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Commit6b5d8e1

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ReleaseRelationBuffers() failed to check for I/O in progress on a buffer
it wants to release. This leads to a race condition: does the backendthat's trying to flush the buffer do so before the one that's deleting therelation does so? Usually no problem, I expect, but on occasion this couldlead to hard-to-reproduce complaints from md.c, especially mdblindwrt.
1 parent610dfa6 commit6b5d8e1

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1 file changed

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‎src/backend/storage/buffer/bufmgr.c‎

Lines changed: 56 additions & 22 deletions
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77
*
88
*
99
* IDENTIFICATION
10-
* $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.66 1999/11/16 04:13:56 momjian Exp $
10+
* $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.67 1999/11/22 01:19:42 tgl Exp $
1111
*
1212
*-------------------------------------------------------------------------
1313
*/
@@ -1056,8 +1056,13 @@ BufferSync()
10561056

10571057

10581058
/*
1059-
* WaitIO -- Block until the IO_IN_PROGRESS flag on 'buf'
1060-
*is cleared. Because IO_IN_PROGRESS conflicts are
1059+
* WaitIO -- Block until the IO_IN_PROGRESS flag on 'buf' is cleared.
1060+
*
1061+
* Should be entered with buffer manager spinlock held; releases it before
1062+
* waiting and re-acquires it afterwards.
1063+
*
1064+
* OLD NOTES:
1065+
*Because IO_IN_PROGRESS conflicts are
10611066
*expected to be rare, there is only one BufferIO
10621067
*lock in the entire system.All processes block
10631068
*on this semaphore when they try to use a buffer
@@ -1069,15 +1074,13 @@ BufferSync()
10691074
*is simple, but efficient enough if WaitIO is
10701075
*rarely called by multiple processes simultaneously.
10711076
*
1072-
*ProcSleep atomically releases the spinlock and goes to
1073-
*sleep.
1074-
*
1075-
*Note: there is an easy fix if the queue becomes long.
1076-
*save the id of the buffer we are waiting for in
1077-
*the queue structure. That way signal can figure
1078-
*out which proc to wake up.
1077+
* NEW NOTES:
1078+
*The above is true only on machines without test-and-set
1079+
*semaphores (which we hope are few, these days). On better
1080+
*hardware, each buffer has a spinlock that we can wait on.
10791081
*/
10801082
#ifdefHAS_TEST_AND_SET
1083+
10811084
staticvoid
10821085
WaitIO(BufferDesc*buf,SPINLOCKspinlock)
10831086
{
@@ -1087,7 +1090,8 @@ WaitIO(BufferDesc *buf, SPINLOCK spinlock)
10871090
SpinAcquire(spinlock);
10881091
}
10891092

1090-
#else/* HAS_TEST_AND_SET */
1093+
#else/* !HAS_TEST_AND_SET */
1094+
10911095
IpcSemaphoreIdWaitIOSemId;
10921096
IpcSemaphoreIdWaitCLSemId;
10931097

@@ -1387,26 +1391,30 @@ RelationGetNumberOfBlocks(Relation relation)
13871391
*
13881392
*this function unmarks all the dirty pages of a relation
13891393
*in the buffer pool so that at the end of transaction
1390-
*these pages will not be flushed.
1394+
*these pages will not be flushed. This is used when the
1395+
*relation is about to be deleted. We assume that the caller
1396+
*holds an exclusive lock on the relation, which should assure
1397+
*that no new buffers will be acquired for the rel meanwhile.
1398+
*
13911399
*XXX currently it sequentially searches the buffer pool, should be
13921400
*changed to more clever ways of searching.
13931401
* --------------------------------------------------------------------
13941402
*/
13951403
void
13961404
ReleaseRelationBuffers(Relationrel)
13971405
{
1406+
Oidrelid=RelationGetRelid(rel);
1407+
boolholding= false;
13981408
inti;
1399-
intholding=0;
14001409
BufferDesc*buf;
14011410

14021411
if (rel->rd_myxactonly)
14031412
{
14041413
for (i=0;i<NLocBuffer;i++)
14051414
{
14061415
buf=&LocalBufferDescriptors[i];
1407-
if ((buf->flags&BM_DIRTY)&&
1408-
(buf->tag.relId.relId==RelationGetRelid(rel)))
1409-
buf->flags &= ~BM_DIRTY;
1416+
if (buf->tag.relId.relId==relid)
1417+
buf->flags &= ~ (BM_DIRTY |BM_JUST_DIRTIED);
14101418
}
14111419
return;
14121420
}
@@ -1417,21 +1425,47 @@ ReleaseRelationBuffers(Relation rel)
14171425
if (!holding)
14181426
{
14191427
SpinAcquire(BufMgrLock);
1420-
holding=1;
1428+
holding=true;
14211429
}
1422-
if ((buf->flags&BM_DIRTY)&&
1423-
(buf->tag.relId.dbId==MyDatabaseId)&&
1424-
(buf->tag.relId.relId==RelationGetRelid(rel)))
1430+
recheck:
1431+
if(buf->tag.relId.dbId==MyDatabaseId&&
1432+
buf->tag.relId.relId==relid)
14251433
{
1426-
buf->flags &= ~BM_DIRTY;
1434+
/*
1435+
* If there is I/O in progress, better wait till it's done;
1436+
* don't want to delete the relation out from under someone
1437+
* who's just trying to flush the buffer!
1438+
*/
1439+
if (buf->flags&BM_IO_IN_PROGRESS)
1440+
{
1441+
WaitIO(buf,BufMgrLock);
1442+
/* By now, the buffer very possibly belongs to some other
1443+
* rel, so check again before proceeding.
1444+
*/
1445+
gotorecheck;
1446+
}
1447+
/* Now we can do what we came for */
1448+
buf->flags &= ~ (BM_DIRTY |BM_JUST_DIRTIED);
1449+
CommitInfoNeedsSave[i-1]=0;
1450+
/*
1451+
* Release any refcount we may have.
1452+
*
1453+
* This is very probably dead code, and if it isn't then it's
1454+
* probably wrong. I added the Assert to find out --- tgl 11/99.
1455+
*/
14271456
if (!(buf->flags&BM_FREE))
14281457
{
1458+
/* Assert checks that buffer will actually get freed! */
1459+
Assert(PrivateRefCount[i-1]==1&&
1460+
buf->refcount==1);
1461+
/* ReleaseBuffer expects we do not hold the lock at entry */
14291462
SpinRelease(BufMgrLock);
1430-
holding=0;
1463+
holding=false;
14311464
ReleaseBuffer(i);
14321465
}
14331466
}
14341467
}
1468+
14351469
if (holding)
14361470
SpinRelease(BufMgrLock);
14371471
}

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