parameter, TRUE if some tables were altered
or deleted and should be reopened by caller.
RETURN
A lock structure pointer on success.
NULL on error or if some tables should be reopen.
*/
/* Map the return value of thr_lock to an error from errmsg.txt */
static int thr_lock_errno_to_mysql[]=
{ 0, 1, ER_LOCK_WAIT_TIMEOUT, ER_LOCK_DEADLOCK };
/**
Reset lock type in lock data and free.
@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_and_free(DrizzleLock **mysql_lock)
{
DrizzleLock *sql_lock= *mysql_lock;
THR_LOCK_DATA **ldata, **ldata_end;
/* 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;
}
free((unsigned char*) sql_lock);
*mysql_lock= 0;
}
DrizzleLock *mysql_lock_tables(Session *session, Table **tables, uint32_t count,
uint32_t flags, bool *need_reopen)
{
DrizzleLock *sql_lock;
Table *write_lock_used;
vector<plugin::StorageEngine *> involved_engines;
int rc;
*need_reopen= false;
for (;;)
{
if (! (sql_lock= get_lock_data(session, tables, count, true,
&write_lock_used)))
break;
if (global_read_lock && write_lock_used &&
! (flags & DRIZZLE_LOCK_IGNORE_GLOBAL_READ_LOCK))
{
/*
Someone has issued LOCK ALL TABLES FOR READ and we want a write lock
Wait until the lock is gone
*/
if (wait_if_global_read_lock(session, 1, 1))
{
/* Clear the lock type of all lock data to avoid reusage. */
reset_lock_data_and_free(&sql_lock);
break;
}
if (session->version != refresh_version)
{
/* Clear the lock type of all lock data to avoid reusage. */
reset_lock_data_and_free(&sql_lock);
goto retry;
}
}
session->set_proc_info("Notify start statement");
/*
* Here, we advise all storage engines involved in the
* statement that we are starting a new statement
*/
if (sql_lock->table_count)
{
size_t num_tables= sql_lock->table_count;
plugin::StorageEngine *engine;
set<size_t> involved_slots;
for (size_t x= 1; x <= num_tables; x++, tables++)
{
engine= (*tables)->cursor->getEngine();
if (involved_slots.count(engine->getId()) > 0)
continue; /* already added to involved engines */
involved_engines.push_back(engine);
involved_slots.insert(engine->getId());
}
for_each(involved_engines.begin(),
involved_engines.end(),
bind2nd(mem_fun(&plugin::StorageEngine::startStatement), session));
}
session->set_proc_info("External lock");
/*
* Here, the call to lock_external() informs the
* all engines for all tables used in this statement
* of the type of lock that Drizzle intends to take on a
* specific table.
*/
if (sql_lock->table_count && lock_external(session, 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);
break;
}
session->set_proc_info("Table lock");
/* Copy the lock data array. thr_multi_lock() reorders its contens. */
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,
session->lock_id)];
if (rc > 1) /* a timeout or a deadlock */
{
if (sql_lock->table_count)
unlock_external(session, sql_lock->table, sql_lock->table_count);
reset_lock_data_and_free(&sql_lock);
my_error(rc, MYF(0));
break;
}
else if (rc == 1) /* aborted */
{
session->some_tables_deleted=1; // Try again
sql_lock->lock_count= 0; // Locks are already freed
// Fall through: unlock, reset lock data, free and retry
}
else if (!session->some_tables_deleted || (flags & DRIZZLE_LOCK_IGNORE_FLUSH))
{
/*
Thread was killed or lock aborted. Let upper level close all
used tables and retry or give error.
*/
break;
}
else if (!session->open_tables)
{
// Only using temporary tables, no need to unlock
session->some_tables_deleted= 0;
break;
}
session->set_proc_info(0);
/* going to retry, unlock all tables */
if (sql_lock->lock_count)
thr_multi_unlock(sql_lock->locks, sql_lock->lock_count);
if (sql_lock->table_count)
unlock_external(session, sql_lock->table, sql_lock->table_count);
/*
If thr_multi_lock fails it resets lock type for tables, which
were locked before (and including) one that caused error. Lock
type for other tables preserved.
*/
reset_lock_data_and_free(&sql_lock);
/*
* Notify all involved engines that the
* SQL statement has ended
*/
for_each(involved_engines.begin(),
involved_engines.end(),
bind2nd(mem_fun(&plugin::StorageEngine::endStatement), session));
retry:
if (flags & DRIZZLE_LOCK_NOTIFY_IF_NEED_REOPEN)
{
*need_reopen= true;
break;
}
if (wait_for_tables(session))
break; // Couldn't open tables
}
session->set_proc_info(0);
if (session->killed)
{
session->send_kill_message();
if (sql_lock)
{
mysql_unlock_tables(session,sql_lock);
sql_lock= NULL;
}
}
session->set_time_after_lock();
return (sql_lock);
}
static int lock_external(Session *session, Table **tables, uint32_t count)
{
register uint32_t i;
int lock_type,error;
for (i=1 ; i <= count ; i++, tables++)
{
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)->cursor->ha_external_lock(session,lock_type)))
{
print_lock_error(error, (*tables)->cursor->getEngine()->getName().c_str());
while (--i)
{
tables--;
(*tables)->cursor->ha_external_lock(session, F_UNLCK);
(*tables)->current_lock=F_UNLCK;
}
return error;
}
else
{
(*tables)->db_stat &= ~ HA_BLOCK_LOCK;
(*tables)->current_lock= lock_type;
}
}
return 0;
}
void mysql_unlock_tables(Session *session, DrizzleLock *sql_lock)
{
if (sql_lock->lock_count)
thr_multi_unlock(sql_lock->locks,sql_lock->lock_count);
if (sql_lock->table_count)
unlock_external(session, sql_lock->table, sql_lock->table_count);
free((unsigned char*) sql_lock);
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(Session *session, Table **table, uint32_t count)
{
DrizzleLock *sql_lock;
Table *write_lock_used;
if ((sql_lock= get_lock_data(session, table, count, false,
&write_lock_used)))
mysql_unlock_tables(session, sql_lock);
}
/**
unlock all tables locked for read.
*/
void mysql_unlock_read_tables(Session *session, DrizzleLock *sql_lock)
{
uint32_t i,found;
/* 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_READ)
{
std::swap(*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++)
{
assert(sql_lock->table[i]->lock_position == i);
if ((uint32_t) sql_lock->table[i]->reginfo.lock_type >= TL_WRITE_ALLOW_READ)
{
std::swap(*table, sql_lock->table[i]);
table++;
found++;
}
}
/* Unlock all read locked tables */
if (i != found)
{
unlock_external(session,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= table - sql_lock->table;
tbl->lock_data_start= found;
found+= tbl->lock_count;
table++;
}
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.
@note This function has a legacy side effect: the table is
unlocked even if it is not found in the locked list.
It's not clear if this side effect is intentional or still
desirable. It might lead to unmatched calls to
unlock_external(). Moreover, a discrepancy can be left
unnoticed by the storage engine, because in
unlock_external() we call handler::external_lock(F_UNLCK) only
if table->current_lock is not F_UNLCK.
@param session thread context
@param locked list of locked tables
@param table the table to unlock
@param always_unlock specify explicitly if the legacy side
effect is desired.
*/
void mysql_lock_remove(Session *session, Table *table)
{
mysql_unlock_some_tables(session, &table, /* table count */ 1);
}
/** Abort all other threads waiting to get lock in table. */
void mysql_lock_abort(Session *session, Table *table)
{
DrizzleLock *locked;
Table *write_lock_used;
if ((locked= get_lock_data(session, &table, 1, false,
&write_lock_used)))
{
for (uint32_t x= 0; x < locked->lock_count; x++)
locked->locks[x]->lock->abort_locks();
free((unsigned char*) locked);
}
}
/**
Abort one thread / table combination.
@param session 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(Session *session, Table *table)
{
DrizzleLock *locked;
Table *write_lock_used;
bool result= false;
if ((locked= get_lock_data(session, &table, 1, false,
&write_lock_used)))
{
for (uint32_t i= 0; i < locked->lock_count; i++)
{
if (locked->locks[i]->lock->abort_locks_for_thread(table->in_use->thread_id))
result= true;
}
free((unsigned char*) locked);
}
return result;
}
/** Unlock a set of external. */
static int unlock_external(Session *session, Table **table,uint32_t count)
{
int error,error_code;
error_code=0;
do
{
if ((*table)->current_lock != F_UNLCK)
{
(*table)->current_lock = F_UNLCK;
if ((error=(*table)->cursor->ha_external_lock(session, F_UNLCK)))
{
error_code=error;
print_lock_error(error_code, (*table)->cursor->getEngine()->getName().c_str());
}
}
table++;
} while (--count);
return error_code;
}
/**
Get lock structures from table structs and initialize locks.
@param session Thread handler
@param table_ptr Pointer to tables that should be locks
@param should_lock One of:
- false : If we should send TL_IGNORE to store lock
- true : Store lock info in Table
@param write_lock_used Store pointer to last table with WRITE_ALLOW_WRITE
*/
static DrizzleLock *get_lock_data(Session *session, Table **table_ptr, uint32_t count,
bool should_lock, Table **write_lock_used)
{
uint32_t tables,lock_count;
DrizzleLock *sql_lock;
THR_LOCK_DATA **locks, **locks_buf, **locks_start;
Table **to, **table_buf;
*write_lock_used=0;
for (uint32_t i= tables= lock_count= 0 ; i < count ; i++)
{
Table *t= table_ptr[i];
if (! (t->getEngine()->check_flag(HTON_BIT_SKIP_STORE_LOCK)))
{
tables++;
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= (DrizzleLock*)
malloc(sizeof(*sql_lock) +
sizeof(THR_LOCK_DATA*) * tables * 2 +
sizeof(table_ptr) * lock_count)))
{
return NULL;
}
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 (uint32_t i= 0; i < count ; i++)
{
Table *table;
enum thr_lock_type lock_type;
if (table_ptr[i]->getEngine()->check_flag(HTON_BIT_SKIP_STORE_LOCK))
continue;
table= table_ptr[i];
lock_type= table->reginfo.lock_type;
assert (lock_type != TL_WRITE_DEFAULT);
if (lock_type >= TL_WRITE_ALLOW_WRITE)
{
*write_lock_used=table;
if (table->db_stat & HA_READ_ONLY)
{
my_error(ER_OPEN_AS_READONLY, MYF(0), table->getAlias());
/* Clear the lock type of the lock data that are stored already. */
sql_lock->lock_count= locks - sql_lock->locks;
reset_lock_data_and_free(&sql_lock);
return NULL;
}
}
locks_start= locks;
locks= table->cursor->store_lock(session, locks,
should_lock == false ? TL_IGNORE : lock_type);
if (should_lock)
{
table->lock_position= (uint32_t) (to - table_buf);
table->lock_data_start= (uint32_t) (locks_start - locks_buf);
table->lock_count= (uint32_t) (locks - locks_start);
assert(table->lock_count == 1);
}
*to++= 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;
return sql_lock;
}
/**
Put a not open table with an old refresh version in the table cache.
@param session Thread handler
@param table_list Lock first table in this list
@param check_in_use Do we need to check if table already in use by us
@note
One must have a lock on LOCK_open!
@warning
If you are going to update the table, you should use
lock_and_wait_for_table_name(removed) instead of this function as this works
together with 'FLUSH TABLES WITH READ LOCK'
@note
This will force any other threads that uses the table to release it
as soon as possible.
@return
< 0 error
@return
== 0 table locked
@return
> 0 table locked, but someone is using it
*/
static int lock_table_name(Session *session, TableList *table_list, bool check_in_use)
{
bool found_locked_table= false;
TableIdentifier identifier(table_list->db, table_list->table_name);
const TableIdentifier::Key &key(identifier.getKey());
if (check_in_use)
{
/* Only insert the table if we haven't insert it already */
TableOpenCacheRange ppp;
ppp= get_open_cache().equal_range(key);
for (TableOpenCache::const_iterator iter= ppp.first;
iter != ppp.second; ++iter)
{
Table *table= (*iter).second;
if (table->reginfo.lock_type < TL_WRITE)
{
if (table->in_use == session)
found_locked_table= true;
continue;
}
if (table->in_use == session)
{
table->getMutableShare()->resetVersion(); // Ensure no one can use this
table->locked_by_name= true;
return 0;
}
}
}
Table *table;
if (!(table= session->table_cache_insert_placeholder(table_list->db, table_list->table_name)))
{
return -1;
}
table_list->table= table;
/* Return 1 if table is in use */
return(test(remove_table_from_cache(session, identifier,
check_in_use ? RTFC_NO_FLAG : RTFC_WAIT_OTHER_THREAD_FLAG)));
}
void unlock_table_name(TableList *table_list)
{
if (table_list->table)
{
remove_table(table_list->table);
broadcast_refresh();
}
}
static bool locked_named_table(TableList *table_list)
{
for (; table_list ; table_list=table_list->next_local)
{
Table *table= table_list->table;
if (table)
{
Table *save_next= table->getNext();
bool result;
table->setNext(NULL);
result= table_is_used(table_list->table, 0);
table->setNext(save_next);
if (result)
return 1;
}
}
return 0; // All tables are locked
}
static bool wait_for_locked_table_names(Session *session, TableList *table_list)
{
bool result= false;
#if 0
assert(ownership of LOCK_open);
#endif
while (locked_named_table(table_list))
{
if (session->killed)
{
result=1;
break;
}
session->wait_for_condition(LOCK_open, COND_refresh);
LOCK_open.lock(); /* Wait for a table to unlock and then lock it */
}
return result;
}
/**
Lock all tables in list with a name lock.
REQUIREMENTS
- One must have a lock on LOCK_open when calling this
@param table_list Names of tables to lock
@retval
0 ok
@retval
1 Fatal error (end of memory ?)
*/
static bool lock_table_names(Session *session, TableList *table_list)
{
bool got_all_locks=1;
TableList *lock_table;
for (lock_table= table_list; lock_table; lock_table= lock_table->next_local)
{
int got_lock;
if ((got_lock= lock_table_name(session, lock_table, true)) < 0)
goto end; // Fatal error
if (got_lock)
got_all_locks=0; // Someone is using table
}
/* If some table was in use, wait until we got the lock */
if (!got_all_locks && wait_for_locked_table_names(session, table_list))
goto end;
return false;
end:
unlock_table_names(table_list, lock_table);
return true;
}
/**
Unlock all tables in list with a name lock.
@param session Thread handle.
@param table_list Names of tables to lock.
@note
This function needs to be protected by LOCK_open. If we're
under LOCK TABLES, this function does not work as advertised. Namely,
it does not exclude other threads from using this table and does not
put an exclusive name lock on this table into the table cache.
@see lock_table_names
@see unlock_table_names
@retval TRUE An error occured.
@retval FALSE Name lock successfully acquired.
*/
bool lock_table_names_exclusively(Session *session, TableList *table_list)
{
if (lock_table_names(session, table_list))
return true;
/*
Upgrade the table name locks from semi-exclusive to exclusive locks.
*/
for (TableList *table= table_list; table; table= table->next_global)
{
if (table->table)
table->table->open_placeholder= 1;
}
return false;
}
/**
Unlock all tables in list with a name lock.
@param
table_list Names of tables to unlock
@param
last_table Don't unlock any tables after this one.
(default 0, which will unlock all tables)
@note
One must have a lock on LOCK_open when calling this.
@note
This function will broadcast refresh signals to inform other threads
that the name locks are removed.
@retval
0 ok
@retval
1 Fatal error (end of memory ?)
*/
void unlock_table_names(TableList *table_list, TableList *last_table)
{
for (TableList *table= table_list;
table != last_table;
table= table->next_local)
unlock_table_name(table);
broadcast_refresh();
}
static void print_lock_error(int error, const char *table)
{
int textno;
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);
}
/****************************************************************************
Handling of global read locks
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().
The global locks are handled through the global variables:
global_read_lock
count of threads which have the global read lock (i.e. have completed at
least the first step above)
global_read_lock_blocks_commit
count of threads which have the global read lock and block
commits (i.e. are in or have completed the second step above)
waiting_for_read_lock
count of threads which want to take a global read lock but cannot
protect_against_global_read_lock
count of threads which have set protection against global read lock.
access to them is protected with a mutex LOCK_global_read_lock
(XXX: one should never take LOCK_open if LOCK_global_read_lock is
taken, otherwise a deadlock may occur. Other mutexes could be a
problem too - grep the code for global_read_lock if you want to use
any other mutex here) Also one must not hold LOCK_open when calling
wait_if_global_read_lock(). When the thread with the global read lock
tries to close its tables, it needs to take LOCK_open in
close_thread_table().
How blocking of threads by global read lock is achieved: that's
advisory. Any piece of code which should be blocked by global read lock must
be designed like this:
- call to wait_if_global_read_lock(). When this returns 0, no global read
lock is owned; if argument abort_on_refresh was 0, none can be obtained.
- job
- if abort_on_refresh was 0, call to start_waiting_global_read_lock() to
allow other threads to get the global read lock. I.e. removal of the
protection.
(Note: it's a bit like an implementation of rwlock).
[ I am sorry to mention some SQL syntaxes below I know I shouldn't but found
no better descriptive way ]
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.
session1: SELECT * FROM t FOR UPDATE;
session2: UPDATE t SET a=1; # blocked by row-level locks of session1
session3: FLUSH TABLES WITH READ LOCK; # blocked in close_cached_tables() by the
table instance of session2
session1: COMMIT; # blocked by session3.
session1 blocks session2 which blocks session3 which blocks session1: 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.
****************************************************************************/
volatile uint32_t global_read_lock=0;
volatile uint32_t global_read_lock_blocks_commit=0;
static volatile uint32_t protect_against_global_read_lock=0;
static volatile uint32_t waiting_for_read_lock=0;
#define GOT_GLOBAL_READ_LOCK 1
#define MADE_GLOBAL_READ_LOCK_BLOCK_COMMIT 2
bool lock_global_read_lock(Session *session)
{
if (!session->global_read_lock)
{
const char *old_message;
LOCK_global_read_lock.lock();
old_message=session->enter_cond(COND_global_read_lock, LOCK_global_read_lock,
"Waiting to get readlock");
waiting_for_read_lock++;
boost::mutex::scoped_lock scopedLock(LOCK_global_read_lock, boost::adopt_lock_t());
while (protect_against_global_read_lock && !session->killed)
COND_global_read_lock.wait(scopedLock);
waiting_for_read_lock--;
scopedLock.release();
if (session->killed)
{
session->exit_cond(old_message);
return true;
}
session->global_read_lock= GOT_GLOBAL_READ_LOCK;
global_read_lock++;
session->exit_cond(old_message); // this unlocks LOCK_global_read_lock
}
/*
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).
*/
return false;
}
void unlock_global_read_lock(Session *session)
{
uint32_t tmp;
{
boost::mutex::scoped_lock scopedLock(LOCK_global_read_lock);
tmp= --global_read_lock;
if (session->global_read_lock == MADE_GLOBAL_READ_LOCK_BLOCK_COMMIT)
--global_read_lock_blocks_commit;
}
/* Send the signal outside the mutex to avoid a context switch */
if (!tmp)
{
COND_global_read_lock.notify_all();
}
session->global_read_lock= 0;
}
static inline bool must_wait(bool is_not_commit)
{
return (global_read_lock &&
(is_not_commit ||
global_read_lock_blocks_commit));
}
bool wait_if_global_read_lock(Session *session, bool abort_on_refresh,
bool is_not_commit)
{
const char *old_message= NULL;
bool result= 0, need_exit_cond;
/*
Assert that we do not own LOCK_open. If we would own it, other
threads could not close their tables. This would make a pretty
deadlock.
*/
safe_mutex_assert_not_owner(LOCK_open.native_handle());
LOCK_global_read_lock.lock();
if ((need_exit_cond= must_wait(is_not_commit)))
{
if (session->global_read_lock) // This thread had the read locks
{
if (is_not_commit)
my_message(ER_CANT_UPDATE_WITH_READLOCK,
ER(ER_CANT_UPDATE_WITH_READLOCK), MYF(0));
LOCK_global_read_lock.unlock();
/*
We allow FLUSHer to COMMIT; we assume FLUSHer knows what it does.
This allowance is needed to not break existing versions of innobackup
which do a BEGIN; INSERT; FLUSH TABLES WITH READ LOCK; COMMIT.
*/
return is_not_commit;
}
old_message=session->enter_cond(COND_global_read_lock, LOCK_global_read_lock,
"Waiting for release of readlock");
while (must_wait(is_not_commit) && ! session->killed &&
(!abort_on_refresh || session->version == refresh_version))
{
boost::mutex::scoped_lock scoped(LOCK_global_read_lock, boost::adopt_lock_t());
COND_global_read_lock.wait(scoped);
scoped.release();
}
if (session->killed)
result=1;
}
if (!abort_on_refresh && !result)
protect_against_global_read_lock++;
/*
The following is only true in case of a global read locks (which is rare)
and if old_message is set
*/
if (unlikely(need_exit_cond))
session->exit_cond(old_message); // this unlocks LOCK_global_read_lock
else
LOCK_global_read_lock.unlock();
return result;
}
void start_waiting_global_read_lock(Session *session)
{
bool tmp;
if (unlikely(session->global_read_lock))
return;
LOCK_global_read_lock.lock();
tmp= (!--protect_against_global_read_lock &&
(waiting_for_read_lock || global_read_lock_blocks_commit));
LOCK_global_read_lock.unlock();
if (tmp)
COND_global_read_lock.notify_all();
return;
}
bool make_global_read_lock_block_commit(Session *session)
{
bool error;
const char *old_message;
/*
If we didn't succeed lock_global_read_lock(), or if we already suceeded
make_global_read_lock_block_commit(), do nothing.
*/
if (session->global_read_lock != GOT_GLOBAL_READ_LOCK)
return false;
LOCK_global_read_lock.lock();
/* increment this BEFORE waiting on cond (otherwise race cond) */
global_read_lock_blocks_commit++;
old_message= session->enter_cond(COND_global_read_lock, LOCK_global_read_lock,
"Waiting for all running commits to finish");
while (protect_against_global_read_lock && !session->killed)
{
boost::mutex::scoped_lock scopedLock(LOCK_global_read_lock, boost::adopt_lock_t());
COND_global_read_lock.wait(scopedLock);
scopedLock.release();
}
if ((error= test(session->killed)))
global_read_lock_blocks_commit--; // undo what we did
else
session->global_read_lock= MADE_GLOBAL_READ_LOCK_BLOCK_COMMIT;
session->exit_cond(old_message); // this unlocks LOCK_global_read_lock
return error;
}
/**
Broadcast COND_refresh and COND_global_read_lock.
Due to a bug in a threading library it could happen that a signal
did not reach its target. A condition for this was that the same
condition variable was used with different mutexes in
pthread_cond_wait(). Some time ago we changed LOCK_open to
LOCK_global_read_lock in global read lock handling. So COND_refresh
was used with LOCK_open and LOCK_global_read_lock.
We did now also change from COND_refresh to COND_global_read_lock
in global read lock handling. But now it is necessary to signal
both conditions at the same time.
@note
When signalling COND_global_read_lock within the global read lock
handling, it is not necessary to also signal COND_refresh.
*/
void broadcast_refresh(void)
{
COND_refresh.notify_all();
COND_global_read_lock.notify_all();
}
/**
@} (end of group Locking)
*/
} /* namespace drizzled */
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