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Commitffafacc

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Repair potential deadlock created by recent changes to recycle btree
index pages: when _bt_getbuf asks the FSM for a free index page, it ispossible (and, in some cases, even moderately likely) that the answerwill be the same page that _bt_split is trying to split. _bt_getbufalready knew that the returned page might not be free, but it wasn'tprepared for the possibility that even trying to lock the page couldbe problematic. Fix by doing a conditional rather than unconditionalgrab of the page lock.
1 parent18c1087 commitffafacc

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3 files changed

+67
-11
lines changed

3 files changed

+67
-11
lines changed

‎src/backend/access/nbtree/nbtpage.c

Lines changed: 33 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -9,7 +9,7 @@
99
*
1010
*
1111
* IDENTIFICATION
12-
* $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtpage.c,v 1.69 2003/08/08 21:41:27 momjian Exp $
12+
* $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtpage.c,v 1.70 2003/08/10 19:48:08 tgl Exp $
1313
*
1414
*NOTES
1515
* Postgres btree pages look like ordinary relation pages.The opaque
@@ -409,23 +409,47 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)
409409
* that the page is still free. (For example, an already-free
410410
* page could have been re-used between the time the last VACUUM
411411
* scanned it and the time the VACUUM made its FSM updates.)
412+
*
413+
* In fact, it's worse than that: we can't even assume that it's
414+
* safe to take a lock on the reported page. If somebody else
415+
* has a lock on it, or even worse our own caller does, we could
416+
* deadlock. (The own-caller scenario is actually not improbable.
417+
* Consider an index on a serial or timestamp column. Nearly all
418+
* splits will be at the rightmost page, so it's entirely likely
419+
* that _bt_split will call us while holding a lock on the page most
420+
* recently acquired from FSM. A VACUUM running concurrently with
421+
* the previous split could well have placed that page back in FSM.)
422+
*
423+
* To get around that, we ask for only a conditional lock on the
424+
* reported page. If we fail, then someone else is using the page,
425+
* and we may reasonably assume it's not free. (If we happen to be
426+
* wrong, the worst consequence is the page will be lost to use till
427+
* the next VACUUM, which is no big problem.)
412428
*/
413429
for (;;)
414430
{
415431
blkno=GetFreeIndexPage(&rel->rd_node);
416432
if (blkno==InvalidBlockNumber)
417433
break;
418434
buf=ReadBuffer(rel,blkno);
419-
LockBuffer(buf,access);
420-
page=BufferGetPage(buf);
421-
if (_bt_page_recyclable(page))
435+
if (ConditionalLockBuffer(buf))
422436
{
423-
/* Okay to use page. Re-initialize and return it */
424-
_bt_pageinit(page,BufferGetPageSize(buf));
425-
returnbuf;
437+
page=BufferGetPage(buf);
438+
if (_bt_page_recyclable(page))
439+
{
440+
/* Okay to use page. Re-initialize and return it */
441+
_bt_pageinit(page,BufferGetPageSize(buf));
442+
returnbuf;
443+
}
444+
elog(DEBUG2,"FSM returned nonrecyclable page");
445+
_bt_relbuf(rel,buf);
446+
}
447+
else
448+
{
449+
elog(DEBUG2,"FSM returned nonlockable page");
450+
/* couldn't get lock, so just drop pin */
451+
ReleaseBuffer(buf);
426452
}
427-
elog(DEBUG2,"FSM returned nonrecyclable page");
428-
_bt_relbuf(rel,buf);
429453
}
430454

431455
/*

‎src/backend/storage/buffer/bufmgr.c

Lines changed: 32 additions & 1 deletion
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 1.139 2003/08/04 02:40:03 momjian Exp $
11+
* $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.140 2003/08/10 19:48:08 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -1936,6 +1936,37 @@ LockBuffer(Buffer buffer, int mode)
19361936
elog(ERROR,"unrecognized buffer lock mode: %d",mode);
19371937
}
19381938

1939+
/*
1940+
* Acquire the cntx_lock for the buffer, but only if we don't have to wait.
1941+
*
1942+
* This assumes the caller wants BUFFER_LOCK_EXCLUSIVE mode.
1943+
*/
1944+
bool
1945+
ConditionalLockBuffer(Bufferbuffer)
1946+
{
1947+
BufferDesc*buf;
1948+
1949+
Assert(BufferIsValid(buffer));
1950+
if (BufferIsLocal(buffer))
1951+
return true;/* act as though we got it */
1952+
1953+
buf=&(BufferDescriptors[buffer-1]);
1954+
1955+
if (LWLockConditionalAcquire(buf->cntx_lock,LW_EXCLUSIVE))
1956+
{
1957+
/*
1958+
* This is not the best place to set cntxDirty flag (eg indices do
1959+
* not always change buffer they lock in excl mode). But please
1960+
* remember that it's critical to set cntxDirty *before* logging
1961+
* changes with XLogInsert() - see comments in BufferSync().
1962+
*/
1963+
buf->cntxDirty= true;
1964+
1965+
return true;
1966+
}
1967+
return false;
1968+
}
1969+
19391970
/*
19401971
* LockBufferForCleanup - lock a buffer in preparation for deleting items
19411972
*

‎src/include/storage/bufmgr.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77
* Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
88
* Portions Copyright (c) 1994, Regents of the University of California
99
*
10-
* $Id: bufmgr.h,v 1.69 2003/08/04 02:40:14 momjian Exp $
10+
* $Id: bufmgr.h,v 1.70 2003/08/10 19:48:08 tgl Exp $
1111
*
1212
*-------------------------------------------------------------------------
1313
*/
@@ -180,6 +180,7 @@ extern void SetBufferCommitInfoNeedsSave(Buffer buffer);
180180

181181
externvoidUnlockBuffers(void);
182182
externvoidLockBuffer(Bufferbuffer,intmode);
183+
externboolConditionalLockBuffer(Bufferbuffer);
183184
externvoidLockBufferForCleanup(Bufferbuffer);
184185

185186
externvoidAbortBufferIO(void);

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