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Commitdab421d

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Seems I was too optimistic in supposing that sinval's maxMsgNum could be
read and written without a lock. The value itself is atomic, sure, but onprocessors with weak memory ordering it's possible for a reader to see thevalue change before it sees the associated message written into the bufferarray. Fix by introducing a spinlock that's used just to read and writemaxMsgNum. (We could do this with less overhead if we recognized a conceptof "memory access barrier"; is it worth introducing such a thing? At themoment probably not --- I can't measure any clear slowdown from adding thespinlock, so this solution is probably fine.) Per buildfarm results.
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‎src/backend/storage/ipc/sinvaladt.c

Lines changed: 48 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/storage/ipc/sinvaladt.c,v 1.71 2008/06/19 21:32:56 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/storage/ipc/sinvaladt.c,v 1.72 2008/06/20 00:24:53 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -23,6 +23,7 @@
2323
#include"storage/proc.h"
2424
#include"storage/shmem.h"
2525
#include"storage/sinvaladt.h"
26+
#include"storage/spin.h"
2627

2728

2829
/*
@@ -84,10 +85,20 @@
8485
* can operate in parallel with one or more readers, because the writer
8586
* has no need to touch anyone's ProcState, except in the infrequent cases
8687
* when SICleanupQueue is needed. The only point of overlap is that
87-
* the writer might change maxMsgNum while readers are looking at it.
88-
* This should be okay: we are assuming that fetching or storing an int
89-
* is atomic, an assumption also made elsewhere in Postgres. However
90-
* readers mustn't assume that maxMsgNum isn't changing under them.
88+
* the writer wants to change maxMsgNum while readers need to read it.
89+
* We deal with that by having a spinlock that readers must take for just
90+
* long enough to read maxMsgNum, while writers take it for just long enough
91+
* to write maxMsgNum. (The exact rule is that you need the spinlock to
92+
* read maxMsgNum if you are not holding SInvalWriteLock, and you need the
93+
* spinlock to write maxMsgNum unless you are holding both locks.)
94+
*
95+
* Note: since maxMsgNum is an int and hence presumably atomically readable/
96+
* writable, the spinlock might seem unnecessary. The reason it is needed
97+
* is to provide a memory barrier: we need to be sure that messages written
98+
* to the array are actually there before maxMsgNum is increased, and that
99+
* readers will see that data after fetching maxMsgNum. Multiprocessors
100+
* that have weak memory-ordering guarantees can fail without the memory
101+
* barrier instructions that are included in the spinlock sequences.
91102
*/
92103

93104

@@ -153,6 +164,8 @@ typedef struct SISeg
153164
intlastBackend;/* index of last active procState entry, +1 */
154165
intmaxBackends;/* size of procState array */
155166

167+
slock_tmsgnumLock;/* spinlock protecting maxMsgNum */
168+
156169
/*
157170
* Circular buffer holding shared-inval messages
158171
*/
@@ -209,12 +222,13 @@ CreateSharedInvalidationState(void)
209222
if (found)
210223
return;
211224

212-
/* Clear message counters, save size of procState array */
225+
/* Clear message counters, save size of procState array, init spinlock */
213226
shmInvalBuffer->minMsgNum=0;
214227
shmInvalBuffer->maxMsgNum=0;
215228
shmInvalBuffer->nextThreshold=CLEANUP_MIN;
216229
shmInvalBuffer->lastBackend=0;
217230
shmInvalBuffer->maxBackends=MaxBackends;
231+
SpinLockInit(&shmInvalBuffer->msgnumLock);
218232

219233
/* The buffer[] array is initially all unused, so we need not fill it */
220234

@@ -365,6 +379,7 @@ SIInsertDataEntries(const SharedInvalidationMessage *data, int n)
365379
{
366380
intnthistime=Min(n,WRITE_QUANTUM);
367381
intnumMsgs;
382+
intmax;
368383

369384
n-=nthistime;
370385

@@ -388,10 +403,21 @@ SIInsertDataEntries(const SharedInvalidationMessage *data, int n)
388403
/*
389404
* Insert new message(s) into proper slot of circular buffer
390405
*/
406+
max=segP->maxMsgNum;
391407
while (nthistime-->0)
392408
{
393-
segP->buffer[segP->maxMsgNum %MAXNUMMESSAGES]=*data++;
394-
segP->maxMsgNum++;
409+
segP->buffer[max %MAXNUMMESSAGES]=*data++;
410+
max++;
411+
}
412+
413+
/* Update current value of maxMsgNum using spinlock */
414+
{
415+
/* use volatile pointer to prevent code rearrangement */
416+
volatileSISeg*vsegP=segP;
417+
418+
SpinLockAcquire(&vsegP->msgnumLock);
419+
vsegP->maxMsgNum=max;
420+
SpinLockRelease(&vsegP->msgnumLock);
395421
}
396422

397423
LWLockRelease(SInvalWriteLock);
@@ -431,21 +457,32 @@ SIGetDataEntries(SharedInvalidationMessage *data, int datasize)
431457
{
432458
SISeg*segP;
433459
ProcState*stateP;
460+
intmax;
434461
intn;
435462

436463
LWLockAcquire(SInvalReadLock,LW_SHARED);
437464

438465
segP=shmInvalBuffer;
439466
stateP=&segP->procState[MyBackendId-1];
440467

468+
/* Fetch current value of maxMsgNum using spinlock */
469+
{
470+
/* use volatile pointer to prevent code rearrangement */
471+
volatileSISeg*vsegP=segP;
472+
473+
SpinLockAcquire(&vsegP->msgnumLock);
474+
max=vsegP->maxMsgNum;
475+
SpinLockRelease(&vsegP->msgnumLock);
476+
}
477+
441478
if (stateP->resetState)
442479
{
443480
/*
444481
* Force reset. We can say we have dealt with any messages added
445482
* since the reset, as well; and that means we should clear the
446483
* signaled flag, too.
447484
*/
448-
stateP->nextMsgNum=segP->maxMsgNum;
485+
stateP->nextMsgNum=max;
449486
stateP->resetState= false;
450487
stateP->signaled= false;
451488
LWLockRelease(SInvalReadLock);
@@ -459,13 +496,9 @@ SIGetDataEntries(SharedInvalidationMessage *data, int datasize)
459496
* There may be other backends that haven't read the message(s), so we
460497
* cannot delete them here. SICleanupQueue() will eventually remove them
461498
* from the queue.
462-
*
463-
* Note: depending on the compiler, we might read maxMsgNum only once
464-
* here, or each time through the loop. It doesn't matter (as long as
465-
* each fetch is atomic).
466499
*/
467500
n=0;
468-
while (n<datasize&&stateP->nextMsgNum<segP->maxMsgNum)
501+
while (n<datasize&&stateP->nextMsgNum<max)
469502
{
470503
data[n++]=segP->buffer[stateP->nextMsgNum %MAXNUMMESSAGES];
471504
stateP->nextMsgNum++;
@@ -474,7 +507,7 @@ SIGetDataEntries(SharedInvalidationMessage *data, int datasize)
474507
/*
475508
* Reset our "signaled" flag whenever we have caught up completely.
476509
*/
477-
if (stateP->nextMsgNum >=segP->maxMsgNum)
510+
if (stateP->nextMsgNum >=max)
478511
stateP->signaled= false;
479512

480513
LWLockRelease(SInvalReadLock);

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