Ejemplo n.º 1
0
/*
 * ProcWakeup -- wake up a process by releasing its private semaphore.
 *
 *	 Also remove the process from the wait queue and set its links invalid.
 *	 RETURN: the next process in the wait queue.
 *
 * The appropriate lock partition lock must be held by caller.
 *
 * XXX: presently, this code is only used for the "success" case, and only
 * works correctly for that case.  To clean up in failure case, would need
 * to twiddle the lock's request counts too --- see RemoveFromWaitQueue.
 * Hence, in practice the waitStatus parameter must be STATUS_OK.
 */
PGPROC *
ProcWakeup(PGPROC *proc, int waitStatus)
{
	PGPROC	   *retProc;

	/* Proc should be sleeping ... */
	if (proc->links.prev == NULL ||
		proc->links.next == NULL)
		return NULL;
	Assert(proc->waitStatus == STATUS_WAITING);

	/* Save next process before we zap the list link */
	retProc = (PGPROC *) proc->links.next;

	/* Remove process from wait queue */
	SHMQueueDelete(&(proc->links));
	(proc->waitLock->waitProcs.size)--;

	/* Clean up process' state and pass it the ok/fail signal */
	proc->waitLock = NULL;
	proc->waitProcLock = NULL;
	proc->waitStatus = waitStatus;

	/* And awaken it */
	PGSemaphoreUnlock(&proc->sem);

	return retProc;
}
Ejemplo n.º 2
0
/*
 * ProcSendSignal - send a signal to a backend identified by PID
 */
void
ProcSendSignal(int pid)
{
	PGPROC	   *proc = NULL;

	if (RecoveryInProgress())
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile PROC_HDR *procglobal = ProcGlobal;

		SpinLockAcquire(ProcStructLock);

		/*
		 * Check to see whether it is the Startup process we wish to signal.
		 * This call is made by the buffer manager when it wishes to wake up a
		 * process that has been waiting for a pin in so it can obtain a
		 * cleanup lock using LockBufferForCleanup(). Startup is not a normal
		 * backend, so BackendPidGetProc() will not return any pid at all. So
		 * we remember the information for this special case.
		 */
		if (pid == procglobal->startupProcPid)
			proc = procglobal->startupProc;

		SpinLockRelease(ProcStructLock);
	}

	if (proc == NULL)
		proc = BackendPidGetProc(pid);

	if (proc != NULL)
		PGSemaphoreUnlock(&proc->sem);
}
Ejemplo n.º 3
0
/*
 * ProcSendSignal - send a signal to a backend identified by PID
 */
void
ProcSendSignal(int pid)
{
	PGPROC	   *proc = BackendPidGetProc(pid);

	if (proc != NULL)
		PGSemaphoreUnlock(&proc->sem);
}
Ejemplo n.º 4
0
void
s_unlock_sema(volatile slock_t *lock)
{
	int			lockndx = *lock;

	if (lockndx <= 0 || lockndx > NUM_SPINLOCK_SEMAPHORES)
		elog(ERROR, "invalid spinlock number: %d", lockndx);
	PGSemaphoreUnlock(&SpinlockSemaArray[lockndx - 1]);
}
Ejemplo n.º 5
0
/*
 * LWLockUpdateVar - Update a variable and wake up waiters atomically
 *
 * Sets *valptr to 'val', and wakes up all processes waiting for us with
 * LWLockWaitForVar().  Setting the value and waking up the processes happen
 * atomically so that any process calling LWLockWaitForVar() on the same lock
 * is guaranteed to see the new value, and act accordingly.
 *
 * The caller must be holding the lock in exclusive mode.
 */
void
LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 val)
{
	PGPROC	   *head;
	PGPROC	   *proc;
	PGPROC	   *next;

	/* Acquire mutex.  Time spent holding mutex should be short! */
	SpinLockAcquire(&lock->mutex);

	/* we should hold the lock */
	Assert(lock->exclusive == 1);

	/* Update the lock's value */
	*valptr = val;

	/*
	 * See if there are any LW_WAIT_UNTIL_FREE waiters that need to be woken
	 * up. They are always in the front of the queue.
	 */
	head = lock->head;

	if (head != NULL && head->lwWaitMode == LW_WAIT_UNTIL_FREE)
	{
		proc = head;
		next = proc->lwWaitLink;
		while (next && next->lwWaitMode == LW_WAIT_UNTIL_FREE)
		{
			proc = next;
			next = next->lwWaitLink;
		}

		/* proc is now the last PGPROC to be released */
		lock->head = next;
		proc->lwWaitLink = NULL;
	}
	else
		head = NULL;

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&lock->mutex);

	/*
	 * Awaken any waiters I removed from the queue.
	 */
	while (head != NULL)
	{
		proc = head;
		head = proc->lwWaitLink;
		proc->lwWaitLink = NULL;
		/* check comment in LWLockRelease() about this barrier */
		pg_write_barrier();
		proc->lwWaiting = false;
		PGSemaphoreUnlock(&proc->sem);
	}
}
Ejemplo n.º 6
0
static void BgwPoolMainLoop(Datum arg)
{
    BgwPoolExecutorCtx* ctx = (BgwPoolExecutorCtx*)arg;
    int id = ctx->id;
    BgwPool* pool = ctx->constructor();
    int size;
    void* work;

    BackgroundWorkerUnblockSignals();
	BackgroundWorkerInitializeConnection(pool->dbname, NULL);

    while(true) { 
        PGSemaphoreLock(&pool->available);
        SpinLockAcquire(&pool->lock);
        size = *(int*)&pool->queue[pool->head];
        Assert(size < pool->size);
        work = malloc(size);
        pool->pending -= 1;
        pool->active += 1;
		if (pool->lastPeakTime == 0 && pool->active == pool->nWorkers && pool->pending != 0) {
			pool->lastPeakTime = MtmGetSystemTime();
		}
        if (pool->head + size + 4 > pool->size) { 
            memcpy(work, pool->queue, size);
            pool->head = INTALIGN(size);
        } else { 
            memcpy(work, &pool->queue[pool->head+4], size);
            pool->head += 4 + INTALIGN(size);
        }
        if (pool->size == pool->head) { 
            pool->head = 0;
        }
        if (pool->producerBlocked) {
            pool->producerBlocked = false;
            PGSemaphoreUnlock(&pool->overflow);
			pool->lastPeakTime = 0;
        }
        SpinLockRelease(&pool->lock);
        pool->executor(id, work, size);
        free(work);
        SpinLockAcquire(&pool->lock);
        pool->active -= 1;
		pool->lastPeakTime = 0;
        SpinLockRelease(&pool->lock);
    }
}
Ejemplo n.º 7
0
void BgwPoolExecute(BgwPool* pool, void* work, size_t size)
{
    if (size+4 > pool->size) {
		/* 
		 * Size of work is larger than size of shared buffer: 
		 * run it immediately
		 */
		pool->executor(0, work, size);
		return;
	}
 
    SpinLockAcquire(&pool->lock);
    while (true) { 
        if ((pool->head <= pool->tail && pool->size - pool->tail < size + 4 && pool->head < size) 
            || (pool->head > pool->tail && pool->head - pool->tail < size + 4))
        {
            if (pool->lastPeakTime == 0) {
				pool->lastPeakTime = MtmGetSystemTime();
			}
			pool->producerBlocked = true;
            SpinLockRelease(&pool->lock);
            PGSemaphoreLock(&pool->overflow);
            SpinLockAcquire(&pool->lock);
        } else {
            pool->pending += 1;
			if (pool->lastPeakTime == 0 && pool->active == pool->nWorkers && pool->pending != 0) {
				pool->lastPeakTime = MtmGetSystemTime();
			}
            *(int*)&pool->queue[pool->tail] = size;
            if (pool->size - pool->tail >= size + 4) { 
                memcpy(&pool->queue[pool->tail+4], work, size);
                pool->tail += 4 + INTALIGN(size);
            } else { 
                memcpy(pool->queue, work, size);
                pool->tail = INTALIGN(size);
            }
            if (pool->tail == pool->size) {
                pool->tail = 0;
            }
            PGSemaphoreUnlock(&pool->available);
            break;
        }
    }
    SpinLockRelease(&pool->lock);            
}
Ejemplo n.º 8
0
/*
 * LWLockRelease - release a previously acquired lock
 */
void
LWLockRelease(LWLockId lockid)
{
	volatile LWLock *lock = &(LWLockArray[lockid].lock);
	PGPROC	   *head;
	PGPROC	   *proc;
	int			i;

	PRINT_LWDEBUG("LWLockRelease", lockid, lock);

	/*
	 * Remove lock from list of locks held.  Usually, but not always, it will
	 * be the latest-acquired lock; so search array backwards.
	 */
	for (i = num_held_lwlocks; --i >= 0;)
	{
		if (lockid == held_lwlocks[i])
			break;
	}
	if (i < 0)
		elog(ERROR, "lock %d is not held", (int) lockid);
	num_held_lwlocks--;
	for (; i < num_held_lwlocks; i++)
		held_lwlocks[i] = held_lwlocks[i + 1];

	/* Acquire mutex.  Time spent holding mutex should be short! */
	SpinLockAcquire(&lock->mutex);

	/* Release my hold on lock */
	if (lock->exclusive > 0)
		lock->exclusive--;
	else
	{
		Assert(lock->shared > 0);
		lock->shared--;
	}

	/*
	 * See if I need to awaken any waiters.  If I released a non-last shared
	 * hold, there cannot be anything to do.  Also, do not awaken any waiters
	 * if someone has already awakened waiters that haven't yet acquired the
	 * lock.
	 */
	head = lock->head;
	if (head != NULL)
	{
		if (lock->exclusive == 0 && lock->shared == 0 && lock->releaseOK)
		{
			/*
			 * Remove the to-be-awakened PGPROCs from the queue.  If the front
			 * waiter wants exclusive lock, awaken him only. Otherwise awaken
			 * as many waiters as want shared access.
			 */
			proc = head;
			if (!proc->lwExclusive)
			{
				while (proc->lwWaitLink != NULL &&
					   !proc->lwWaitLink->lwExclusive)
					proc = proc->lwWaitLink;
			}
			/* proc is now the last PGPROC to be released */
			lock->head = proc->lwWaitLink;
			proc->lwWaitLink = NULL;
			/* prevent additional wakeups until retryer gets to run */
			lock->releaseOK = false;
		}
		else
		{
			/* lock is still held, can't awaken anything */
			head = NULL;
		}
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&lock->mutex);

	TRACE_POSTGRESQL_LWLOCK_RELEASE(lockid);

	/*
	 * Awaken any waiters I removed from the queue.
	 */
	while (head != NULL)
	{
		LOG_LWDEBUG("LWLockRelease", lockid, "release waiter");
		proc = head;
		head = proc->lwWaitLink;
		proc->lwWaitLink = NULL;
		proc->lwWaiting = false;
		PGSemaphoreUnlock(&proc->sem);
	}

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	RESUME_INTERRUPTS();
}
Ejemplo n.º 9
0
/*
 * LWLockAcquire - acquire a lightweight lock in the specified mode
 *
 * If the lock is not available, sleep until it is.
 *
 * Side effect: cancel/die interrupts are held off until lock release.
 */
void
LWLockAcquire(LWLockId lockid, LWLockMode mode)
{
	volatile LWLock *lock = &(LWLockArray[lockid].lock);
	PGPROC	   *proc = MyProc;
	bool		retry = false;
	int			extraWaits = 0;

	PRINT_LWDEBUG("LWLockAcquire", lockid, lock);

#ifdef LWLOCK_STATS
	/* Set up local count state first time through in a given process */
	if (counts_for_pid != MyProcPid)
	{
		int		   *LWLockCounter = (int *) ((char *) LWLockArray - 2 * sizeof(int));
		int			numLocks = LWLockCounter[1];

		sh_acquire_counts = calloc(numLocks, sizeof(int));
		ex_acquire_counts = calloc(numLocks, sizeof(int));
		block_counts = calloc(numLocks, sizeof(int));
		counts_for_pid = MyProcPid;
		on_shmem_exit(print_lwlock_stats, 0);
	}
	/* Count lock acquisition attempts */
	if (mode == LW_EXCLUSIVE)
		ex_acquire_counts[lockid]++;
	else
		sh_acquire_counts[lockid]++;
#endif   /* LWLOCK_STATS */

	/*
	 * We can't wait if we haven't got a PGPROC.  This should only occur
	 * during bootstrap or shared memory initialization.  Put an Assert here
	 * to catch unsafe coding practices.
	 */
	Assert(!(proc == NULL && IsUnderPostmaster));

	/* 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();

	/*
	 * Loop here to try to acquire lock after each time we are signaled by
	 * LWLockRelease.
	 *
	 * NOTE: it might seem better to have LWLockRelease actually grant us the
	 * lock, rather than retrying and possibly having to go back to sleep. But
	 * in practice that is no good because it means a process swap for every
	 * lock acquisition when two or more processes are contending for the same
	 * lock.  Since LWLocks are normally used to protect not-very-long
	 * sections of computation, a process needs to be able to acquire and
	 * release the same lock many times during a single CPU time slice, even
	 * in the presence of contention.  The efficiency of being able to do that
	 * outweighs the inefficiency of sometimes wasting a process dispatch
	 * cycle because the lock is not free when a released waiter finally gets
	 * to run.	See pgsql-hackers archives for 29-Dec-01.
	 */
	for (;;)
	{
		bool		mustwait;

		/* Acquire mutex.  Time spent holding mutex should be short! */
		SpinLockAcquire(&lock->mutex);

		/* If retrying, allow LWLockRelease to release waiters again */
		if (retry)
			lock->releaseOK = true;

		/* 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;
		}

		if (!mustwait)
			break;				/* got the lock */

		/*
		 * Add myself to wait queue.
		 *
		 * If we don't have a PGPROC structure, there's no way to wait. This
		 * should never occur, since MyProc should only be null during shared
		 * memory initialization.
		 */
		if (proc == NULL)
			elog(PANIC, "cannot wait without a PGPROC structure");

		proc->lwWaiting = true;
		proc->lwExclusive = (mode == LW_EXCLUSIVE);
		proc->lwWaitLink = NULL;
		if (lock->head == NULL)
			lock->head = proc;
		else
			lock->tail->lwWaitLink = proc;
		lock->tail = proc;

		/* Can release the mutex now */
		SpinLockRelease(&lock->mutex);

		/*
		 * Wait until awakened.
		 *
		 * Since we share the process wait semaphore with the regular lock
		 * manager and ProcWaitForSignal, and we may need to acquire an LWLock
		 * while one of those is pending, it is possible that we get awakened
		 * for a reason other than being signaled by LWLockRelease. If so,
		 * loop back and wait again.  Once we've gotten the LWLock,
		 * re-increment the sema by the number of additional signals received,
		 * so that the lock manager or signal manager will see the received
		 * signal when it next waits.
		 */
		LOG_LWDEBUG("LWLockAcquire", lockid, "waiting");

#ifdef LWLOCK_STATS
		block_counts[lockid]++;
#endif

		TRACE_POSTGRESQL_LWLOCK_WAIT_START(lockid, mode);

		for (;;)
		{
			/* "false" means cannot accept cancel/die interrupt here. */
			PGSemaphoreLock(&proc->sem, false);
			if (!proc->lwWaiting)
				break;
			extraWaits++;
		}

		TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(lockid, mode);

		LOG_LWDEBUG("LWLockAcquire", lockid, "awakened");

		/* Now loop back and try to acquire lock again. */
		retry = true;
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&lock->mutex);

	TRACE_POSTGRESQL_LWLOCK_ACQUIRE(lockid, mode);

	/* Add lock to list of locks held by this backend */
	held_lwlocks[num_held_lwlocks++] = lockid;

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(&proc->sem);
}
Ejemplo n.º 10
0
/*
 * LWLockWaitForVar - Wait until lock is free, or a variable is updated.
 *
 * If the lock is held and *valptr equals oldval, waits until the lock is
 * either freed, or the lock holder updates *valptr by calling
 * LWLockUpdateVar.  If the lock is free on exit (immediately or after
 * waiting), returns true.  If the lock is still held, but *valptr no longer
 * matches oldval, returns false and sets *newval to the current value in
 * *valptr.
 *
 * It's possible that the lock holder releases the lock, but another backend
 * acquires it again before we get a chance to observe that the lock was
 * momentarily released.  We wouldn't need to wait for the new lock holder,
 * but we cannot distinguish that case, so we will have to wait.
 *
 * Note: this function ignores shared lock holders; if the lock is held
 * in shared mode, returns 'true'.
 */
bool
LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval)
{
	PGPROC	   *proc = MyProc;
	int			extraWaits = 0;
	bool		result = false;
#ifdef LWLOCK_STATS
	lwlock_stats *lwstats;
#endif

	PRINT_LWDEBUG("LWLockWaitForVar", lock);

#ifdef LWLOCK_STATS
	lwstats = get_lwlock_stats_entry(lock);
#endif   /* LWLOCK_STATS */

	/*
	 * Quick test first to see if it the slot is free right now.
	 *
	 * XXX: the caller uses a spinlock before this, so we don't need a memory
	 * barrier here as far as the current usage is concerned.  But that might
	 * not be safe in general.
	 */
	if (lock->exclusive == 0)
		return true;

	/*
	 * Lock out cancel/die interrupts while we sleep on the lock.  There is no
	 * cleanup mechanism to remove us from the wait queue if we got
	 * interrupted.
	 */
	HOLD_INTERRUPTS();

	/*
	 * Loop here to check the lock's status after each time we are signaled.
	 */
	for (;;)
	{
		bool		mustwait;
		uint64		value;

		/* Acquire mutex.  Time spent holding mutex should be short! */
#ifdef LWLOCK_STATS
		lwstats->spin_delay_count += SpinLockAcquire(&lock->mutex);
#else
		SpinLockAcquire(&lock->mutex);
#endif

		/* Is the lock now free, and if not, does the value match? */
		if (lock->exclusive == 0)
		{
			result = true;
			mustwait = false;
		}
		else
		{
			value = *valptr;
			if (value != oldval)
			{
				result = false;
				mustwait = false;
				*newval = value;
			}
			else
				mustwait = true;
		}

		if (!mustwait)
			break;				/* the lock was free or value didn't match */

		/*
		 * Add myself to wait queue.
		 */
		proc->lwWaiting = true;
		proc->lwWaitMode = LW_WAIT_UNTIL_FREE;
		/* waiters are added to the front of the queue */
		proc->lwWaitLink = lock->head;
		if (lock->head == NULL)
			lock->tail = proc;
		lock->head = proc;

		/*
		 * Set releaseOK, to make sure we get woken up as soon as the lock is
		 * released.
		 */
		lock->releaseOK = true;

		/* Can release the mutex now */
		SpinLockRelease(&lock->mutex);

		/*
		 * Wait until awakened.
		 *
		 * Since we share the process wait semaphore with the regular lock
		 * manager and ProcWaitForSignal, and we may need to acquire an LWLock
		 * while one of those is pending, it is possible that we get awakened
		 * for a reason other than being signaled by LWLockRelease. If so,
		 * loop back and wait again.  Once we've gotten the LWLock,
		 * re-increment the sema by the number of additional signals received,
		 * so that the lock manager or signal manager will see the received
		 * signal when it next waits.
		 */
		LOG_LWDEBUG("LWLockWaitForVar", T_NAME(lock), T_ID(lock), "waiting");

#ifdef LWLOCK_STATS
		lwstats->block_count++;
#endif

		TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), T_ID(lock),
										   LW_EXCLUSIVE);

		for (;;)
		{
			/* "false" means cannot accept cancel/die interrupt here. */
			PGSemaphoreLock(&proc->sem, false);
			if (!proc->lwWaiting)
				break;
			extraWaits++;
		}

		TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), T_ID(lock),
										  LW_EXCLUSIVE);

		LOG_LWDEBUG("LWLockWaitForVar", T_NAME(lock), T_ID(lock), "awakened");

		/* Now loop back and check the status of the lock again. */
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&lock->mutex);

	TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), T_ID(lock), LW_EXCLUSIVE);

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(&proc->sem);

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	RESUME_INTERRUPTS();

	return result;
}
Ejemplo n.º 11
0
/*
 * LWLockAcquireOrWait - Acquire lock, or wait until it's free
 *
 * The semantics of this function are a bit funky.  If the lock is currently
 * free, it is acquired in the given mode, and the function returns true.  If
 * the lock isn't immediately free, the function waits until it is released
 * and returns false, but does not acquire the lock.
 *
 * This is currently used for WALWriteLock: when a backend flushes the WAL,
 * holding WALWriteLock, it can flush the commit records of many other
 * backends as a side-effect.  Those other backends need to wait until the
 * flush finishes, but don't need to acquire the lock anymore.  They can just
 * wake up, observe that their records have already been flushed, and return.
 */
bool
LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
{
	PGPROC	   *proc = MyProc;
	bool		mustwait;
	int			extraWaits = 0;
#ifdef LWLOCK_STATS
	lwlock_stats *lwstats;
#endif

	PRINT_LWDEBUG("LWLockAcquireOrWait", lock);

#ifdef LWLOCK_STATS
	lwstats = get_lwlock_stats_entry(lock);
#endif

	/* 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;
	}

	if (mustwait)
	{
		/*
		 * Add myself to wait queue.
		 *
		 * If we don't have a PGPROC structure, there's no way to wait.  This
		 * should never occur, since MyProc should only be null during shared
		 * memory initialization.
		 */
		if (proc == NULL)
			elog(PANIC, "cannot wait without a PGPROC structure");

		proc->lwWaiting = true;
		proc->lwWaitMode = LW_WAIT_UNTIL_FREE;
		proc->lwWaitLink = NULL;
		if (lock->head == NULL)
			lock->head = proc;
		else
			lock->tail->lwWaitLink = proc;
		lock->tail = proc;

		/* Can release the mutex now */
		SpinLockRelease(&lock->mutex);

		/*
		 * Wait until awakened.  Like in LWLockAcquire, be prepared for bogus
		 * wakups, because we share the semaphore with ProcWaitForSignal.
		 */
		LOG_LWDEBUG("LWLockAcquireOrWait", T_NAME(lock), T_ID(lock),
					"waiting");

#ifdef LWLOCK_STATS
		lwstats->block_count++;
#endif

		TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), T_ID(lock), mode);

		for (;;)
		{
			/* "false" means cannot accept cancel/die interrupt here. */
			PGSemaphoreLock(&proc->sem, false);
			if (!proc->lwWaiting)
				break;
			extraWaits++;
		}

		TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), T_ID(lock), mode);

		LOG_LWDEBUG("LWLockAcquireOrWait", T_NAME(lock), T_ID(lock),
					"awakened");
	}
	else
	{
		/* We are done updating shared state of the lock itself. */
		SpinLockRelease(&lock->mutex);
	}

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(&proc->sem);

	if (mustwait)
	{
		/* Failed to get lock, so release interrupt holdoff */
		RESUME_INTERRUPTS();
		LOG_LWDEBUG("LWLockAcquireOrWait", T_NAME(lock), T_ID(lock), "failed");
		TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL(T_NAME(lock), T_ID(lock),
													 mode);
	}
	else
	{
		/* Add lock to list of locks held by this backend */
		held_lwlocks[num_held_lwlocks++] = lock;
		TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT(T_NAME(lock), T_ID(lock),
												mode);
	}

	return !mustwait;
}
Ejemplo n.º 12
0
/*
 * LWLockRelease - release a previously acquired lock
 */
void
LWLockRelease(LWLock *lock)
{
	PGPROC	   *head;
	PGPROC	   *proc;
	int			i;

	PRINT_LWDEBUG("LWLockRelease", lock);

	/*
	 * Remove lock from list of locks held.  Usually, but not always, it will
	 * be the latest-acquired lock; so search array backwards.
	 */
	for (i = num_held_lwlocks; --i >= 0;)
	{
		if (lock == held_lwlocks[i])
			break;
	}
	if (i < 0)
		elog(ERROR, "lock %s %d is not held", T_NAME(lock), T_ID(lock));
	num_held_lwlocks--;
	for (; i < num_held_lwlocks; i++)
		held_lwlocks[i] = held_lwlocks[i + 1];

	/* Acquire mutex.  Time spent holding mutex should be short! */
	SpinLockAcquire(&lock->mutex);

	/* Release my hold on lock */
	if (lock->exclusive > 0)
		lock->exclusive--;
	else
	{
		Assert(lock->shared > 0);
		lock->shared--;
	}

	/*
	 * See if I need to awaken any waiters.  If I released a non-last shared
	 * hold, there cannot be anything to do.  Also, do not awaken any waiters
	 * if someone has already awakened waiters that haven't yet acquired the
	 * lock.
	 */
	head = lock->head;
	if (head != NULL)
	{
		if (lock->exclusive == 0 && lock->shared == 0 && lock->releaseOK)
		{
			/*
			 * Remove the to-be-awakened PGPROCs from the queue.
			 */
			bool		releaseOK = true;

			proc = head;

			/*
			 * First wake up any backends that want to be woken up without
			 * acquiring the lock.
			 */
			while (proc->lwWaitMode == LW_WAIT_UNTIL_FREE && proc->lwWaitLink)
				proc = proc->lwWaitLink;

			/*
			 * If the front waiter wants exclusive lock, awaken him only.
			 * Otherwise awaken as many waiters as want shared access.
			 */
			if (proc->lwWaitMode != LW_EXCLUSIVE)
			{
				while (proc->lwWaitLink != NULL &&
					   proc->lwWaitLink->lwWaitMode != LW_EXCLUSIVE)
				{
					if (proc->lwWaitMode != LW_WAIT_UNTIL_FREE)
						releaseOK = false;
					proc = proc->lwWaitLink;
				}
			}
			/* proc is now the last PGPROC to be released */
			lock->head = proc->lwWaitLink;
			proc->lwWaitLink = NULL;

			/*
			 * Prevent additional wakeups until retryer gets to run. Backends
			 * that are just waiting for the lock to become free don't retry
			 * automatically.
			 */
			if (proc->lwWaitMode != LW_WAIT_UNTIL_FREE)
				releaseOK = false;

			lock->releaseOK = releaseOK;
		}
		else
		{
			/* lock is still held, can't awaken anything */
			head = NULL;
		}
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&lock->mutex);

	TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock), T_ID(lock));

	/*
	 * Awaken any waiters I removed from the queue.
	 */
	while (head != NULL)
	{
		LOG_LWDEBUG("LWLockRelease", T_NAME(lock), T_ID(lock),
					"release waiter");
		proc = head;
		head = proc->lwWaitLink;
		proc->lwWaitLink = NULL;
		/*
		 * Guarantee that lwWaiting being unset only becomes visible once the
		 * unlink from the link has completed. Otherwise the target backend
		 * could be woken up for other reason and enqueue for a new lock - if
		 * that happens before the list unlink happens, the list would end up
		 * being corrupted.
		 *
		 * The barrier pairs with the SpinLockAcquire() when enqueing for
		 * another lock.
		 */
		pg_write_barrier();
		proc->lwWaiting = false;
		PGSemaphoreUnlock(&proc->sem);
	}

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	RESUME_INTERRUPTS();
}
Ejemplo n.º 13
0
int
main(int argc, char **argv)
{
	MyStorage  *storage;
	int			cpid;

	printf("Creating shared memory ... ");
	fflush(stdout);

	storage = (MyStorage *) PGSharedMemoryCreate(8192, false, 5433);

	storage->flag = 1234;

	printf("OK\n");

	printf("Creating semaphores ... ");
	fflush(stdout);

	PGReserveSemaphores(2, 5433);

	PGSemaphoreCreate(&storage->sem);

	printf("OK\n");

	/* sema initial value is 1, so lock should work */

	printf("Testing Lock ... ");
	fflush(stdout);

	PGSemaphoreLock(&storage->sem, false);

	printf("OK\n");

	/* now sema value is 0, so trylock should fail */

	printf("Testing TryLock ... ");
	fflush(stdout);

	if (PGSemaphoreTryLock(&storage->sem))
		printf("unexpected result!\n");
	else
		printf("OK\n");

	/* unlocking twice and then locking twice should work... */

	printf("Testing Multiple Lock ... ");
	fflush(stdout);

	PGSemaphoreUnlock(&storage->sem);
	PGSemaphoreUnlock(&storage->sem);

	PGSemaphoreLock(&storage->sem, false);
	PGSemaphoreLock(&storage->sem, false);

	printf("OK\n");

	/* check Reset too */

	printf("Testing Reset ... ");
	fflush(stdout);

	PGSemaphoreUnlock(&storage->sem);

	PGSemaphoreReset(&storage->sem);

	if (PGSemaphoreTryLock(&storage->sem))
		printf("unexpected result!\n");
	else
		printf("OK\n");

	/* Fork a child process and see if it can communicate */

	printf("Forking child process ... ");
	fflush(stdout);

	cpid = fork();
	if (cpid == 0)
	{
		/* In child */
		on_exit_reset();
		sleep(3);
		storage->flag++;
		PGSemaphoreUnlock(&storage->sem);
		proc_exit(0);
	}
	if (cpid < 0)
	{
		/* Fork failed */
		printf("failed: %s\n", strerror(errno));
		proc_exit(1);
	}

	printf("forked child pid %d OK\n", cpid);

	if (storage->flag != 1234)
		printf("Wrong value found in shared memory!\n");

	printf("Waiting for child (should wait 3 sec here) ... ");
	fflush(stdout);

	PGSemaphoreLock(&storage->sem, false);

	printf("OK\n");

	if (storage->flag != 1235)
		printf("Wrong value found in shared memory!\n");

	/* Test shutdown */

	printf("Running shmem_exit processing ... ");
	fflush(stdout);

	shmem_exit(0);

	printf("OK\n");

	printf("Tests complete.\n");

	proc_exit(0);

	return 0;					/* not reached */
}
Ejemplo n.º 14
0
/*
 * LWLockAcquire - acquire a lightweight lock in the specified mode
 *
 * If the lock is not available, sleep until it is.
 *
 * Side effect: cancel/die interrupts are held off until lock release.
 */
void
LWLockAcquire(LWLockId lockid, LWLockMode mode)
{
	volatile LWLock *lock = &(LWLockArray[lockid].lock);
#if LWLOCK_LOCK_PARTS > 1
	volatile LWLockPart *part = LWLOCK_PART(lock, lockid, MyBackendId);
#endif
	PGPROC	   *proc = MyProc;
	bool		retry = false;
	int			extraWaits = 0;

	PRINT_LWDEBUG("LWLockAcquire", lockid, lock);

#ifdef LWLOCK_STATS
	/* Set up local count state first time through in a given process */
	if (counts_for_pid != MyProcPid)
	{
		int		   *LWLockCounter = (int *) ((char *) LWLockArray - 2 * sizeof(int));
		int			numLocks = LWLockCounter[1];

		sh_acquire_counts = calloc(numLocks, sizeof(int));
		ex_acquire_counts = calloc(numLocks, sizeof(int));
		block_counts = calloc(numLocks, sizeof(int));
		counts_for_pid = MyProcPid;
		on_shmem_exit(print_lwlock_stats, 0);
	}
	/* Count lock acquisition attempts */
	if (mode == LW_EXCLUSIVE)
		ex_acquire_counts[lockid]++;
	else
		sh_acquire_counts[lockid]++;
#endif   /* LWLOCK_STATS */

	/*
	 * We can't wait if we haven't got a PGPROC.  This should only occur
	 * during bootstrap or shared memory initialization.  Put an Assert here
	 * to catch unsafe coding practices.
	 */
	Assert(!(proc == NULL && IsUnderPostmaster));

	/* 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();

	/*
	 * Loop here to try to acquire lock after each time we are signaled by
	 * LWLockRelease.
	 *
	 * NOTE: it might seem better to have LWLockRelease actually grant us the
	 * lock, rather than retrying and possibly having to go back to sleep. But
	 * in practice that is no good because it means a process swap for every
	 * lock acquisition when two or more processes are contending for the same
	 * lock.  Since LWLocks are normally used to protect not-very-long
	 * sections of computation, a process needs to be able to acquire and
	 * release the same lock many times during a single CPU time slice, even
	 * in the presence of contention.  The efficiency of being able to do that
	 * outweighs the inefficiency of sometimes wasting a process dispatch
	 * cycle because the lock is not free when a released waiter finally gets
	 * to run.	See pgsql-hackers archives for 29-Dec-01.
	 */
	for (;;)
	{
		bool		mustwait;
		
		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)) {
				/* If retrying, allow LWLockRelease to release waiters again.
				 * Usually this happens after we acquired the mutex, but if
				 * we skip that, we still need to set releaseOK.
				 *
				 * Acquiring the mutex here is not really an option - if many
				 * reader are awoken simultaneously by an exclusive unlock,
				 * that would be a source of considerable contention.
				 *
				 * Fotunately, this is safe even without the mutex. First,
				 * there actually cannot be any non-fast path unlocking
				 * attempt in progress, because we'd then either still see
				 * the exclusive flag set or the mutex being taken. And
				 * even if there was, and such an attempt cleared the flag
				 * immediately after we set it, it'd also wake up some waiter
				 * who'd then re-set the flag.
				 *
				 * The only reason to do this here, and not directly
				 * after returning from PGSemaphoreLock(), is that it seems
				 * benefical to make SpinLockAcquire() the first thing to
				 * touch the lock if possible, in case we acquire the spin
				 * lock at all. That way, the cache line doesn't go through
				 * a possible shared state, but instead directly to exclusive.
				 * On Opterons at least, there seems to be a difference, c.f.
				 * the comment above tas() for x86_64 in s_lock.h
				 */
				if (retry && !lock->releaseOK)
					lock->releaseOK = true;
					
				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;
		}
		
		/* If retrying, allow LWLockRelease to release waiters again.
		 * This is also separately done in the LW_SHARED early exit case
		 * above, and in contrast to there we don't hold the mutex there.
		 * See the comment there for why this is safe
		 */
		if (retry)
			lock->releaseOK = true;
		
		if (!mustwait)
			break;				/* got the lock */
			
		/*
		 * Step (3b). Add myself to wait queue.
		 *
		 * If we don't have a PGPROC structure, there's no way to wait. This
		 * should never occur, since MyProc should only be null during shared
		 * memory initialization.
		 */
		if (proc == NULL)
			elog(PANIC, "cannot wait without a PGPROC structure");

		proc->lwWaiting = true;
		proc->lwExclusive = (mode == LW_EXCLUSIVE);
		proc->lwWaitLink = NULL;
		if (lock->head == NULL)
			lock->head = proc;
		else
			lock->tail->lwWaitLink = proc;
		lock->tail = proc;

		/* Can release the mutex now */
		SpinLockRelease(&lock->mutex);

		/*
		 * Wait until awakened.
		 *
		 * Since we share the process wait semaphore with the regular lock
		 * manager and ProcWaitForSignal, and we may need to acquire an LWLock
		 * while one of those is pending, it is possible that we get awakened
		 * for a reason other than being signaled by LWLockRelease. If so,
		 * loop back and wait again.  Once we've gotten the LWLock,
		 * re-increment the sema by the number of additional signals received,
		 * so that the lock manager or signal manager will see the received
		 * signal when it next waits.
		 */
		LOG_LWDEBUG("LWLockAcquire", lockid, "waiting");

#ifdef LWLOCK_STATS
		block_counts[lockid]++;
#endif

		TRACE_POSTGRESQL_LWLOCK_WAIT_START(lockid, mode);

		for (;;)
		{
			/* "false" means cannot accept cancel/die interrupt here. */
			PGSemaphoreLock(&proc->sem, false);
			if (!proc->lwWaiting)
				break;
			extraWaits++;
		}

		TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(lockid, mode);

		LOG_LWDEBUG("LWLockAcquire", lockid, "awakened");

		/* Now loop back and try to acquire lock again. */
		retry = true;
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&lock->mutex);
	
	/* Step 4. Enter protected section. This surely happens after (3),
	 * this time because lock release semantics demand that no store
	 * must be moved from before a lock release to after the release,
	 * again for obvious reasons
	 */

#ifdef LWLOCK_PART_SHARED_OPS_ATOMIC
lock_acquired:
#endif

	TRACE_POSTGRESQL_LWLOCK_ACQUIRE(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;

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(&proc->sem);
}
Ejemplo n.º 15
0
/*
 * CheckDeadLock
 *
 * We only get to this routine if the DEADLOCK_TIMEOUT fired
 * while waiting for a lock to be released by some other process.  Look
 * to see if there's a deadlock; if not, just return and continue waiting.
 * (But signal ProcSleep to log a message, if log_lock_waits is true.)
 * If we have a real deadlock, remove ourselves from the lock's wait queue
 * and signal an error to ProcSleep.
 *
 * NB: this is run inside a signal handler, so be very wary about what is done
 * here or in called routines.
 */
void
CheckDeadLock(void)
{
	int			i;

	/*
	 * Acquire exclusive lock on the entire shared lock data structures. Must
	 * grab LWLocks in partition-number order to avoid LWLock deadlock.
	 *
	 * Note that the deadlock check interrupt had better not be enabled
	 * anywhere that this process itself holds lock partition locks, else this
	 * will wait forever.  Also note that LWLockAcquire creates a critical
	 * section, so that this routine cannot be interrupted by cancel/die
	 * interrupts.
	 */
	for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
		LWLockAcquire(FirstLockMgrLock + i, LW_EXCLUSIVE);

	/*
	 * Check to see if we've been awoken by anyone in the interim.
	 *
	 * If we have, we can return and resume our transaction -- happy day.
	 * Before we are awoken the process releasing the lock grants it to us so
	 * we know that we don't have to wait anymore.
	 *
	 * We check by looking to see if we've been unlinked from the wait queue.
	 * This is quicker than checking our semaphore's state, since no kernel
	 * call is needed, and it is safe because we hold the lock partition lock.
	 */
	if (MyProc->links.prev == NULL ||
		MyProc->links.next == NULL)
		goto check_done;

#ifdef LOCK_DEBUG
	if (Debug_deadlocks)
		DumpAllLocks();
#endif

	/* Run the deadlock check, and set deadlock_state for use by ProcSleep */
	deadlock_state = DeadLockCheck(MyProc);

	if (deadlock_state == DS_HARD_DEADLOCK)
	{
		/*
		 * Oops.  We have a deadlock.
		 *
		 * Get this process out of wait state. (Note: we could do this more
		 * efficiently by relying on lockAwaited, but use this coding to
		 * preserve the flexibility to kill some other transaction than the
		 * one detecting the deadlock.)
		 *
		 * RemoveFromWaitQueue sets MyProc->waitStatus to STATUS_ERROR, so
		 * ProcSleep will report an error after we return from the signal
		 * handler.
		 */
		Assert(MyProc->waitLock != NULL);
		RemoveFromWaitQueue(MyProc, LockTagHashCode(&(MyProc->waitLock->tag)));

		/*
		 * Unlock my semaphore so that the interrupted ProcSleep() call can
		 * finish.
		 */
		PGSemaphoreUnlock(&MyProc->sem);

		/*
		 * We're done here.  Transaction abort caused by the error that
		 * ProcSleep will raise will cause any other locks we hold to be
		 * released, thus allowing other processes to wake up; we don't need
		 * to do that here.  NOTE: an exception is that releasing locks we
		 * hold doesn't consider the possibility of waiters that were blocked
		 * behind us on the lock we just failed to get, and might now be
		 * wakable because we're not in front of them anymore.  However,
		 * RemoveFromWaitQueue took care of waking up any such processes.
		 */
	}
	else if (log_lock_waits || deadlock_state == DS_BLOCKED_BY_AUTOVACUUM)
	{
		/*
		 * Unlock my semaphore so that the interrupted ProcSleep() call can
		 * print the log message (we daren't do it here because we are inside
		 * a signal handler).  It will then sleep again until someone releases
		 * the lock.
		 *
		 * If blocked by autovacuum, this wakeup will enable ProcSleep to send
		 * the canceling signal to the autovacuum worker.
		 */
		PGSemaphoreUnlock(&MyProc->sem);
	}

	/*
	 * And release locks.  We do this in reverse order for two reasons: (1)
	 * Anyone else who needs more than one of the locks will be trying to lock
	 * them in increasing order; we don't want to release the other process
	 * until it can get all the locks it needs. (2) This avoids O(N^2)
	 * behavior inside LWLockRelease.
	 */
check_done:
	for (i = NUM_LOCK_PARTITIONS; --i >= 0;)
		LWLockRelease(FirstLockMgrLock + i);
}
Ejemplo n.º 16
0
Archivo: lwlock.c Proyecto: 50wu/gpdb
/*
 * LWLockRelease - release a previously acquired lock
 */
void
LWLockRelease(LWLockId lockid)
{
	volatile LWLock *lock = &(LWLockArray[lockid].lock);
	PGPROC	   *head;
	PGPROC	   *proc;
	int			i;
	bool		saveExclusive;

	PRINT_LWDEBUG("LWLockRelease", lockid, lock);

	/*
	 * Remove lock from list of locks held.  Usually, but not always, it will
	 * be the latest-acquired lock; so search array backwards.
	 */
	for (i = num_held_lwlocks; --i >= 0;)
	{
		if (lockid == held_lwlocks[i])
			break;
	}
	if (i < 0)
		elog(ERROR, "lock %d is not held", (int) lockid);

	saveExclusive = held_lwlocks_exclusive[i];
	if (InterruptHoldoffCount <= 0)
		elog(PANIC, "upon entering lock release, the interrupt holdoff count is bad (%d) for release of lock %d (%s)", 
			 InterruptHoldoffCount,
			 (int)lockid,
			 (saveExclusive ? "Exclusive" : "Shared"));

#ifdef LWLOCK_TRACE_MIRROREDLOCK
	if (lockid == MirroredLock)
		elog(LOG, 
			 "LWLockRelease: release for MirroredLock by PID %u in held_lwlocks[%d] %s", 
			 MyProcPid, 
			 i,
			 (held_lwlocks_exclusive[i] ? "Exclusive" : "Shared"));
#endif
	
	num_held_lwlocks--;
	for (; i < num_held_lwlocks; i++)
	{
		held_lwlocks_exclusive[i] = held_lwlocks_exclusive[i + 1];
		held_lwlocks[i] = held_lwlocks[i + 1];
#ifdef USE_TEST_UTILS_X86
		/* shift stack traces */
		held_lwlocks_depth[i] = held_lwlocks_depth[i + 1];
		memcpy
			(
			held_lwlocks_addresses[i],
			held_lwlocks_addresses[i + 1],
			held_lwlocks_depth[i] * sizeof(*held_lwlocks_depth)
			)
			;
#endif /* USE_TEST_UTILS_X86 */
	}

	// Clear out old last entry.
	held_lwlocks_exclusive[num_held_lwlocks] = false;
	held_lwlocks[num_held_lwlocks] = 0;
#ifdef USE_TEST_UTILS_X86
	held_lwlocks_depth[num_held_lwlocks] = 0;
#endif /* USE_TEST_UTILS_X86 */

	/* Acquire mutex.  Time spent holding mutex should be short! */
	SpinLockAcquire(&lock->mutex);

	/* Release my hold on lock */
	if (lock->exclusive > 0)
	{
		lock->exclusive--;
		lock->exclusivePid = 0;
	}
	else
	{
		Assert(lock->shared > 0);
		lock->shared--;
	}

	/*
	 * See if I need to awaken any waiters.  If I released a non-last shared
	 * hold, there cannot be anything to do.  Also, do not awaken any waiters
	 * if someone has already awakened waiters that haven't yet acquired the
	 * lock.
	 */
	head = lock->head;
	if (head != NULL)
	{
		if (lock->exclusive == 0 && lock->shared == 0 && lock->releaseOK)
		{
			/*
			 * Remove the to-be-awakened PGPROCs from the queue.  If the front
			 * waiter wants exclusive lock, awaken him only. Otherwise awaken
			 * as many waiters as want shared access.
			 */
			proc = head;
			if (!proc->lwExclusive)
			{
				while (proc->lwWaitLink != NULL &&
					   !proc->lwWaitLink->lwExclusive)
				{
					proc = proc->lwWaitLink;
					if (proc->pid != 0)
					{
						lock->releaseOK = false;
					}					
				}
			}
			/* proc is now the last PGPROC to be released */
			lock->head = proc->lwWaitLink;
			proc->lwWaitLink = NULL;
			
			/* proc->pid can be 0 if process exited while waiting for lock */
			if (proc->pid != 0)
			{
				/* prevent additional wakeups until retryer gets to run */
				lock->releaseOK = false;
			}
		}
		else
		{
			/* lock is still held, can't awaken anything */
			head = NULL;
		}
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&lock->mutex);

	PG_TRACE1(lwlock__release, lockid);

	/*
	 * Awaken any waiters I removed from the queue.
	 */
	while (head != NULL)
	{
#ifdef LWLOCK_TRACE_MIRROREDLOCK
		if (lockid == MirroredLock)
			elog(LOG, "LWLockRelease: release waiter for MirroredLock (this PID %u", MyProcPid);
#endif
		LOG_LWDEBUG("LWLockRelease", lockid, "release waiter");
		proc = head;
		head = proc->lwWaitLink;
		proc->lwWaitLink = NULL;
		pg_write_barrier();
		proc->lwWaiting = false;
		PGSemaphoreUnlock(&proc->sem);
	}

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	if (InterruptHoldoffCount <= 0)
		elog(PANIC, "upon exiting lock release, the interrupt holdoff count is bad (%d) for release of lock %d (%s)", 
			 InterruptHoldoffCount,
			 (int)lockid,
			 (saveExclusive ? "Exclusive" : "Shared"));
	RESUME_INTERRUPTS();
}
Ejemplo n.º 17
0
/*
 * LWLockAcquire - acquire a lightweight lock in the specified mode
 *
 * If the lock is not available, sleep until it is.
 *
 * Side effect: cancel/die interrupts are held off until lock release.
 */
void
LWLockAcquire(LWLockId lockid, LWLockMode mode)
{
	volatile LWLock *lock = LWLockArray + lockid;
	PGPROC	   *proc = MyProc;
	bool		retry = false;
	int			extraWaits = 0;

	PRINT_LWDEBUG("LWLockAcquire", lockid, lock);

	/*
	 * We can't wait if we haven't got a PGPROC.  This should only occur
	 * during bootstrap or shared memory initialization.  Put an Assert
	 * here to catch unsafe coding practices.
	 */
	Assert(!(proc == NULL && IsUnderPostmaster));

	/*
	 * 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();

	/*
	 * Loop here to try to acquire lock after each time we are signaled by
	 * LWLockRelease.
	 *
	 * NOTE: it might seem better to have LWLockRelease actually grant us the
	 * lock, rather than retrying and possibly having to go back to sleep.
	 * But in practice that is no good because it means a process swap for
	 * every lock acquisition when two or more processes are contending
	 * for the same lock.  Since LWLocks are normally used to protect
	 * not-very-long sections of computation, a process needs to be able
	 * to acquire and release the same lock many times during a single CPU
	 * time slice, even in the presence of contention.	The efficiency of
	 * being able to do that outweighs the inefficiency of sometimes
	 * wasting a process dispatch cycle because the lock is not free when
	 * a released waiter finally gets to run.  See pgsql-hackers archives
	 * for 29-Dec-01.
	 */
	for (;;)
	{
		bool		mustwait;

		/* Acquire mutex.  Time spent holding mutex should be short! */
		SpinLockAcquire_NoHoldoff(&lock->mutex);

		/* If retrying, allow LWLockRelease to release waiters again */
		if (retry)
			lock->releaseOK = true;

		/* 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;
		}

		if (!mustwait)
			break;				/* got the lock */

		/*
		 * Add myself to wait queue.
		 *
		 * If we don't have a PGPROC structure, there's no way to wait. This
		 * should never occur, since MyProc should only be null during
		 * shared memory initialization.
		 */
		if (proc == NULL)
			elog(FATAL, "cannot wait without a PGPROC structure");

		proc->lwWaiting = true;
		proc->lwExclusive = (mode == LW_EXCLUSIVE);
		proc->lwWaitLink = NULL;
		if (lock->head == NULL)
			lock->head = proc;
		else
			lock->tail->lwWaitLink = proc;
		lock->tail = proc;

		/* Can release the mutex now */
		SpinLockRelease_NoHoldoff(&lock->mutex);

		/*
		 * Wait until awakened.
		 *
		 * Since we share the process wait semaphore with the regular lock
		 * manager and ProcWaitForSignal, and we may need to acquire an
		 * LWLock while one of those is pending, it is possible that we
		 * get awakened for a reason other than being signaled by
		 * LWLockRelease. If so, loop back and wait again.	Once we've
		 * gotten the LWLock, re-increment the sema by the number of
		 * additional signals received, so that the lock manager or signal
		 * manager will see the received signal when it next waits.
		 */
		LOG_LWDEBUG("LWLockAcquire", lockid, "waiting");

		for (;;)
		{
			/* "false" means cannot accept cancel/die interrupt here. */
			PGSemaphoreLock(&proc->sem, false);
			if (!proc->lwWaiting)
				break;
			extraWaits++;
		}

		LOG_LWDEBUG("LWLockAcquire", lockid, "awakened");

		/* Now loop back and try to acquire lock again. */
		retry = true;
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease_NoHoldoff(&lock->mutex);

	/* Add lock to list of locks held by this backend */
	Assert(num_held_lwlocks < MAX_SIMUL_LWLOCKS);
	held_lwlocks[num_held_lwlocks++] = lockid;

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(&proc->sem);
}
Ejemplo n.º 18
0
/*
 * CheckDeadLock
 *
 * We only get to this routine if we got SIGALRM after DeadlockTimeout
 * while waiting for a lock to be released by some other process.  Look
 * to see if there's a deadlock; if not, just return and continue waiting.
 * (But signal ProcSleep to log a message, if log_lock_waits is true.)
 * If we have a real deadlock, remove ourselves from the lock's wait queue
 * and signal an error to ProcSleep.
 *
 * NB: this is run inside a signal handler, so be very wary about what is done
 * here or in called routines.
 */
static void
CheckDeadLock(void)
{
	int			i;

	/*
	 * Acquire exclusive lock on the entire shared lock data structures. Must
	 * grab LWLocks in partition-number order to avoid LWLock deadlock.
	 *
	 * Note that the deadlock check interrupt had better not be enabled
	 * anywhere that this process itself holds lock partition locks, else this
	 * will wait forever.  Also note that LWLockAcquire creates a critical
	 * section, so that this routine cannot be interrupted by cancel/die
	 * interrupts.
	 */
	for (i = 0; i < NUM_LOCK_PARTITIONS; i++)
		LWLockAcquire(FirstLockMgrLock + i, LW_EXCLUSIVE);

	/*
	 * Check to see if we've been awoken by anyone in the interim.
	 *
	 * If we have, we can return and resume our transaction -- happy day.
	 * Before we are awoken the process releasing the lock grants it to us
	 * so we know that we don't have to wait anymore.
	 *
	 * We check by looking to see if we've been unlinked from the wait queue.
	 * This is quicker than checking our semaphore's state, since no kernel
	 * call is needed, and it is safe because we hold the lock partition lock.
	 */
	if (MyProc->links.prev == INVALID_OFFSET ||
		MyProc->links.next == INVALID_OFFSET)
		goto check_done;

#ifdef LOCK_DEBUG
	if (Debug_deadlocks)
		DumpAllLocks();
#endif

	if (!DeadLockCheck(MyProc))
	{
		/* No deadlock, so keep waiting */
		goto check_done;
	}

	/*
	 * Unlock my semaphore so that the interrupted ProcSleep() call can
	 * finish.
	 */
	PGSemaphoreUnlock(&MyProc->sem);

	/*
	 * We're done here.  Transaction abort caused by the error that ProcSleep
	 * will raise will cause any other locks we hold to be released, thus
	 * allowing other processes to wake up; we don't need to do that here.
	 * NOTE: an exception is that releasing locks we hold doesn't consider the
	 * possibility of waiters that were blocked behind us on the lock we just
	 * failed to get, and might now be wakable because we're not in front of
	 * them anymore.  However, RemoveFromWaitQueue took care of waking up any
	 * such processes.
	 */

	/*
	 * Release locks acquired at head of routine.  Order is not critical, so
	 * do it back-to-front to avoid waking another CheckDeadLock instance
	 * before it can get all the locks.
	 */
check_done:
	for (i = NUM_LOCK_PARTITIONS; --i >= 0;)
		LWLockRelease(FirstLockMgrLock + i);
}
Ejemplo n.º 19
0
/*
 * LWLockRelease - release a previously acquired lock
 */
void
LWLockRelease(LWLockId lockid)
{
	volatile LWLock *lock = &(LWLockArray[lockid].lock);
#if LWLOCK_LOCK_PARTS > 1
	volatile LWLockPart *part = LWLOCK_PART(lock, lockid, MyBackendId);
#endif
	PGPROC	   *head = NULL;
	PGPROC	   *proc;
	LWLockMode	mode;
	int			i;

	PRINT_LWDEBUG("LWLockRelease", lockid, lock);

	/*
	 * Remove lock from list of locks held.  Usually, but not always, it will
	 * be the latest-acquired lock; so search array backwards.
	 */
	 
	for (i = num_held_lwlocks; --i >= 0;)
	{
		if (lockid == held_lwlocks[i])
			break;
	}
	if (i < 0)
		elog(ERROR, "lock %d is not held", (int) lockid);
	
	mode = held_lwlocks_mode[i];
	
	num_held_lwlocks--;
	for (; i < num_held_lwlocks; i++) {
		held_lwlocks[i] = held_lwlocks[i + 1];
		held_lwlocks_mode[i] = held_lwlocks_mode[i + 1];
	}
		
	if (mode == LW_SHARED) {
#ifdef LWLOCK_PART_SHARED_OPS_ATOMIC
		int shared_pre;

		/* Release my hold on lock */
		Assert(lock->exclusive == 0);
		shared_pre = LWLOCK_PART_SHARED_POSTDEC_ATOMIC(lock, lockid, part, MyBackendId);
		Assert(shared_pre > 0);
		
		/* If the count didn't drop to zero (i.e., there are more lockers
		 * using the same shared counter partition), we can leave waiting
		 * up blocked exclusive locking attempts to them. Note that there
		 * may also be shared lockers using a *different* partition, so
		 * we're not necessarily the last share lockers, even if we continue.
		 * Still, it's an easy optimization, so we got for it
		 */
		if (shared_pre > 1)
			goto lock_released;
			
		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
		 * Assume now that we're that last share lock holder. Then, the
		 * fence after the atomic add above ensures that no further such
		 * concurrent exclusive locking attempts will proceed to (3a) and
		 * thus block. There may be such attempts currently blocking or
		 * about to block, but we can recognize those by either wait queue
		 * being non-empty or the mutex being taken. Conversely, if we see
		 * neither, we may assume that nobody needs to be signalled.
		 *
		 * Note that if two shared lockers release their lock while
		 * an exclusive locking attempt is in progress, both may decide
		 * they need to signal here. Taking the mutex below will sort that
		 * out, but it's a bit unfortunate that they have to race for the
		 * mutex here. Also, taking the mutex will force *other* shared
		 * lockers to take the mutex also in their release path.
		 * XXX: We may be able to improve that if we could dinstinguish
		 * been mutexed held for the purpose of unlocking and mutexes
		 * held for the purpose of locking.
		 *
		 * FIXME: This doesn't work if slock_t is a struct.
		 */
		
		if ((lock->mutex == 0) && (lock->head == NULL))
			goto lock_released;
			
		/* At this point, we don't know if the concurrent exclusive locker
		 * has seen on-zero in our shared counter partition in his step
		 * (2) or not. We must thus take the mutex and re-check.
		 */
#endif /* LWLOCK_PART_SHARED_OPS_ATOMIC */
		
		/* Acquire mutex.  Time spent holding mutex should be short! */
		SpinLockAcquire(&lock->mutex);
		
#ifdef LWLOCK_PART_SHARED_OPS_ATOMIC
		/* Already decremented the shared counter partition above */
#else
		/* Release my hold on lock */
		lock->shared--;
#endif
	}
	else {
		/* Acquire mutex.  Time spent holding mutex should be short! */
		SpinLockAcquire(&lock->mutex);
		
		/* Release my hold on lock */
		lock->exclusive--;
		Assert(lock->exclusive == 0);
	}

	/*
	 * See if I need to awaken any waiters.  If I released a non-last shared
	 * hold, there cannot be anything to do.  Also, do not awaken any waiters
	 * if someone has already awakened waiters that haven't yet acquired the
	 * lock.
	 */
	head = lock->head;
	if (head != NULL)
	{
		bool		is_shared;
		
		if (mode == LW_SHARED)
			LWLOCK_IS_SHARED(is_shared, lock, lockid);
		else
			is_shared = false;
			
		if (lock->exclusive == 0 && !is_shared && lock->releaseOK)
		{
			/*
			 * Remove the to-be-awakened PGPROCs from the queue.  If the front
			 * waiter wants exclusive lock, awaken him only. Otherwise awaken
			 * as many waiters as want shared access.
			 */
			proc = head;
			if (!proc->lwExclusive)
			{
				while (proc->lwWaitLink != NULL &&
					   !proc->lwWaitLink->lwExclusive)
					proc = proc->lwWaitLink;
			}
			/* proc is now the last PGPROC to be released */
			lock->head = proc->lwWaitLink;
			proc->lwWaitLink = NULL;
			/* prevent additional wakeups until retryer gets to run */
			lock->releaseOK = false;
		}
		else
		{
			/* lock is still held, can't awaken anything */
			head = NULL;
		}
	}

	/* We are done updating shared state of the lock itself. */
	SpinLockRelease(&lock->mutex);

#ifdef LWLOCK_PART_SHARED_OPS_ATOMIC
lock_released:
#endif

	TRACE_POSTGRESQL_LWLOCK_RELEASE(lockid);

	/*
	 * Awaken any waiters I removed from the queue.
	 */
	while (head != NULL)
	{
		LOG_LWDEBUG("LWLockRelease", lockid, "release waiter");
		proc = head;
		head = proc->lwWaitLink;
		proc->lwWaitLink = NULL;
		proc->lwWaiting = false;
		PGSemaphoreUnlock(&proc->sem);
	}

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	RESUME_INTERRUPTS();
}
Ejemplo n.º 20
0
void
s_unlock_sema(volatile slock_t *lock)
{
	PGSemaphoreUnlock((PGSemaphore) lock);
}
Ejemplo n.º 21
0
/*
 * Create a semaphore set with the given number of useful semaphores
 * (an additional sema is actually allocated to serve as identifier).
 * Dead Postgres sema sets are recycled if found, but we do not fail
 * upon collision with non-Postgres sema sets.
 *
 * The idea here is to detect and re-use keys that may have been assigned
 * by a crashed postmaster or backend.
 */
static IpcSemaphoreId
IpcSemaphoreCreate(int numSems)
{
	IpcSemaphoreId semId;
	union semun semun;
	PGSemaphoreData mysema;

	/* Loop till we find a free IPC key */
	for (nextSemaKey++;; nextSemaKey++)
	{
		pid_t		creatorPID;

		/* Try to create new semaphore set */
		semId = InternalIpcSemaphoreCreate(nextSemaKey, numSems + 1);
		if (semId >= 0)
			break;				/* successful create */

		/* See if it looks to be leftover from a dead Postgres process */
		semId = semget(nextSemaKey, numSems + 1, 0);
		if (semId < 0)
			continue;			/* failed: must be some other app's */
		if (IpcSemaphoreGetValue(semId, numSems) != PGSemaMagic)
			continue;			/* sema belongs to a non-Postgres app */

		/*
		 * If the creator PID is my own PID or does not belong to any extant
		 * process, it's safe to zap it.
		 */
		creatorPID = IpcSemaphoreGetLastPID(semId, numSems);
		if (creatorPID <= 0)
			continue;			/* oops, GETPID failed */
		if (creatorPID != getpid())
		{
			if (kill(creatorPID, 0) == 0 || errno != ESRCH)
				continue;		/* sema belongs to a live process */
		}

		/*
		 * The sema set appears to be from a dead Postgres process, or from a
		 * previous cycle of life in this same process.  Zap it, if possible.
		 * This probably shouldn't fail, but if it does, assume the sema set
		 * belongs to someone else after all, and continue quietly.
		 */
		semun.val = 0;			/* unused, but keep compiler quiet */
		if (semctl(semId, 0, IPC_RMID, semun) < 0)
			continue;

		/*
		 * Now try again to create the sema set.
		 */
		semId = InternalIpcSemaphoreCreate(nextSemaKey, numSems + 1);
		if (semId >= 0)
			break;				/* successful create */

		/*
		 * Can only get here if some other process managed to create the same
		 * sema key before we did.  Let him have that one, loop around to try
		 * next key.
		 */
	}

	/*
	 * OK, we created a new sema set.  Mark it as created by this process. We
	 * do this by setting the spare semaphore to PGSemaMagic-1 and then
	 * incrementing it with semop().  That leaves it with value PGSemaMagic
	 * and sempid referencing this process.
	 */
	IpcSemaphoreInitialize(semId, numSems, PGSemaMagic - 1);
	mysema.semId = semId;
	mysema.semNum = numSems;
	PGSemaphoreUnlock(&mysema);

	return semId;
}
Ejemplo n.º 22
0
void
s_unlock_sema(volatile slock_t *lock)
{
	PGSemaphoreUnlock(&SpinlockSemaArray[*lock]);
}