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Commita8405cf

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committed
Acquire read lock on a buffer while writing it out, to prevent
concurrent modifications to the page by other backends.
1 parent384d94e commita8405cf

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1 file changed

+45
-32
lines changed

1 file changed

+45
-32
lines changed

‎src/backend/storage/buffer/bufmgr.c‎

Lines changed: 45 additions & 32 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.82 2000/06/08 22:37:20 momjian Exp $
11+
* $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.83 2000/09/25 04:11:09 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -429,17 +429,10 @@ BufferAlloc(Relation reln,
429429
inProgress= FALSE;
430430
for (buf= (BufferDesc*)NULL;buf== (BufferDesc*)NULL;)
431431
{
432-
433-
/* GetFreeBuffer will abort if it can't find a free buffer */
434432
buf=GetFreeBuffer();
435433

436-
/*
437-
* But it can return buf == NULL if we are in aborting transaction
438-
* now and so elog(ERROR,...) in GetFreeBuffer will not abort
439-
* again.
440-
*/
441-
if (buf==NULL)
442-
returnNULL;
434+
/* GetFreeBuffer will abort if it can't find a free buffer */
435+
Assert(buf);
443436

444437
/*
445438
* There should be exactly one pin on the buffer after it is
@@ -789,11 +782,21 @@ FlushBuffer(Buffer buffer, bool release)
789782
WaitIO(bufHdr,BufMgrLock);/* confirm end of IO */
790783
bufHdr->flags &= ~BM_JUST_DIRTIED;
791784
StartBufferIO(bufHdr, false);/* output IO start */
785+
792786
SpinRelease(BufMgrLock);
793787

788+
/*
789+
* Grab a read lock on the buffer to ensure that no
790+
* other backend changes its contents while we write it;
791+
* see comments in BufferSync().
792+
*/
793+
LockBuffer(BufferDescriptorGetBuffer(bufHdr),BUFFER_LOCK_SHARE);
794+
794795
status=smgrflush(DEFAULT_SMGR,bufrel,bufHdr->tag.blockNum,
795796
(char*)MAKE_PTR(bufHdr->data));
796797

798+
LockBuffer(BufferDescriptorGetBuffer(bufHdr),BUFFER_LOCK_UNLOCK);
799+
797800
/* drop relcache refcnt incremented by RelationIdCacheGetRelation */
798801
RelationDecrementReferenceCount(bufrel);
799802

@@ -1017,19 +1020,6 @@ ClearBufferDirtiedByMe(Buffer buffer, BufferDesc *bufHdr)
10171020
*that have been dirtied by the current xact and flush them to disk.
10181021
*We do *not* flush dirty buffers that have been dirtied by other xacts.
10191022
*(This is a substantial change from pre-7.0 behavior.)
1020-
*
1021-
*OLD COMMENTS (do these still apply?)
1022-
*
1023-
*Also, we need to be sure that no other transaction is
1024-
*modifying the page as we flush it.This is only a problem for objects
1025-
*that use a non-two-phase locking protocol, like btree indices.For
1026-
*those objects, we would like to set a write lock for the duration of
1027-
*our IO. Another possibility is to code updates to btree pages
1028-
*carefully, so that writing them out out of order cannot cause
1029-
*any unrecoverable errors.
1030-
*
1031-
*I don't want to think hard about this right now, so I will try
1032-
*to come back to it later.
10331023
*/
10341024
staticvoid
10351025
BufferSync()
@@ -1112,15 +1102,28 @@ BufferSync()
11121102
bufHdr->flags &= ~BM_JUST_DIRTIED;
11131103
StartBufferIO(bufHdr, false);/* output IO start */
11141104

1105+
SpinRelease(BufMgrLock);
1106+
1107+
/*
1108+
* Grab a read lock on the buffer to ensure that no
1109+
* other backend changes its contents while we write it;
1110+
* otherwise we could write a non-self-consistent page
1111+
* image to disk, which'd be bad news if the other
1112+
* transaction aborts before writing its changes.
1113+
*
1114+
* Note that we still need the BM_JUST_DIRTIED mechanism
1115+
* in case someone dirties the buffer just before we
1116+
* grab this lock or just after we release it.
1117+
*/
1118+
LockBuffer(BufferDescriptorGetBuffer(bufHdr),
1119+
BUFFER_LOCK_SHARE);
1120+
11151121
/*
11161122
* If we didn't have the reldesc in our local cache,
11171123
* write this page out using the 'blind write' storage
11181124
* manager routine. If we did find it, use the
11191125
* standard interface.
11201126
*/
1121-
#ifndefOPTIMIZE_SINGLE
1122-
SpinRelease(BufMgrLock);
1123-
#endif/* OPTIMIZE_SINGLE */
11241127
if (reln== (Relation)NULL)
11251128
{
11261129
status=smgrblindwrt(DEFAULT_SMGR,
@@ -1137,9 +1140,14 @@ BufferSync()
11371140
bufHdr->tag.blockNum,
11381141
(char*)MAKE_PTR(bufHdr->data));
11391142
}
1140-
#ifndefOPTIMIZE_SINGLE
1143+
1144+
/*
1145+
* Release the per-buffer readlock, reacquire BufMgrLock.
1146+
*/
1147+
LockBuffer(BufferDescriptorGetBuffer(bufHdr),
1148+
BUFFER_LOCK_UNLOCK);
1149+
11411150
SpinAcquire(BufMgrLock);
1142-
#endif/* OPTIMIZE_SINGLE */
11431151

11441152
UnpinBuffer(bufHdr);
11451153
if (status==SM_FAIL)
@@ -1522,9 +1530,14 @@ BufferReplace(BufferDesc *bufHdr)
15221530
/* To check if block content changed while flushing. - vadim 01/17/97 */
15231531
bufHdr->flags &= ~BM_JUST_DIRTIED;
15241532

1525-
#ifndefOPTIMIZE_SINGLE
15261533
SpinRelease(BufMgrLock);
1527-
#endif/* OPTIMIZE_SINGLE */
1534+
1535+
/*
1536+
* Grab a read lock on the buffer to ensure that no
1537+
* other backend changes its contents while we write it;
1538+
* see comments in BufferSync().
1539+
*/
1540+
LockBuffer(BufferDescriptorGetBuffer(bufHdr),BUFFER_LOCK_SHARE);
15281541

15291542
if (reln!= (Relation)NULL)
15301543
{
@@ -1540,9 +1553,9 @@ BufferReplace(BufferDesc *bufHdr)
15401553
false);/* no fsync */
15411554
}
15421555

1543-
#ifndefOPTIMIZE_SINGLE
1556+
LockBuffer(BufferDescriptorGetBuffer(bufHdr),BUFFER_LOCK_UNLOCK);
1557+
15441558
SpinAcquire(BufMgrLock);
1545-
#endif/* OPTIMIZE_SINGLE */
15461559

15471560
/* drop relcache refcnt incremented by RelationIdCacheGetRelation */
15481561
if (reln!= (Relation)NULL)

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