deadlock.c

PostgreSQL7.4.6 for Linux

/*-------------------------------------------------------------------------
 *
 * deadlock.c
 *	  POSTGRES deadlock detection code
 *
 * See src/backend/storage/lmgr/README for a description of the deadlock
 * detection and resolution algorithms.
 *
 *
 * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  $Header: /cvsroot/pgsql/src/backend/storage/lmgr/deadlock.c,v 1.25 2003/09/25 06:58:02 petere Exp $
 *
 *	Interface:
 *
 *	DeadLockCheck()
 *	DeadLockReport()
 *	RememberSimpleDeadLock()
 *	InitDeadLockChecking()
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "lib/stringinfo.h"
#include "miscadmin.h"
#include "storage/proc.h"
#include "utils/memutils.h"


/* One edge in the waits-for graph */
typedef struct
{
	PGPROC	   *waiter;			/* the waiting process */
	PGPROC	   *blocker;		/* the process it is waiting for */
	int			pred;			/* workspace for TopoSort */
	int			link;			/* workspace for TopoSort */
} EDGE;

/* One potential reordering of a lock's wait queue */
typedef struct
{
	LOCK	   *lock;			/* the lock whose wait queue is described */
	PGPROC	  **procs;			/* array of PGPROC *'s in new wait order */
	int			nProcs;
} WAIT_ORDER;

/*
 * Information saved about each edge in a detected deadlock cycle.	This
 * is used to print a diagnostic message upon failure.
 *
 * Note: because we want to examine this info after releasing the LockMgrLock,
 * we can't just store LOCK and PGPROC pointers; we must extract out all the
 * info we want to be able to print.
 */
typedef struct
{
	LOCKTAG		locktag;		/* ID of awaited lock object */
	LOCKMODE	lockmode;		/* type of lock we're waiting for */
	int			pid;			/* PID of blocked backend */
} DEADLOCK_INFO;


static bool DeadLockCheckRecurse(PGPROC *proc);
static int	TestConfiguration(PGPROC *startProc);
static bool FindLockCycle(PGPROC *checkProc,
			  EDGE *softEdges, int *nSoftEdges);
static bool FindLockCycleRecurse(PGPROC *checkProc, int depth,
					 EDGE *softEdges, int *nSoftEdges);
static bool ExpandConstraints(EDGE *constraints, int nConstraints);
static bool TopoSort(LOCK *lock, EDGE *constraints, int nConstraints,
		 PGPROC **ordering);

#ifdef DEBUG_DEADLOCK
static void PrintLockQueue(LOCK *lock, const char *info);
#endif


/*
 * Working space for the deadlock detector
 */

/* Workspace for FindLockCycle */
static PGPROC **visitedProcs;	/* Array of visited procs */
static int	nVisitedProcs;

/* Workspace for TopoSort */
static PGPROC **topoProcs;		/* Array of not-yet-output procs */
static int *beforeConstraints;	/* Counts of remaining before-constraints */
static int *afterConstraints;	/* List head for after-constraints */

/* Output area for ExpandConstraints */
static WAIT_ORDER *waitOrders;	/* Array of proposed queue rearrangements */
static int	nWaitOrders;
static PGPROC **waitOrderProcs; /* Space for waitOrders queue contents */

/* Current list of constraints being considered */
static EDGE *curConstraints;
static int	nCurConstraints;
static int	maxCurConstraints;

/* Storage space for results from FindLockCycle */
static EDGE *possibleConstraints;
static int	nPossibleConstraints;
static int	maxPossibleConstraints;
static DEADLOCK_INFO *deadlockDetails;
static int	nDeadlockDetails;


/*
 * InitDeadLockChecking -- initialize deadlock checker during backend startup
 *
 * This does per-backend initialization of the deadlock checker; primarily,
 * allocation of working memory for DeadLockCheck.	We do this per-backend
 * since there's no percentage in making the kernel do copy-on-write
 * inheritance of workspace from the postmaster.  We want to allocate the
 * space at startup because the deadlock checker might be invoked when there's
 * no free memory left.
 */
void
InitDeadLockChecking(void)
{
	MemoryContext oldcxt;

	/* Make sure allocations are permanent */
	oldcxt = MemoryContextSwitchTo(TopMemoryContext);

	/*
	 * FindLockCycle needs at most MaxBackends entries in visitedProcs[]
	 * and deadlockDetails[].
	 */
	visitedProcs = (PGPROC **) palloc(MaxBackends * sizeof(PGPROC *));
	deadlockDetails = (DEADLOCK_INFO *) palloc(MaxBackends * sizeof(DEADLOCK_INFO));

	/*
	 * TopoSort needs to consider at most MaxBackends wait-queue entries,
	 * and it needn't run concurrently with FindLockCycle.
	 */
	topoProcs = visitedProcs;	/* re-use this space */
	beforeConstraints = (int *) palloc(MaxBackends * sizeof(int));
	afterConstraints = (int *) palloc(MaxBackends * sizeof(int));

	/*
	 * We need to consider rearranging at most MaxBackends/2 wait queues
	 * (since it takes at least two waiters in a queue to create a soft
	 * edge), and the expanded form of the wait queues can't involve more
	 * than MaxBackends total waiters.	(But avoid palloc(0) if
	 * MaxBackends = 1.)
	 */
	waitOrders = (WAIT_ORDER *)
		palloc(((MaxBackends + 1) / 2) * sizeof(WAIT_ORDER));
	waitOrderProcs = (PGPROC **) palloc(MaxBackends * sizeof(PGPROC *));

	/*
	 * Allow at most MaxBackends distinct constraints in a configuration.
	 * (Is this enough?  In practice it seems it should be, but I don't
	 * quite see how to prove it.  If we run out, we might fail to find a
	 * workable wait queue rearrangement even though one exists.)  NOTE
	 * that this number limits the maximum recursion depth of
	 * DeadLockCheckRecurse. Making it really big might potentially allow
	 * a stack-overflow problem.
	 */
	maxCurConstraints = MaxBackends;
	curConstraints = (EDGE *) palloc(maxCurConstraints * sizeof(EDGE));

	/*
	 * Allow up to 3*MaxBackends constraints to be saved without having to
	 * re-run TestConfiguration.  (This is probably more than enough, but
	 * we can survive if we run low on space by doing excess runs of
	 * TestConfiguration to re-compute constraint lists each time needed.)
	 * The last MaxBackends entries in possibleConstraints[] are reserved
	 * as output workspace for FindLockCycle.
	 */
	maxPossibleConstraints = MaxBackends * 4;
	possibleConstraints =
		(EDGE *) palloc(maxPossibleConstraints * sizeof(EDGE));

	MemoryContextSwitchTo(oldcxt);
}

/*
 * DeadLockCheck -- Checks for deadlocks for a given process
 *
 * This code looks for deadlocks involving the given process.  If any
 * are found, it tries to rearrange lock wait queues to resolve the
 * deadlock.  If resolution is impossible, return TRUE --- the caller
 * is then expected to abort the given proc's transaction.
 *
 * We can't block on user locks, so no sense testing for deadlock
 * because there is no blocking, and no timer for the block.  So,
 * only look at regular locks.
 *
 * We must have already locked the master lock before being called.
 * NOTE: although the lockmethod structure appears to allow each lock
 * table to have a different masterLock, all locks that can block had
 * better use the same LWLock, else this code will not be adequately
 * interlocked!
 *
 * On failure, deadlock details are recorded in deadlockDetails[] for
 * subsequent printing by DeadLockReport().  That activity is separate
 * because we don't want to do it while holding the master lock.
 */
bool
DeadLockCheck(PGPROC *proc)
{
	int			i,
				j;

	/* Initialize to "no constraints" */
	nCurConstraints = 0;
	nPossibleConstraints = 0;
	nWaitOrders = 0;

	/* Search for deadlocks and possible fixes */
	if (DeadLockCheckRecurse(proc))
	{
		/*
		 * Call FindLockCycle one more time, to record the correct
		 * deadlockDetails[] for the basic state with no rearrangements.
		 */
		int			nSoftEdges;

		nWaitOrders = 0;
		if (!FindLockCycle(proc, possibleConstraints, &nSoftEdges))
			elog(FATAL, "deadlock seems to have disappeared");

		return true;			/* cannot find a non-deadlocked state */
	}

	/* Apply any needed rearrangements of wait queues */
	for (i = 0; i < nWaitOrders; i++)
	{
		LOCK	   *lock = waitOrders[i].lock;
		PGPROC	  **procs = waitOrders[i].procs;
		int			nProcs = waitOrders[i].nProcs;
		PROC_QUEUE *waitQueue = &(lock->waitProcs);

		Assert(nProcs == waitQueue->size);

#ifdef DEBUG_DEADLOCK
		PrintLockQueue(lock, "DeadLockCheck:");
#endif

		/* Reset the queue and re-add procs in the desired order */
		ProcQueueInit(waitQueue);
		for (j = 0; j < nProcs; j++)
		{
			SHMQueueInsertBefore(&(waitQueue->links), &(procs[j]->links));
			waitQueue->size++;
		}

#ifdef DEBUG_DEADLOCK
		PrintLockQueue(lock, "rearranged to:");
#endif

		/* See if any waiters for the lock can be woken up now */
		ProcLockWakeup(GetLocksMethodTable(lock), lock);
	}
	return false;
}

/*
 * DeadLockCheckRecurse -- recursively search for valid orderings
 *
 * curConstraints[] holds the current set of constraints being considered
 * by an outer level of recursion.	Add to this each possible solution
 * constraint for any cycle detected at this level.
 *
 * Returns TRUE if no solution exists.	Returns FALSE if a deadlock-free
 * state is attainable, in which case waitOrders[] shows the required
 * rearrangements of lock wait queues (if any).
 */
static bool
DeadLockCheckRecurse(PGPROC *proc)
{
	int			nEdges;
	int			oldPossibleConstraints;
	bool		savedList;
	int			i;

	nEdges = TestConfiguration(proc);
	if (nEdges < 0)
		return true;			/* hard deadlock --- no solution */
	if (nEdges == 0)
		return false;			/* good configuration found */
	if (nCurConstraints >= maxCurConstraints)
		return true;			/* out of room for active constraints? */
	oldPossibleConstraints = nPossibleConstraints;
	if (nPossibleConstraints + nEdges + MaxBackends <= maxPossibleConstraints)
	{
		/* We can save the edge list in possibleConstraints[] */
		nPossibleConstraints += nEdges;
		savedList = true;
	}
	else
	{
		/* Not room; will need to regenerate the edges on-the-fly */
		savedList = false;
	}

	/*
	 * Try each available soft edge as an addition to the configuration.
	 */
	for (i = 0; i < nEdges; i++)
	{
		if (!savedList && i > 0)
		{
			/* Regenerate the list of possible added constraints */
			if (nEdges != TestConfiguration(proc))
				elog(FATAL, "inconsistent results during deadlock check");
		}
		curConstraints[nCurConstraints] =
			possibleConstraints[oldPossibleConstraints + i];
		nCurConstraints++;
		if (!DeadLockCheckRecurse(proc))
			return false;		/* found a valid solution! */
		/* give up on that added constraint, try again */
		nCurConstraints--;
	}
	nPossibleConstraints = oldPossibleConstraints;
	return true;				/* no solution found */
}


/*--------------------
 * Test a configuration (current set of constraints) for validity.
 *
 * Returns:
 *		0: the configuration is good (no deadlocks)
 *	   -1: the configuration has a hard deadlock or is not self-consistent
 *		>0: the configuration has one or more soft deadlocks
 *
 * In the soft-deadlock case, one of the soft cycles is chosen arbitrarily
 * and a list of its soft edges is returned beginning at
 * possibleConstraints+nPossibleConstraints.  The return value is the
 * number of soft edges.
 *--------------------
 */
static int
TestConfiguration(PGPROC *startProc)
{
	int			softFound = 0;
	EDGE	   *softEdges = possibleConstraints + nPossibleConstraints;
	int			nSoftEdges;
	int			i;

	/*
	 * Make sure we have room for FindLockCycle's output.
	 */
	if (nPossibleConstraints + MaxBackends > maxPossibleConstraints)
		return -1;

	/*
	 * Expand current constraint set into wait orderings.  Fail if the
	 * constraint set is not self-consistent.
	 */
	if (!ExpandConstraints(curConstraints, nCurConstraints))
		return -1;

	/*
	 * Check for cycles involving startProc or any of the procs mentioned
	 * in constraints.	We check startProc last because if it has a soft
	 * cycle still to be dealt with, we want to deal with that first.
	 */
	for (i = 0; i < nCurConstraints; i++)
	{
		if (FindLockCycle(curConstraints[i].waiter, softEdges, &nSoftEdges))
		{
			if (nSoftEdges == 0)
				return -1;		/* hard deadlock detected */
			softFound = nSoftEdges;
		}
		if (FindLockCycle(curConstraints[i].blocker, softEdges, &nSoftEdges))
		{
			if (nSoftEdges == 0)
				return -1;		/* hard deadlock detected */
			softFound = nSoftEdges;
		}
	}
	if (FindLockCycle(startProc, softEdges, &nSoftEdges))
	{
		if (nSoftEdges == 0)
			return -1;			/* hard deadlock detected */
		softFound = nSoftEdges;
	}
	return softFound;
}


/*
 * FindLockCycle -- basic check for deadlock cycles
 *
 * Scan outward from the given proc to see if there is a cycle in the
 * waits-for graph that includes this proc.  Return TRUE if a cycle
 * is found, else FALSE.  If a cycle is found, we return a list of
 * the "soft edges", if any, included in the cycle.  These edges could
 * potentially be eliminated by rearranging wait queues.  We also fill
 * deadlockDetails[] with information about the detected cycle; this info
 * is not used by the deadlock algorithm itself, only to print a useful
 * message after failing.
 *
 * Since we need to be able to check hypothetical configurations that would
 * exist after wait queue rearrangement, the routine pays attention to the
 * table of hypothetical queue orders in waitOrders[].	These orders will
 * be believed in preference to the actual ordering seen in the locktable.
 */
static bool
FindLockCycle(PGPROC *checkProc,
			  EDGE *softEdges,	/* output argument */
			  int *nSoftEdges)	/* output argument */
{
	nVisitedProcs = 0;
	nDeadlockDetails = 0;
	*nSoftEdges = 0;
	return FindLockCycleRecurse(checkProc, 0, softEdges, nSoftEdges);
}

static bool
FindLockCycleRecurse(PGPROC *checkProc,
					 int depth,
					 EDGE *softEdges,	/* output argument */
					 int *nSoftEdges)	/* output argument */
{
	PGPROC	   *proc;
	LOCK	   *lock;
	PROCLOCK   *proclock;
	SHM_QUEUE  *lockHolders;
	LOCKMETHODTABLE *lockMethodTable;
	PROC_QUEUE *waitQueue;
	int			queue_size;
	int			conflictMask;
	int			i;
	int			numLockModes,
				lm;

	/*
	 * Have we already seen this proc?
	 */
	for (i = 0; i < nVisitedProcs; i++)
	{
		if (visitedProcs[i] == checkProc)
		{
			/* If we return to starting point, we have a deadlock cycle */
			if (i == 0)
			{
				/*
				 * record total length of cycle --- outer levels will now
				 * fill deadlockDetails[]
				 */
				Assert(depth <= MaxBackends);
				nDeadlockDetails = depth;

				return true;
			}