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Bug#43758 Query cache can lock up threads in 'freeing items' state

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Early patch submitted for discussion.

It is possible for more than one thread to enter the condition
in query_cache_insert(), but the condition predicate is to
signal one thread each time the cache status changes between
the following states: {NO_FLUSH_IN_PROGRESS,FLUSH_IN_PROGRESS,
TABLE_FLUSH_IN_PROGRESS}

Consider three threads THD1, THD2, THD3

   THD2: select ... => Got a writer in ::store_query
   THD3: select ... => Got a writer in ::store_query
   THD1: flush tables => qc status= FLUSH_IN_PROGRESS;
                      new writers are blocked.
   THD2: select ... => Still got a writer and enters cond in
                       query_cache_insert
   THD3: select ... => Still got a writer and enters cond in
                       query_cache_insert
   THD1: flush tables => finished and signal status change.
   THD2: select ... => Wakes up and completes the insert.
   THD3: select ... => Happily waiting for better times. Why hurry?

This patch is a refactoring of this lock system. It introduces four new methods:
   Query_cache::try_lock()
   Query_cache::lock()
   Query_cache::lock_and_suspend()
   Query_cache::unlock()

This change also deprecates wait_while_table_flush_is_in_progress(). All threads are
queued and put on a conditional wait. On each unlock the queue is signalled. This resolve
the issues with left over threads. To assure that no threads are spending unnecessary
time waiting a signal broadcast is issued every time a lock is taken before a full
cache flush.

mysql-test/r/query_cache_debug.result:
  * Added test case for bug43758
mysql-test/t/query_cache_debug.test:
  * Added test case for bug43758
sql/sql_cache.cc:
  * Replaced calls to wait_while_table_flush_is_in_progress() with
    calls to try_lock(), lock_and_suspend() and unlock().
  * Renamed enumeration Cache_status to Cache_lock_status.
  * Renamed enumeration items to UNLOCKED, LOCKED_NO_WAIT and LOCKED.
    If the LOCKED_NO_WAIT lock type is used to lock the query cache, other
    threads using try_lock() will fail to acquire the lock.
    This is useful if the query cache is temporary disabled due to 
    a full table flush.
sql/sql_cache.h:
  * Replaced calls to wait_while_table_flush_is_in_progress() with
    calls to try_lock(), lock_and_suspend() and unlock().
  * Renamed enumeration Cache_status to Cache_lock_status.
  * Renamed enumeration items to UNLOCKED, LOCKED_NO_WAIT and LOCKED.
    If the LOCKED_NO_WAIT lock type is used to lock the query cache, other
    threads using try_lock() will fail to acquire the lock.
    This is useful if the query cache is temporary disabled due to 
    a full table flush.
This commit is contained in:
Kristofer Pettersson
2009-06-16 10:34:47 +02:00
parent 62a32540fc
commit 02e5ad9881
4 changed files with 472 additions and 214 deletions
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22
sql/sql_cache.h
View File

@@ -272,12 +272,12 @@ public:
private:
#ifndef DBUG_OFF
my_thread_id m_cache_lock_thread_id;
#endif
pthread_cond_t COND_cache_status_changed;
enum Cache_status { NO_FLUSH_IN_PROGRESS, FLUSH_IN_PROGRESS,
TABLE_FLUSH_IN_PROGRESS };
Cache_status m_cache_status;
enum Cache_lock_status { UNLOCKED, LOCKED_NO_WAIT, LOCKED };
Cache_lock_status m_cache_lock_status;
void free_query_internal(Query_cache_block *point);
void invalidate_table_internal(THD *thd, uchar *key, uint32 key_length);
@@ -380,8 +380,6 @@ protected:
Query_cache_block *pprev);
my_bool join_results(ulong join_limit);
void wait_while_table_flush_is_in_progress(bool *interrupt);
/*
Following function control structure_guard_mutex
by themself or don't need structure_guard_mutex
@@ -469,11 +467,6 @@ protected:
friend void query_cache_end_of_result(THD *thd);
friend void query_cache_abort(NET *net);
bool is_flushing(void)
{
return (m_cache_status != Query_cache::NO_FLUSH_IN_PROGRESS);
}
/*
The following functions are only used when debugging
We don't protect these with ifndef DBUG_OFF to not have to recompile
@@ -491,6 +484,11 @@ protected:
Query_cache_block_table * point,
const char *name);
my_bool in_blocks(Query_cache_block * point);
bool try_lock(void);
void lock(void);
void lock_and_suspend(void);
void unlock(void);
};
extern Query_cache query_cache;