From 763554393a9c816a8b95d068a8137e18e33f824c Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 4 Sep 2001 21:42:17 +0000
Subject: [PATCH] Fix code so that we recover cleanly if there are no free
semaphores available in freeSemMap. As noted by Tatsuo, this is now a likely
scenario for detecting MaxBackends-exceeded; if MaxBackends is a multiple of
PROC_NSEMS_PER_SET then we will fail here and not in sinval.c. The cleanup
path did not work correctly before, anyway.
---
src/backend/storage/lmgr/proc.c | 101 +++++++++++++++++++-------------
1 file changed, 60 insertions(+), 41 deletions(-)
diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c
index f491bc16f70..71f85cbe58f 100644
--- a/src/backend/storage/lmgr/proc.c
+++ b/src/backend/storage/lmgr/proc.c
@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
- * $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.105 2001/09/04 02:26:57 tgl Exp $
+ * $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.106 2001/09/04 21:42:17 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -186,6 +186,17 @@ InitProcess(void)
unsigned long location,
myOffset;
+ /*
+ * ProcStructLock protects the freelist of PROC entries and the map
+ * of free semaphores. Note that when we acquire it here, we do not
+ * have a PROC entry and so the ownership of the spinlock is not
+ * recorded anywhere; even if it was, until we register ProcKill as
+ * an on_shmem_exit callback, there is no exit hook that will cause
+ * owned spinlocks to be released. Upshot: during the first part of
+ * this routine, be careful to release the lock manually before any
+ * elog(), else you'll have a stuck spinlock to add to your woes.
+ */
+
SpinAcquire(ProcStructLock);
/* attach to the ProcGlobal structure */
@@ -194,13 +205,14 @@ InitProcess(void)
if (!found)
{
/* this should not happen. InitProcGlobal() is called before this. */
+ SpinRelease(ProcStructLock);
elog(STOP, "InitProcess: Proc Header uninitialized");
}
if (MyProc != NULL)
{
SpinRelease(ProcStructLock);
- elog(ERROR, "ProcInit: you already exist");
+ elog(ERROR, "InitProcess: you already exist");
}
/* try to get a proc struct from the free list first */
@@ -214,14 +226,12 @@ InitProcess(void)
}
else
{
-
/*
* have to allocate one. We can't use the normal shmem index
* table mechanism because the proc structure is stored by PID
* instead of by a global name (need to look it up by PID when we
* cleanup dead processes).
*/
-
MyProc = (PROC *) ShmemAlloc(sizeof(PROC));
if (!MyProc)
{
@@ -231,44 +241,32 @@ InitProcess(void)
}
/*
- * zero out the spin lock counts and set the sLocks field for
- * ProcStructLock to 1 as we have acquired this spinlock above but
- * didn't record it since we didn't have MyProc until now.
- */
- MemSet(MyProc->sLocks, 0, sizeof(MyProc->sLocks));
- MyProc->sLocks[ProcStructLock] = 1;
-
- /*
- * Set up a wait-semaphore for the proc.
+ * Initialize all fields of MyProc.
*/
- if (IsUnderPostmaster)
- {
- ProcGetNewSemIdAndNum(&MyProc->sem.semId, &MyProc->sem.semNum);
-
- /*
- * we might be reusing a semaphore that belongs to a dead backend.
- * So be careful and reinitialize its value here.
- */
- ZeroProcSemaphore(MyProc);
- }
- else
- {
- MyProc->sem.semId = -1;
- MyProc->sem.semNum = -1;
- }
-
SHMQueueElemInit(&(MyProc->links));
+ MyProc->sem.semId = -1; /* no wait-semaphore acquired yet */
+ MyProc->sem.semNum = -1;
MyProc->errType = STATUS_OK;
- MyProc->pid = MyProcPid;
- MyProc->databaseId = MyDatabaseId;
MyProc->xid = InvalidTransactionId;
MyProc->xmin = InvalidTransactionId;
+ MyProc->logRec.xrecoff = 0;
MyProc->waitLock = NULL;
MyProc->waitHolder = NULL;
+ MyProc->pid = MyProcPid;
+ MyProc->databaseId = MyDatabaseId;
SHMQueueInit(&(MyProc->procHolders));
+ /*
+ * Zero out the spin lock counts and set the sLocks field for
+ * ProcStructLock to 1 as we have acquired this spinlock above but
+ * didn't record it since we didn't have MyProc until now.
+ */
+ MemSet(MyProc->sLocks, 0, sizeof(MyProc->sLocks));
+ MyProc->sLocks[ProcStructLock] = 1;
/*
- * Release the lock.
+ * Release the lock while accessing shmem index; we still haven't
+ * installed ProcKill and so we don't want to hold lock if there's
+ * an error.
*/
SpinRelease(ProcStructLock);
@@ -283,10 +281,28 @@ InitProcess(void)
elog(STOP, "InitProcess: ShmemPID table broken");
/*
- * Arrange to clean up at backend exit.
+ * Arrange to clean up at backend exit. Once we do this, owned
+ * spinlocks will be released on exit, and so we can be a lot less
+ * tense about errors.
*/
on_shmem_exit(ProcKill, 0);
+ /*
+ * Set up a wait-semaphore for the proc. (Do this last so that we
+ * can rely on ProcKill to clean up if it fails.)
+ */
+ if (IsUnderPostmaster)
+ {
+ SpinAcquire(ProcStructLock);
+ ProcGetNewSemIdAndNum(&MyProc->sem.semId, &MyProc->sem.semNum);
+ SpinRelease(ProcStructLock);
+ /*
+ * We might be reusing a semaphore that belongs to a dead backend.
+ * So be careful and reinitialize its value here.
+ */
+ ZeroProcSemaphore(MyProc);
+ }
+
/*
* Now that we have a PROC, we could try to acquire locks, so
* initialize the deadlock checker.
@@ -425,8 +441,9 @@ ProcKill(void)
SpinAcquire(ProcStructLock);
- /* Free up my wait semaphore */
- ProcFreeSem(MyProc->sem.semId, MyProc->sem.semNum);
+ /* Free up my wait semaphore, if I got one */
+ if (MyProc->sem.semId >= 0)
+ ProcFreeSem(MyProc->sem.semId, MyProc->sem.semNum);
/* Add PROC struct to freelist so space can be recycled in future */
MyProc->links.next = ProcGlobal->freeProcs;
@@ -993,10 +1010,7 @@ ProcGetNewSemIdAndNum(IpcSemaphoreId *semId, int *semNum)
{
if ((freeSemMap[i] & mask) == 0)
{
-
- /*
- * a free semaphore found. Mark it as allocated.
- */
+ /* A free semaphore found. Mark it as allocated. */
freeSemMap[i] |= mask;
*semId = procSemIds[i];
@@ -1007,8 +1021,13 @@ ProcGetNewSemIdAndNum(IpcSemaphoreId *semId, int *semNum)
}
}
- /* if we reach here, all the semaphores are in use. */
- elog(ERROR, "ProcGetNewSemIdAndNum: cannot allocate a free semaphore");
+ /*
+ * If we reach here, all the semaphores are in use. This is one of the
+ * possible places to detect "too many backends", so give the standard
+ * error message. (Whether we detect it here or in sinval.c depends on
+ * whether MaxBackends is a multiple of PROC_NSEMS_PER_SET.)
+ */
+ elog(FATAL, "Sorry, too many clients already");
}
/*
--
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