/*
* LWLockConditionalAcquire - acquire a lightweight lock in the specified mode
*
* If the lock is not available, return FALSE with no side-effects.
*
* If successful, cancel/die interrupts are held off until lock release.
*/
bool
LWLockConditionalAcquire(LWLockId lockid, LWLockMode mode)
{
volatile LWLock *lock = &(LWLockArray[lockid].lock);
#if LWLOCK_LOCK_PARTS > 1
volatile LWLockPart *part = LWLOCK_PART(lock, lockid, MyBackendId);
#endif
bool mustwait;
PRINT_LWDEBUG("LWLockConditionalAcquire", lockid, lock);
/* Ensure we will have room to remember the lock */
if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
elog(ERROR, "too many LWLocks taken");
/*
* Lock out cancel/die interrupts until we exit the code section protected
* by the LWLock. This ensures that interrupts will not interfere with
* manipulations of data structures in shared memory.
*/
HOLD_INTERRUPTS();
if (mode == LW_SHARED)
{
#ifdef LWLOCK_PART_SHARED_OPS_ATOMIC
/* Increment shared counter partition. If there's no contention,
* this is sufficient to take the lock
*/
LWLOCK_PART_SHARED_POSTINC_ATOMIC(lock, lockid, part, MyBackendId);
LWLOCK_PART_SHARED_FENCE();
/* A concurrent exclusive locking attempt does the following
* three steps
* 1) Acquire mutex
* 2) Check shared counter partitions for readers.
* 3a) If found add proc to wait queue, block, restart at (1)
* 3b) If not found, set exclusive flag, continue with (4)
* 4) Enter protected section
* The fence after the atomic add above ensures that no further
* such attempt can proceed to (3b) or beyond. There may be
* pre-existing exclusive locking attempts at step (3b) or beyond,
* but we can recognize those by either the mutex being taken, or
* the exclusive flag being set. Conversely, if we see neither, we
* may proceed and enter the protected section.
*
* FIXME: This doesn't work if slock_t is a struct or doesn't
* use 0 for state "unlocked".
*/
if ((lock->mutex == 0) && (lock->exclusive == 0))
goto lock_acquired;
/* At this point, we don't know if the concurrent exclusive locker
* has proceeded to (3b) or blocked. We must take the mutex and
* re-check
*/
#endif /* LWLOCK_PART_SHARED_OPS_ATOMIC */
/* Acquire mutex. Time spent holding mutex should be short! */
SpinLockAcquire(&lock->mutex);
if (lock->exclusive == 0)
{
#ifdef LWLOCK_PART_SHARED_OPS_ATOMIC
/* Already incremented the shared counter partition above */
#else
lock->shared++;
#endif
mustwait = false;
}
else
{
#ifdef LWLOCK_PART_SHARED_OPS_ATOMIC
/* Must undo shared counter partition increment. Note that
* we *need* to do that while holding the mutex. Otherwise,
* the exclusive lock could be released and attempted to be
* re-acquired before we undo the increment. That attempt
* would then block, even though there'd be no lock holder
* left
*/
LWLOCK_PART_SHARED_POSTDEC_ATOMIC(lock, lockid, part, MyBackendId);
#endif
mustwait = true;
}
}
else
{
/* Step (1). Acquire mutex. Time spent holding mutex should be
* short!
*/
SpinLockAcquire(&lock->mutex);
if (lock->exclusive == 0)
{
/* Step (2). Check for shared lockers. This surely happens
* after (1), otherwise SpinLockAcquire() is broken. Lock
* acquire semantics demand that no load must be re-ordered
* from after a lock acquisition to before, for obvious
* reasons.
*/
LWLOCK_IS_SHARED(mustwait, lock, lockid);
if (!mustwait) {
/* Step (3a). Set exclusive flag. This surely happens
* after (2) because it depends on the result of (2),
* no matter how much reordering is going on here.
*/
lock->exclusive++;
}
}
else
mustwait = true;
}
/* We are done updating shared state of the lock itself. */
SpinLockRelease(&lock->mutex);
if (mustwait)
{
/* Failed to get lock, so release interrupt holdoff */
RESUME_INTERRUPTS();
LOG_LWDEBUG("LWLockConditionalAcquire", lockid, "failed");
TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(lockid, mode);
return false;
}
#ifdef LWLOCK_PART_SHARED_OPS_ATOMIC
lock_acquired:
#endif
TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE(lockid, mode);
/* Add lock to list of locks held by this backend */
held_lwlocks[num_held_lwlocks] = lockid;
held_lwlocks_mode[num_held_lwlocks] = mode;
++num_held_lwlocks;
return true;
}
/*
* LWLockConditionalAcquire - acquire a lightweight lock in the specified mode
*
* If the lock is not available, return FALSE with no side-effects.
*
* If successful, cancel/die interrupts are held off until lock release.
*/
bool
LWLockConditionalAcquire(LWLockId lockid, LWLockMode mode)
{
volatile LWLock *lock = &(LWLockArray[lockid].lock);
bool mustwait;
PRINT_LWDEBUG("LWLockConditionalAcquire", lockid, lock);
/* Ensure we will have room to remember the lock */
if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
elog(ERROR, "too many LWLocks taken");
/*
* Lock out cancel/die interrupts until we exit the code section protected
* by the LWLock. This ensures that interrupts will not interfere with
* manipulations of data structures in shared memory.
*/
HOLD_INTERRUPTS();
/* Acquire mutex. Time spent holding mutex should be short! */
SpinLockAcquire(&lock->mutex);
/* If I can get the lock, do so quickly. */
if (mode == LW_EXCLUSIVE)
{
if (lock->exclusive == 0 && lock->shared == 0)
{
lock->exclusive++;
mustwait = false;
}
else
mustwait = true;
}
else
{
if (lock->exclusive == 0)
{
lock->shared++;
mustwait = false;
}
else
mustwait = true;
}
/* We are done updating shared state of the lock itself. */
SpinLockRelease(&lock->mutex);
if (mustwait)
{
/* Failed to get lock, so release interrupt holdoff */
RESUME_INTERRUPTS();
LOG_LWDEBUG("LWLockConditionalAcquire", lockid, "failed");
TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(lockid, mode);
}
else
{
/* Add lock to list of locks held by this backend */
held_lwlocks[num_held_lwlocks++] = lockid;
TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE(lockid, mode);
}
return !mustwait;
}
