diff options
Diffstat (limited to 'mysys/thr_lock.c')
| -rw-r--r-- | mysys/thr_lock.c | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/mysys/thr_lock.c b/mysys/thr_lock.c index 3eabde85a58..c830c4ed0cc 100644 --- a/mysys/thr_lock.c +++ b/mysys/thr_lock.c @@ -420,6 +420,28 @@ wait_for_lock(struct st_lock_list *wait, THR_LOCK_DATA *data, my_bool can_deadlock= test(data->owner->info->n_cursors); DBUG_ENTER("wait_for_lock"); + /* + One can use this to signal when a thread is going to wait for a lock. + See debug_sync.cc. + + Beware of waiting for a signal here. The lock has aquired its mutex. + While waiting on a signal here, the locking thread could not aquire + the mutex to release the lock. One could lock up the table + completely. + + In detail it works so: When thr_lock() tries to acquire a table + lock, it locks the lock->mutex, checks if it can have the lock, and + if not, it calls wait_for_lock(). Here it unlocks the table lock + while waiting on a condition. The sync point is located before this + wait for condition. If we have a waiting action here, we hold the + the table locks mutex all the time. Any attempt to look at the table + lock by another thread blocks it immediately on lock->mutex. This + can easily become an unexpected and unobvious blockage. So be + warned: Do not request a WAIT_FOR action for the 'wait_for_lock' + sync point unless you really know what you do. + */ + DEBUG_SYNC_C("wait_for_lock"); + if (!in_wait_list) { (*wait->last)=data; /* Wait for lock */ |