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Documentation improvement and minor code cleanups for the latch facility.

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Improve the documentation around weak-memory-ordering risks, and do a pass
of general editorialization on the comments in the latch code.  Make the
Windows latch code more like the Unix latch code where feasible; in
particular provide the same Assert checks in both implementations.
Fix poorly-placed WaitLatch call in syncrep.c.

This patch resolves, for the moment, concerns around weak-memory-ordering
bugs in latch-related code: we have documented the restrictions and checked
that existing calls meet them.  In 9.2 I hope that we will install suitable
memory barrier instructions in SetLatch/ResetLatch, so that their callers
don't need to be quite so careful.
This commit is contained in:
Tom Lane
2011-08-09 15:30:45 -04:00
parent cff60f2dfa
commit 4e15a4db5e
5 changed files with 185 additions and 124 deletions
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20
src/backend/replication/syncrep.c
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@@ -166,13 +166,6 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
{
int syncRepState;
/*
* Wait on latch for up to 60 seconds. This allows us to check for
* postmaster death regularly while waiting. Note that timeout here
* does not necessarily release from loop.
*/
WaitLatch(&MyProc->waitLatch, WL_LATCH_SET | WL_TIMEOUT, 60000000L);
/* Must reset the latch before testing state. */
ResetLatch(&MyProc->waitLatch);
@@ -184,6 +177,12 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
* walsender changes the state to SYNC_REP_WAIT_COMPLETE, it will
* never update it again, so we can't be seeing a stale value in that
* case.
*
* Note: on machines with weak memory ordering, the acquisition of
* the lock is essential to avoid race conditions: we cannot be sure
* the sender's state update has reached main memory until we acquire
* the lock. We could get rid of this dance if SetLatch/ResetLatch
* contained memory barriers.
*/
syncRepState = MyProc->syncRepState;
if (syncRepState == SYNC_REP_WAITING)
@@ -246,6 +245,13 @@ SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
SyncRepCancelWait();
break;
}
/*
* Wait on latch for up to 60 seconds. This allows us to check for
* cancel/die signal or postmaster death regularly while waiting. Note
* that timeout here does not necessarily release from loop.
*/
WaitLatch(&MyProc->waitLatch, WL_LATCH_SET | WL_TIMEOUT, 60000000L);
}
/*