proc.c

PostgreSQL7.4.6 for Linux

/*
 * ProcKill() -- Destroy the per-proc data structure for
 *		this process. Release any of its held LW locks.
 */
static void
ProcKill(void)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile PROC_HDR *procglobal = ProcGlobal;

	Assert(MyProc != NULL);

	/* Release any LW locks I am holding */
	LWLockReleaseAll();

	/*
	 * Make real sure we release any buffer locks and pins we might be
	 * holding, too.  It is pretty ugly to do this here and not in a
	 * shutdown callback registered by the bufmgr ... but we must do this
	 * *after* LWLockReleaseAll and *before* zapping MyProc.
	 */
	AbortBufferIO();
	UnlockBuffers();
	AtEOXact_Buffers(false);

	/* Get off any wait queue I might be on */
	LockWaitCancel();

	/* Remove from the standard lock table */
	LockReleaseAll(DEFAULT_LOCKMETHOD, MyProc, true, InvalidTransactionId);

#ifdef USER_LOCKS
	/* Remove from the user lock table */
	LockReleaseAll(USER_LOCKMETHOD, MyProc, true, InvalidTransactionId);
#endif

	SpinLockAcquire(ProcStructLock);

	/* Return PGPROC structure (and semaphore) to freelist */
	MyProc->links.next = procglobal->freeProcs;
	procglobal->freeProcs = MAKE_OFFSET(MyProc);

	/* PGPROC struct isn't mine anymore */
	MyProc = NULL;

	SpinLockRelease(ProcStructLock);
}

/*
 * DummyProcKill() -- Cut-down version of ProcKill for dummy (checkpoint)
 *		processes.	The PGPROC and sema are not released, only marked
 *		as not-in-use.
 */
static void
DummyProcKill(void)
{
	Assert(MyProc != NULL && MyProc == DummyProc);

	/* Release any LW locks I am holding */
	LWLockReleaseAll();

	/* Release buffer locks and pins, too */
	AbortBufferIO();
	UnlockBuffers();
	AtEOXact_Buffers(false);

	/* I can't be on regular lock queues, so needn't check */

	/* Mark DummyProc no longer in use */
	MyProc->pid = 0;

	/* PGPROC struct isn't mine anymore */
	MyProc = NULL;
}


/*
 * ProcQueue package: routines for putting processes to sleep
 *		and  waking them up
 */

/*
 * ProcQueueAlloc -- alloc/attach to a shared memory process queue
 *
 * Returns: a pointer to the queue or NULL
 * Side Effects: Initializes the queue if we allocated one
 */
#ifdef NOT_USED
PROC_QUEUE *
ProcQueueAlloc(char *name)
{
	bool		found;
	PROC_QUEUE *queue = (PROC_QUEUE *)
	ShmemInitStruct(name, sizeof(PROC_QUEUE), &found);

	if (!queue)
		return NULL;
	if (!found)
		ProcQueueInit(queue);
	return queue;
}
#endif

/*
 * ProcQueueInit -- initialize a shared memory process queue
 */
void
ProcQueueInit(PROC_QUEUE *queue)
{
	SHMQueueInit(&(queue->links));
	queue->size = 0;
}


/*
 * ProcSleep -- put a process to sleep
 *
 * Caller must have set MyProc->heldLocks to reflect locks already held
 * on the lockable object by this process (under all XIDs).
 *
 * Locktable's masterLock must be held at entry, and will be held
 * at exit.
 *
 * Result: STATUS_OK if we acquired the lock, STATUS_ERROR if not (deadlock).
 *
 * ASSUME: that no one will fiddle with the queue until after
 *		we release the masterLock.
 *
 * NOTES: The process queue is now a priority queue for locking.
 *
 * P() on the semaphore should put us to sleep.  The process
 * semaphore is normally zero, so when we try to acquire it, we sleep.
 */
int
ProcSleep(LOCKMETHODTABLE *lockMethodTable,
		  LOCKMODE lockmode,
		  LOCK *lock,
		  PROCLOCK *proclock)
{
	LWLockId	masterLock = lockMethodTable->masterLock;
	PROC_QUEUE *waitQueue = &(lock->waitProcs);
	int			myHeldLocks = MyProc->heldLocks;
	bool		early_deadlock = false;
	PGPROC	   *proc;
	int			i;

	/*
	 * Determine where to add myself in the wait queue.
	 *
	 * Normally I should go at the end of the queue.  However, if I already
	 * hold locks that conflict with the request of any previous waiter,
	 * put myself in the queue just in front of the first such waiter.
	 * This is not a necessary step, since deadlock detection would move
	 * me to before that waiter anyway; but it's relatively cheap to
	 * detect such a conflict immediately, and avoid delaying till
	 * deadlock timeout.
	 *
	 * Special case: if I find I should go in front of some waiter, check to
	 * see if I conflict with already-held locks or the requests before
	 * that waiter.  If not, then just grant myself the requested lock
	 * immediately.  This is the same as the test for immediate grant in
	 * LockAcquire, except we are only considering the part of the wait
	 * queue before my insertion point.
	 */
	if (myHeldLocks != 0)
	{
		int			aheadRequests = 0;

		proc = (PGPROC *) MAKE_PTR(waitQueue->links.next);
		for (i = 0; i < waitQueue->size; i++)
		{
			/* Must he wait for me? */
			if (lockMethodTable->conflictTab[proc->waitLockMode] & myHeldLocks)
			{
				/* Must I wait for him ? */
				if (lockMethodTable->conflictTab[lockmode] & proc->heldLocks)
				{
					/*
					 * Yes, so we have a deadlock.	Easiest way to clean
					 * up correctly is to call RemoveFromWaitQueue(), but
					 * we can't do that until we are *on* the wait queue.
					 * So, set a flag to check below, and break out of
					 * loop.  Also, record deadlock info for later
					 * message.
					 */
					RememberSimpleDeadLock(MyProc, lockmode, lock, proc);
					early_deadlock = true;
					break;
				}
				/* I must go before this waiter.  Check special case. */
				if ((lockMethodTable->conflictTab[lockmode] & aheadRequests) == 0 &&
					LockCheckConflicts(lockMethodTable,
									   lockmode,
									   lock,
									   proclock,
									   MyProc,
									   NULL) == STATUS_OK)
				{
					/* Skip the wait and just grant myself the lock. */
					GrantLock(lock, proclock, lockmode);
					return STATUS_OK;
				}
				/* Break out of loop to put myself before him */
				break;
			}
			/* Nope, so advance to next waiter */
			aheadRequests |= (1 << proc->waitLockMode);
			proc = (PGPROC *) MAKE_PTR(proc->links.next);
		}

		/*
		 * If we fall out of loop normally, proc points to waitQueue head,
		 * so we will insert at tail of queue as desired.
		 */
	}
	else
	{
		/* I hold no locks, so I can't push in front of anyone. */
		proc = (PGPROC *) &(waitQueue->links);
	}

	/*
	 * Insert self into queue, ahead of the given proc (or at tail of
	 * queue).
	 */
	SHMQueueInsertBefore(&(proc->links), &(MyProc->links));
	waitQueue->size++;

	lock->waitMask |= (1 << lockmode);

	/* Set up wait information in PGPROC object, too */
	MyProc->waitLock = lock;
	MyProc->waitHolder = proclock;
	MyProc->waitLockMode = lockmode;

	MyProc->errType = STATUS_OK;	/* initialize result for success */

	/*
	 * If we detected deadlock, give up without waiting.  This must agree
	 * with CheckDeadLock's recovery code, except that we shouldn't
	 * release the semaphore since we haven't tried to lock it yet.
	 */
	if (early_deadlock)
	{
		RemoveFromWaitQueue(MyProc);
		MyProc->errType = STATUS_ERROR;
		return STATUS_ERROR;
	}

	/* mark that we are waiting for a lock */
	waitingForLock = true;

	/*
	 * Release the locktable's masterLock.
	 *
	 * NOTE: this may also cause us to exit critical-section state, possibly
	 * allowing a cancel/die interrupt to be accepted. This is OK because
	 * we have recorded the fact that we are waiting for a lock, and so
	 * LockWaitCancel will clean up if cancel/die happens.
	 */
	LWLockRelease(masterLock);

	/*
	 * Set timer so we can wake up after awhile and check for a deadlock.
	 * If a deadlock is detected, the handler releases the process's
	 * semaphore and sets MyProc->errType = STATUS_ERROR, allowing us to
	 * know that we must report failure rather than success.
	 *
	 * By delaying the check until we've waited for a bit, we can avoid
	 * running the rather expensive deadlock-check code in most cases.
	 */
	if (!enable_sig_alarm(DeadlockTimeout, false))
		elog(FATAL, "could not set timer for process wakeup");

	/*
	 * If someone wakes us between LWLockRelease and PGSemaphoreLock,
	 * PGSemaphoreLock will not block.	The wakeup is "saved" by the
	 * semaphore implementation.  Note also that if CheckDeadLock is
	 * invoked but does not detect a deadlock, PGSemaphoreLock() will
	 * continue to wait.  There used to be a loop here, but it was useless
	 * code...
	 *
	 * We pass interruptOK = true, which eliminates a window in which
	 * cancel/die interrupts would be held off undesirably.  This is a
	 * promise that we don't mind losing control to a cancel/die interrupt
	 * here.  We don't, because we have no state-change work to do after
	 * being granted the lock (the grantor did it all).
	 */
	PGSemaphoreLock(&MyProc->sem, true);

	/*
	 * Disable the timer, if it's still running
	 */
	if (!disable_sig_alarm(false))
		elog(FATAL, "could not disable timer for process wakeup");

	/*
	 * Now there is nothing for LockWaitCancel to do.
	 */
	waitingForLock = false;

	/*
	 * Re-acquire the locktable's masterLock.
	 */
	LWLockAcquire(masterLock, LW_EXCLUSIVE);

	/*
	 * We don't have to do anything else, because the awaker did all the
	 * necessary update of the lock table and MyProc.
	 */
	return MyProc->errType;
}


/*
 * 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.
 *
 * 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.
 */
PGPROC *
ProcWakeup(PGPROC *proc, int errType)
{
	PGPROC	   *retProc;

	/* assume that masterLock has been acquired */

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

	/* Save next process before we zap the list link */
	retProc = (PGPROC *) MAKE_PTR(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->waitHolder = NULL;
	proc->errType = errType;

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

	return retProc;
}

/*
 * ProcLockWakeup -- routine for waking up processes when a lock is
 *		released (or a prior waiter is aborted).  Scan all waiters
 *		for lock, waken any that are no longer blocked.
 */
void
ProcLockWakeup(LOCKMETHODTABLE *lockMethodTable, LOCK *lock)
{
	PROC_QUEUE *waitQueue = &(lock->waitProcs);
	int			queue_size = waitQueue->size;
	PGPROC	   *proc;
	int			aheadRequests = 0;

	Assert(queue_size >= 0);

	if (queue_size == 0)
		return;

	proc = (PGPROC *) MAKE_PTR(waitQueue->links.next);

	while (queue_size-- > 0)
	{
		LOCKMODE	lockmode = proc->waitLockMode;

		/*
		 * Waken if (a) doesn't conflict with requests of earlier waiters,
		 * and (b) doesn't conflict with already-held locks.
		 */
		if ((lockMethodTable->conflictTab[lockmode] & aheadRequests) == 0 &&
			LockCheckConflicts(lockMethodTable,
							   lockmode,
							   lock,
							   proc->waitHolder,
							   proc,
							   NULL) == STATUS_OK)
		{
			/* OK to waken */
			GrantLock(lock, proc->waitHolder, lockmode);