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Commitc3a8e2a

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Use 64-bit atomics for xlblocks array elements.
In preparation for reading the contents of WAL buffers without alock. Also, avoids the previously-needed comment in GetXLogBuffer()explaining why it's safe from torn reads.Author: Bharath RupireddyDiscussion:https://postgr.es/m/CALj2ACVfFMfqD5oLzZSQQZWfXiJqd-NdX0_317veP6FuB31QWA@mail.gmail.comReviewed-by: Andres Freund
1 parent1301c80 commitc3a8e2a

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1 file changed

+19
-21
lines changed
  • src/backend/access/transam

1 file changed

+19
-21
lines changed

‎src/backend/access/transam/xlog.c

Lines changed: 19 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -501,7 +501,7 @@ typedef struct XLogCtlData
501501
* WALBufMappingLock.
502502
*/
503503
char*pages;/* buffers for unwritten XLOG pages */
504-
XLogRecPtr*xlblocks;/* 1st byte ptr-s + XLOG_BLCKSZ */
504+
pg_atomic_uint64*xlblocks;/* 1st byte ptr-s + XLOG_BLCKSZ */
505505
intXLogCacheBlck;/* highest allocated xlog buffer index */
506506

507507
/*
@@ -1636,20 +1636,16 @@ GetXLogBuffer(XLogRecPtr ptr, TimeLineID tli)
16361636
* out to disk and evicted, and the caller is responsible for making sure
16371637
* that doesn't happen.
16381638
*
1639-
* However, we don't hold a lock while we read the value. If someone has
1640-
* just initialized the page, it's possible that we get a "torn read" of
1641-
* the XLogRecPtr if 64-bit fetches are not atomic on this platform. In
1642-
* that case we will see a bogus value. That's ok, we'll grab the mapping
1643-
* lock (in AdvanceXLInsertBuffer) and retry if we see anything else than
1644-
* the page we're looking for. But it means that when we do this unlocked
1645-
* read, we might see a value that appears to be ahead of the page we're
1646-
* looking for. Don't PANIC on that, until we've verified the value while
1647-
* holding the lock.
1639+
* We don't hold a lock while we read the value. If someone is just about
1640+
* to initialize or has just initialized the page, it's possible that we
1641+
* get InvalidXLogRecPtr. That's ok, we'll grab the mapping lock (in
1642+
* AdvanceXLInsertBuffer) and retry if we see anything other than the page
1643+
* we're looking for.
16481644
*/
16491645
expectedEndPtr=ptr;
16501646
expectedEndPtr+=XLOG_BLCKSZ-ptr %XLOG_BLCKSZ;
16511647

1652-
endptr=XLogCtl->xlblocks[idx];
1648+
endptr=pg_atomic_read_u64(&XLogCtl->xlblocks[idx]);
16531649
if (expectedEndPtr!=endptr)
16541650
{
16551651
XLogRecPtrinitializedUpto;
@@ -1680,7 +1676,7 @@ GetXLogBuffer(XLogRecPtr ptr, TimeLineID tli)
16801676
WALInsertLockUpdateInsertingAt(initializedUpto);
16811677

16821678
AdvanceXLInsertBuffer(ptr,tli, false);
1683-
endptr=XLogCtl->xlblocks[idx];
1679+
endptr=pg_atomic_read_u64(&XLogCtl->xlblocks[idx]);
16841680

16851681
if (expectedEndPtr!=endptr)
16861682
elog(PANIC,"could not find WAL buffer for %X/%X",
@@ -1867,7 +1863,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
18671863
* be zero if the buffer hasn't been used yet). Fall through if it's
18681864
* already written out.
18691865
*/
1870-
OldPageRqstPtr=XLogCtl->xlblocks[nextidx];
1866+
OldPageRqstPtr=pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]);
18711867
if (LogwrtResult.Write<OldPageRqstPtr)
18721868
{
18731869
/*
@@ -1989,8 +1985,7 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
19891985
*/
19901986
pg_write_barrier();
19911987

1992-
*((volatileXLogRecPtr*)&XLogCtl->xlblocks[nextidx])=NewPageEndPtr;
1993-
1988+
pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx],NewPageEndPtr);
19941989
XLogCtl->InitializedUpTo=NewPageEndPtr;
19951990

19961991
npages++;
@@ -2208,7 +2203,7 @@ XLogWrite(XLogwrtRqst WriteRqst, TimeLineID tli, bool flexible)
22082203
* if we're passed a bogus WriteRqst.Write that is past the end of the
22092204
* last page that's been initialized by AdvanceXLInsertBuffer.
22102205
*/
2211-
XLogRecPtrEndPtr=XLogCtl->xlblocks[curridx];
2206+
XLogRecPtrEndPtr=pg_atomic_read_u64(&XLogCtl->xlblocks[curridx]);
22122207

22132208
if (LogwrtResult.Write >=EndPtr)
22142209
elog(PANIC,"xlog write request %X/%X is past end of log %X/%X",
@@ -4632,7 +4627,7 @@ XLOGShmemSize(void)
46324627
/* WAL insertion locks, plus alignment */
46334628
size=add_size(size,mul_size(sizeof(WALInsertLockPadded),NUM_XLOGINSERT_LOCKS+1));
46344629
/* xlblocks array */
4635-
size=add_size(size,mul_size(sizeof(XLogRecPtr),XLOGbuffers));
4630+
size=add_size(size,mul_size(sizeof(pg_atomic_uint64),XLOGbuffers));
46364631
/* extra alignment padding for XLOG I/O buffers */
46374632
size=add_size(size,Max(XLOG_BLCKSZ,PG_IO_ALIGN_SIZE));
46384633
/* and the buffers themselves */
@@ -4710,10 +4705,13 @@ XLOGShmemInit(void)
47104705
* needed here.
47114706
*/
47124707
allocptr= ((char*)XLogCtl)+sizeof(XLogCtlData);
4713-
XLogCtl->xlblocks= (XLogRecPtr*)allocptr;
4714-
memset(XLogCtl->xlblocks,0,sizeof(XLogRecPtr)*XLOGbuffers);
4715-
allocptr+=sizeof(XLogRecPtr)*XLOGbuffers;
4708+
XLogCtl->xlblocks= (pg_atomic_uint64*)allocptr;
4709+
allocptr+=sizeof(pg_atomic_uint64)*XLOGbuffers;
47164710

4711+
for (i=0;i<XLOGbuffers;i++)
4712+
{
4713+
pg_atomic_init_u64(&XLogCtl->xlblocks[i],InvalidXLogRecPtr);
4714+
}
47174715

47184716
/* WAL insertion locks. Ensure they're aligned to the full padded size */
47194717
allocptr+=sizeof(WALInsertLockPadded)-
@@ -5750,7 +5748,7 @@ StartupXLOG(void)
57505748
memcpy(page,endOfRecoveryInfo->lastPage,len);
57515749
memset(page+len,0,XLOG_BLCKSZ-len);
57525750

5753-
XLogCtl->xlblocks[firstIdx]=endOfRecoveryInfo->lastPageBeginPtr+XLOG_BLCKSZ;
5751+
pg_atomic_write_u64(&XLogCtl->xlblocks[firstIdx],endOfRecoveryInfo->lastPageBeginPtr+XLOG_BLCKSZ);
57545752
XLogCtl->InitializedUpTo=endOfRecoveryInfo->lastPageBeginPtr+XLOG_BLCKSZ;
57555753
}
57565754
else

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