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Commit7635543

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Fix code so that we recover cleanly if there are no free semaphores
available in freeSemMap. As noted by Tatsuo, this is now a likelyscenario for detecting MaxBackends-exceeded; if MaxBackends is a multipleof PROC_NSEMS_PER_SET then we will fail here and not in sinval.c. Thecleanup path did not work correctly before, anyway.
1 parent29c22ee commit7635543

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1 file changed

+60
-41
lines changed
  • src/backend/storage/lmgr

1 file changed

+60
-41
lines changed

‎src/backend/storage/lmgr/proc.c‎

Lines changed: 60 additions & 41 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.105 2001/09/0402:26:57 tgl Exp $
11+
* $Header: /cvsroot/pgsql/src/backend/storage/lmgr/proc.c,v 1.106 2001/09/0421:42:17 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -186,6 +186,17 @@ InitProcess(void)
186186
unsigned longlocation,
187187
myOffset;
188188

189+
/*
190+
* ProcStructLock protects the freelist of PROC entries and the map
191+
* of free semaphores. Note that when we acquire it here, we do not
192+
* have a PROC entry and so the ownership of the spinlock is not
193+
* recorded anywhere; even if it was, until we register ProcKill as
194+
* an on_shmem_exit callback, there is no exit hook that will cause
195+
* owned spinlocks to be released. Upshot: during the first part of
196+
* this routine, be careful to release the lock manually before any
197+
* elog(), else you'll have a stuck spinlock to add to your woes.
198+
*/
199+
189200
SpinAcquire(ProcStructLock);
190201

191202
/* attach to the ProcGlobal structure */
@@ -194,13 +205,14 @@ InitProcess(void)
194205
if (!found)
195206
{
196207
/* this should not happen. InitProcGlobal() is called before this. */
208+
SpinRelease(ProcStructLock);
197209
elog(STOP,"InitProcess: Proc Header uninitialized");
198210
}
199211

200212
if (MyProc!=NULL)
201213
{
202214
SpinRelease(ProcStructLock);
203-
elog(ERROR,"ProcInit: you already exist");
215+
elog(ERROR,"InitProcess: you already exist");
204216
}
205217

206218
/* try to get a proc struct from the free list first */
@@ -214,14 +226,12 @@ InitProcess(void)
214226
}
215227
else
216228
{
217-
218229
/*
219230
* have to allocate one. We can't use the normal shmem index
220231
* table mechanism because the proc structure is stored by PID
221232
* instead of by a global name (need to look it up by PID when we
222233
* cleanup dead processes).
223234
*/
224-
225235
MyProc= (PROC*)ShmemAlloc(sizeof(PROC));
226236
if (!MyProc)
227237
{
@@ -231,44 +241,32 @@ InitProcess(void)
231241
}
232242

233243
/*
234-
* zero out the spin lock counts and set the sLocks field for
235-
* ProcStructLock to 1 as we have acquired this spinlock above but
236-
* didn't record it since we didn't have MyProc until now.
237-
*/
238-
MemSet(MyProc->sLocks,0,sizeof(MyProc->sLocks));
239-
MyProc->sLocks[ProcStructLock]=1;
240-
241-
/*
242-
* Set up a wait-semaphore for the proc.
244+
* Initialize all fields of MyProc.
243245
*/
244-
if (IsUnderPostmaster)
245-
{
246-
ProcGetNewSemIdAndNum(&MyProc->sem.semId,&MyProc->sem.semNum);
247-
248-
/*
249-
* we might be reusing a semaphore that belongs to a dead backend.
250-
* So be careful and reinitialize its value here.
251-
*/
252-
ZeroProcSemaphore(MyProc);
253-
}
254-
else
255-
{
256-
MyProc->sem.semId=-1;
257-
MyProc->sem.semNum=-1;
258-
}
259-
260246
SHMQueueElemInit(&(MyProc->links));
247+
MyProc->sem.semId=-1;/* no wait-semaphore acquired yet */
248+
MyProc->sem.semNum=-1;
261249
MyProc->errType=STATUS_OK;
262-
MyProc->pid=MyProcPid;
263-
MyProc->databaseId=MyDatabaseId;
264250
MyProc->xid=InvalidTransactionId;
265251
MyProc->xmin=InvalidTransactionId;
252+
MyProc->logRec.xrecoff=0;
266253
MyProc->waitLock=NULL;
267254
MyProc->waitHolder=NULL;
255+
MyProc->pid=MyProcPid;
256+
MyProc->databaseId=MyDatabaseId;
268257
SHMQueueInit(&(MyProc->procHolders));
258+
/*
259+
* Zero out the spin lock counts and set the sLocks field for
260+
* ProcStructLock to 1 as we have acquired this spinlock above but
261+
* didn't record it since we didn't have MyProc until now.
262+
*/
263+
MemSet(MyProc->sLocks,0,sizeof(MyProc->sLocks));
264+
MyProc->sLocks[ProcStructLock]=1;
269265

270266
/*
271-
* Release the lock.
267+
* Release the lock while accessing shmem index; we still haven't
268+
* installed ProcKill and so we don't want to hold lock if there's
269+
* an error.
272270
*/
273271
SpinRelease(ProcStructLock);
274272

@@ -283,10 +281,28 @@ InitProcess(void)
283281
elog(STOP,"InitProcess: ShmemPID table broken");
284282

285283
/*
286-
* Arrange to clean up at backend exit.
284+
* Arrange to clean up at backend exit. Once we do this, owned
285+
* spinlocks will be released on exit, and so we can be a lot less
286+
* tense about errors.
287287
*/
288288
on_shmem_exit(ProcKill,0);
289289

290+
/*
291+
* Set up a wait-semaphore for the proc. (Do this last so that we
292+
* can rely on ProcKill to clean up if it fails.)
293+
*/
294+
if (IsUnderPostmaster)
295+
{
296+
SpinAcquire(ProcStructLock);
297+
ProcGetNewSemIdAndNum(&MyProc->sem.semId,&MyProc->sem.semNum);
298+
SpinRelease(ProcStructLock);
299+
/*
300+
* We might be reusing a semaphore that belongs to a dead backend.
301+
* So be careful and reinitialize its value here.
302+
*/
303+
ZeroProcSemaphore(MyProc);
304+
}
305+
290306
/*
291307
* Now that we have a PROC, we could try to acquire locks, so
292308
* initialize the deadlock checker.
@@ -425,8 +441,9 @@ ProcKill(void)
425441

426442
SpinAcquire(ProcStructLock);
427443

428-
/* Free up my wait semaphore */
429-
ProcFreeSem(MyProc->sem.semId,MyProc->sem.semNum);
444+
/* Free up my wait semaphore, if I got one */
445+
if (MyProc->sem.semId >=0)
446+
ProcFreeSem(MyProc->sem.semId,MyProc->sem.semNum);
430447

431448
/* Add PROC struct to freelist so space can be recycled in future */
432449
MyProc->links.next=ProcGlobal->freeProcs;
@@ -993,10 +1010,7 @@ ProcGetNewSemIdAndNum(IpcSemaphoreId *semId, int *semNum)
9931010
{
9941011
if ((freeSemMap[i]&mask)==0)
9951012
{
996-
997-
/*
998-
* a free semaphore found. Mark it as allocated.
999-
*/
1013+
/* A free semaphore found. Mark it as allocated. */
10001014
freeSemMap[i] |=mask;
10011015

10021016
*semId=procSemIds[i];
@@ -1007,8 +1021,13 @@ ProcGetNewSemIdAndNum(IpcSemaphoreId *semId, int *semNum)
10071021
}
10081022
}
10091023

1010-
/* if we reach here, all the semaphores are in use. */
1011-
elog(ERROR,"ProcGetNewSemIdAndNum: cannot allocate a free semaphore");
1024+
/*
1025+
* If we reach here, all the semaphores are in use. This is one of the
1026+
* possible places to detect "too many backends", so give the standard
1027+
* error message. (Whether we detect it here or in sinval.c depends on
1028+
* whether MaxBackends is a multiple of PROC_NSEMS_PER_SET.)
1029+
*/
1030+
elog(FATAL,"Sorry, too many clients already");
10121031
}
10131032

10141033
/*

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