}
/*
- * Emit a WAL record if acquisition of this lock needs to be replayed in a
- * standby server. Only AccessExclusiveLocks can conflict with lock types
- * that read-only transactions can acquire in a standby server.
+ * Prepare to emit a WAL record if acquisition of this lock needs to be
+ * replayed in a standby server.
*
- * Make sure this definition matches the one in
- * GetRunningTransactionLocks().
+ * Here we prepare to log; after lock is acquired we'll issue log record.
+ * This arrangement simplifies error recovery in case the preparation step
+ * fails.
*
- * First we prepare to log, then after lock acquired we issue log record.
+ * Only AccessExclusiveLocks can conflict with lock types that read-only
+ * transactions can acquire in a standby server. Make sure this definition
+ * matches the one in GetRunningTransactionLocks().
*/
if (lockmode >= AccessExclusiveLock &&
locktag->locktag_type == LOCKTAG_RELATION &&
* lock type on a relation we have already locked using the fast-path, but
* for now we don't worry about that case either.
*/
- if (EligibleForRelationFastPath(locktag, lockmode)
- && FastPathLocalUseCount < FP_LOCK_SLOTS_PER_BACKEND)
+ if (EligibleForRelationFastPath(locktag, lockmode) &&
+ FastPathLocalUseCount < FP_LOCK_SLOTS_PER_BACKEND)
{
uint32 fasthashcode = FastPathStrongLockHashPartition(hashcode);
bool acquired;
LWLockRelease(MyProc->backendLock);
if (acquired)
{
+ /*
+ * The locallock might contain stale pointers to some old shared
+ * objects; we MUST reset these to null before considering the
+ * lock to be acquired via fast-path.
+ */
+ locallock->lock = NULL;
+ locallock->proclock = NULL;
GrantLockLocal(locallock, owner);
return LOCKACQUIRE_OK;
}
LWLockAcquire(partitionLock, LW_EXCLUSIVE);
/*
- * Find or create a proclock entry with this tag
+ * Find or create lock and proclock entries with this tag
+ *
+ * Note: if the locallock object already existed, it might have a pointer
+ * to the lock already ... but we should not assume that that pointer is
+ * valid, since a lock object with zero hold and request counts can go
+ * away anytime. So we have to use SetupLockInTable() to recompute the
+ * lock and proclock pointers, even if they're already set.
*/
proclock = SetupLockInTable(lockMethodTable, MyProc, locktag,
hashcode, lockmode);
LWLockRelease(partitionLock);
/*
- * Emit a WAL record if acquisition of this lock need to be replayed in a
+ * Emit a WAL record if acquisition of this lock needs to be replayed in a
* standby server.
*/
if (log_lock)
/*
* Find or create a lock with this tag.
- *
- * Note: if the locallock object already existed, it might have a pointer
- * to the lock already ... but we probably should not assume that that
- * pointer is valid, since a lock object with no locks can go away
- * anytime.
*/
lock = (LOCK *) hash_search_with_hash_value(LockMethodLockHash,
(const void *) locktag,
return TRUE;
/* Attempt fast release of any lock eligible for the fast path. */
- if (EligibleForRelationFastPath(locktag, lockmode)
- && FastPathLocalUseCount > 0)
+ if (EligibleForRelationFastPath(locktag, lockmode) &&
+ FastPathLocalUseCount > 0)
{
bool released;
* Normally, we don't need to re-find the lock or proclock, since we kept
* their addresses in the locallock table, and they couldn't have been
* removed while we were holding a lock on them. But it's possible that
- * the locks have been moved to the main hash table by another backend, in
- * which case we might need to go look them up after all.
+ * the lock was taken fast-path and has since been moved to the main hash
+ * table by another backend, in which case we will need to look up the
+ * objects here. We assume the lock field is NULL if so.
*/
lock = locallock->lock;
if (!lock)
{
PROCLOCKTAG proclocktag;
- bool found;
Assert(EligibleForRelationFastPath(locktag, lockmode));
lock = (LOCK *) hash_search_with_hash_value(LockMethodLockHash,
(const void *) locktag,
locallock->hashcode,
HASH_FIND,
- &found);
- Assert(found && lock != NULL);
+ NULL);
+ if (!lock)
+ elog(ERROR, "failed to re-find shared lock object");
locallock->lock = lock;
proclocktag.myLock = lock;
proclocktag.myProc = MyProc;
locallock->proclock = (PROCLOCK *) hash_search(LockMethodProcLockHash,
(void *) &proclocktag,
- HASH_FIND, &found);
- Assert(found);
+ HASH_FIND,
+ NULL);
+ if (!locallock->proclock)
+ elog(ERROR, "failed to re-find shared proclock object");
}
LOCK_PRINT("LockRelease: found", lock, lockmode);
proclock = locallock->proclock;
* entries, then we scan the process's proclocks and get rid of those. We
* do this separately because we may have multiple locallock entries
* pointing to the same proclock, and we daren't end up with any dangling
- * pointers.
+ * pointers. Fast-path locks are cleaned up during the locallock table
+ * scan, though.
*/
hash_seq_init(&status, LockMethodLocalHash);
/*
* If the lock or proclock pointers are NULL, this lock was taken via
- * the relation fast-path.
+ * the relation fast-path (and is not known to have been transferred).
*/
if (locallock->proclock == NULL || locallock->lock == NULL)
{
/*
* If we don't currently hold the LWLock that protects our
* fast-path data structures, we must acquire it before attempting
- * to release the lock via the fast-path.
+ * to release the lock via the fast-path. We will continue to
+ * hold the LWLock until we're done scanning the locallock table,
+ * unless we hit a transferred fast-path lock. (XXX is this
+ * really such a good idea? There could be a lot of entries ...)
*/
if (!have_fast_path_lwlock)
{
RemoveLocalLock(locallock);
}
+ /* Done with the fast-path data structures */
if (have_fast_path_lwlock)
LWLockRelease(MyProc->backendLock);
Assert(!result);
FAST_PATH_CLEAR_LOCKMODE(MyProc, f, lockmode);
result = true;
+ /* we continue iterating so as to update FastPathLocalUseCount */
}
if (FAST_PATH_GET_BITS(MyProc, f) != 0)
++FastPathLocalUseCount;
FAST_PATH_CLEAR_LOCKMODE(proc, f, lockmode);
}
LWLockRelease(partitionLock);
+
+ /* No need to examine remaining slots. */
+ break;
}
LWLockRelease(proc->backendLock);
}
* FastPathGetLockEntry
* Return the PROCLOCK for a lock originally taken via the fast-path,
* transferring it to the primary lock table if necessary.
+ *
+ * Note: caller takes care of updating the locallock object.
*/
static PROCLOCK *
FastPathGetRelationLockEntry(LOCALLOCK *locallock)
FAST_PATH_CLEAR_LOCKMODE(MyProc, f, lockmode);
LWLockRelease(partitionLock);
+
+ /* No need to examine remaining slots. */
+ break;
}
LWLockRelease(MyProc->backendLock);
*/
if (VirtualTransactionIdIsValid(vxid))
vxids[count++] = vxid;
+
+ /* No need to examine remaining slots. */
break;
}
* shared memory. We also track the number of lock acquisitions per
* ResourceOwner, so that we can release just those locks belonging to a
* particular ResourceOwner.
+ *
+ * When holding a lock taken "normally", the lock and proclock fields always
+ * point to the associated objects in shared memory. However, if we acquired
+ * the lock via the fast-path mechanism, the lock and proclock fields are set
+ * to NULL, since there probably aren't any such objects in shared memory.
+ * (If the lock later gets promoted to normal representation, we may eventually
+ * update our locallock's lock/proclock fields after finding the shared
+ * objects.)
+ *
+ * Caution: a locallock object can be left over from a failed lock acquisition
+ * attempt. In this case its lock/proclock fields are untrustworthy, since
+ * the shared lock object is neither held nor awaited, and hence is available
+ * to be reclaimed. If nLocks > 0 then these pointers must either be valid or
+ * NULL, but when nLocks == 0 they should be considered garbage.
*/
typedef struct LOCALLOCKTAG
{
LOCALLOCKTAG tag; /* unique identifier of locallock entry */
/* data */
- LOCK *lock; /* associated LOCK object in shared mem */
- PROCLOCK *proclock; /* associated PROCLOCK object in shmem */
+ LOCK *lock; /* associated LOCK object, if any */
+ PROCLOCK *proclock; /* associated PROCLOCK object, if any */
uint32 hashcode; /* copy of LOCKTAG's hash value */
int64 nLocks; /* total number of times lock is held */
int numLockOwners; /* # of relevant ResourceOwners */
int maxLockOwners; /* allocated size of array */
- bool holdsStrongLockCount; /* bumped FastPathStrongRelatonLocks? */
+ bool holdsStrongLockCount; /* bumped FastPathStrongRelationLocks */
LOCALLOCKOWNER *lockOwners; /* dynamically resizable array */
} LOCALLOCK;
projects / postgresql / commitdiff