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Repair a low-probability race condition identified by Qingqing Zhou.

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If a process abandons a wait in LockBufferForCleanup (in practice,
only happens if someone cancels a VACUUM) just before someone else
sends it a signal indicating the buffer is available, it was possible
for the wakeup to remain in the process' semaphore, causing misbehavior
next time the process waited for an lmgr lock.  Rather than try to
prevent the race condition directly, it seems best to make the lock
manager robust against leftover wakeups, by having it repeat waiting
on the semaphore if the lock has not actually been granted or denied
yet.
This commit is contained in:
Tom Lane
2006-04-14 03:38:56 +00:00
parent cc39aca7d4
commit 0fcc3c2f1d
5 changed files with 44 additions and 53 deletions
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3
src/include/c.h
View File

@ -12,7 +12,7 @@
* Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/c.h,v 1.199 2006/03/05 15:58:52 momjian Exp $
* $PostgreSQL: pgsql/src/include/c.h,v 1.200 2006/04/14 03:38:56 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -710,6 +710,7 @@ typedef NameData *Name;
#define STATUS_ERROR (-1)
#define STATUS_EOF (-2)
#define STATUS_FOUND (1)
#define STATUS_WAITING (2)
/* ----------------------------------------------------------------

5
src/include/storage/proc.h
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@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/storage/proc.h,v 1.87 2006/03/05 15:59:00 momjian Exp $
* $PostgreSQL: pgsql/src/include/storage/proc.h,v 1.88 2006/04/14 03:38:56 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -61,7 +61,7 @@ struct PGPROC
SHM_QUEUE links; /* list link if process is in a list */
PGSemaphoreData sem; /* ONE semaphore to sleep on */
int waitStatus; /* STATUS_OK or STATUS_ERROR after wakeup */
int waitStatus; /* STATUS_WAITING, STATUS_OK or STATUS_ERROR */
TransactionId xid; /* transaction currently being executed by
* this proc */
@ -147,7 +147,6 @@ extern void ProcLockWakeup(LockMethod lockMethodTable, LOCK *lock);
extern bool LockWaitCancel(void);
extern void ProcWaitForSignal(void);
extern void ProcCancelWaitForSignal(void);
extern void ProcSendSignal(int pid);
extern bool enable_sig_alarm(int delayms, bool is_statement_timeout);