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Commit3fb5862

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Revert: Get rid of WALBufMappingLock
This commit reverts6a2275b. Buildfarm failure on batta spots someconcurrency issue, which requires further investigation.
1 parent75dfde1 commit3fb5862

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3 files changed

+48
-132
lines changed

3 files changed

+48
-132
lines changed

‎src/backend/access/transam/xlog.c

Lines changed: 46 additions & 130 deletions
Original file line numberDiff line numberDiff line change
@@ -302,6 +302,11 @@ static bool doPageWrites;
302302
* so it's a plain spinlock. The other locks are held longer (potentially
303303
* over I/O operations), so we use LWLocks for them. These locks are:
304304
*
305+
* WALBufMappingLock: must be held to replace a page in the WAL buffer cache.
306+
* It is only held while initializing and changing the mapping. If the
307+
* contents of the buffer being replaced haven't been written yet, the mapping
308+
* lock is released while the write is done, and reacquired afterwards.
309+
*
305310
* WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
306311
* XLogFlush).
307312
*
@@ -468,32 +473,21 @@ typedef struct XLogCtlData
468473
pg_atomic_uint64logFlushResult;/* last byte + 1 flushed */
469474

470475
/*
471-
* Latest reserved for inititalization page in the cache (last byte
472-
* position + 1).
476+
* Latest initialized page in the cache (last byte position + 1).
473477
*
474-
* To change the identity of a buffer, you need to advance
475-
*InitializeReserved first. To change the identity of a buffer that's
478+
* To change the identity of a buffer (and InitializedUpTo), you need to
479+
*hold WALBufMappingLock. To change the identity of a buffer that's
476480
* still dirty, the old page needs to be written out first, and for that
477481
* you need WALWriteLock, and you need to ensure that there are no
478482
* in-progress insertions to the page by calling
479483
* WaitXLogInsertionsToFinish().
480484
*/
481-
pg_atomic_uint64InitializeReserved;
482-
483-
/*
484-
* Latest initialized page in the cache (last byte position + 1).
485-
*
486-
* InitializedUpTo is updated after the buffer initialization. After
487-
* update, waiters got notification using InitializedUpToCondVar.
488-
*/
489-
pg_atomic_uint64InitializedUpTo;
490-
ConditionVariableInitializedUpToCondVar;
485+
XLogRecPtrInitializedUpTo;
491486

492487
/*
493488
* These values do not change after startup, although the pointed-to pages
494-
* and xlblocks values certainly do. xlblocks values are changed
495-
* lock-free according to the check for the xlog write position and are
496-
* accompanied by changes of InitializeReserved and InitializedUpTo.
489+
* and xlblocks values certainly do. xlblocks values are protected by
490+
* WALBufMappingLock.
497491
*/
498492
char*pages;/* buffers for unwritten XLOG pages */
499493
pg_atomic_uint64*xlblocks;/* 1st byte ptr-s + XLOG_BLCKSZ */
@@ -816,9 +810,9 @@ XLogInsertRecord(XLogRecData *rdata,
816810
* fullPageWrites from changing until the insertion is finished.
817811
*
818812
* Step 2 can usually be done completely in parallel. If the required WAL
819-
* page is not initialized yet, you have togo through AdvanceXLInsertBuffer,
820-
*which will ensure it is initialized. Butthe WAL writer tries to do that
821-
*ahead of insertionsto avoid that from happening in the critical path.
813+
* page is not initialized yet, you have tograb WALBufMappingLock to
814+
*initialize it, butthe WAL writer tries to do that ahead of insertions
815+
* to avoid that from happening in the critical path.
822816
*
823817
*----------
824818
*/
@@ -1997,70 +1991,32 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
19971991
XLogRecPtrNewPageEndPtr=InvalidXLogRecPtr;
19981992
XLogRecPtrNewPageBeginPtr;
19991993
XLogPageHeaderNewPage;
2000-
XLogRecPtrReservedPtr;
20011994
intnpagespg_attribute_unused()=0;
20021995

2003-
/*
2004-
* We must run the loop below inside the critical section as we expect
2005-
* XLogCtl->InitializedUpTo to eventually keep up. The most of callers
2006-
* already run inside the critical section. Except for WAL writer, which
2007-
* passed 'opportunistic == true', and therefore we don't perform
2008-
* operations that could error out.
2009-
*
2010-
* Start an explicit critical section anyway though.
2011-
*/
2012-
Assert(CritSectionCount>0||opportunistic);
2013-
START_CRIT_SECTION();
1996+
LWLockAcquire(WALBufMappingLock,LW_EXCLUSIVE);
20141997

2015-
/*--
2016-
* Loop till we get all the pages in WAL buffer before 'upto' reserved for
2017-
* initialization. Multiple process can initialize different buffers with
2018-
* this loop in parallel as following.
2019-
*
2020-
* 1. Reserve page for initialization using XLogCtl->InitializeReserved.
2021-
* 2. Initialize the reserved page.
2022-
* 3. Attempt to advance XLogCtl->InitializedUpTo,
1998+
/*
1999+
* Now that we have the lock, check if someone initialized the page
2000+
* already.
20232001
*/
2024-
ReservedPtr=pg_atomic_read_u64(&XLogCtl->InitializeReserved);
2025-
while (upto >=ReservedPtr||opportunistic)
2002+
while (upto >=XLogCtl->InitializedUpTo||opportunistic)
20262003
{
2027-
Assert(ReservedPtr %XLOG_BLCKSZ==0);
2004+
nextidx=XLogRecPtrToBufIdx(XLogCtl->InitializedUpTo);
20282005

20292006
/*
2030-
* Get ending-offset of the buffer page we need to replace.
2031-
*
2032-
* We don't lookup into xlblocks, but rather calculate position we
2033-
* must wait to be written. If it was written, xlblocks will have this
2034-
* position (or uninitialized)
2007+
* Get ending-offset of the buffer page we need to replace (this may
2008+
* be zero if the buffer hasn't been used yet). Fall through if it's
2009+
* already written out.
20352010
*/
2036-
if (ReservedPtr+XLOG_BLCKSZ>XLOG_BLCKSZ*XLOGbuffers)
2037-
OldPageRqstPtr=ReservedPtr+XLOG_BLCKSZ-XLOG_BLCKSZ*XLOGbuffers;
2038-
else
2039-
OldPageRqstPtr=InvalidXLogRecPtr;
2040-
2041-
if (LogwrtResult.Write<OldPageRqstPtr&&opportunistic)
2011+
OldPageRqstPtr=pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]);
2012+
if (LogwrtResult.Write<OldPageRqstPtr)
20422013
{
20432014
/*
2044-
* If we just want to pre-initialize as much as we can without
2045-
* flushing, give up now.
2015+
*Nope, got work to do.If we just want to pre-initialize as much
2016+
*as we can withoutflushing, give up now.
20462017
*/
2047-
upto=ReservedPtr-1;
2048-
break;
2049-
}
2050-
2051-
/*
2052-
* Attempt to reserve the page for initialization. Failure means that
2053-
* this page got reserved by another process.
2054-
*/
2055-
if (!pg_atomic_compare_exchange_u64(&XLogCtl->InitializeReserved,
2056-
&ReservedPtr,
2057-
ReservedPtr+XLOG_BLCKSZ))
2058-
continue;
2059-
2060-
/* Fall through if it's already written out. */
2061-
if (LogwrtResult.Write<OldPageRqstPtr)
2062-
{
2063-
/* Nope, got work to do. */
2018+
if (opportunistic)
2019+
break;
20642020

20652021
/* Advance shared memory write request position */
20662022
SpinLockAcquire(&XLogCtl->info_lck);
@@ -2075,6 +2031,14 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
20752031
RefreshXLogWriteResult(LogwrtResult);
20762032
if (LogwrtResult.Write<OldPageRqstPtr)
20772033
{
2034+
/*
2035+
* Must acquire write lock. Release WALBufMappingLock first,
2036+
* to make sure that all insertions that we need to wait for
2037+
* can finish (up to this same position). Otherwise we risk
2038+
* deadlock.
2039+
*/
2040+
LWLockRelease(WALBufMappingLock);
2041+
20782042
WaitXLogInsertionsToFinish(OldPageRqstPtr);
20792043

20802044
LWLockAcquire(WALWriteLock,LW_EXCLUSIVE);
@@ -2096,24 +2060,20 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
20962060
pgWalUsage.wal_buffers_full++;
20972061
TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
20982062
}
2063+
/* Re-acquire WALBufMappingLock and retry */
2064+
LWLockAcquire(WALBufMappingLock,LW_EXCLUSIVE);
2065+
continue;
20992066
}
21002067
}
21012068

21022069
/*
21032070
* Now the next buffer slot is free and we can set it up to be the
21042071
* next output page.
21052072
*/
2106-
NewPageBeginPtr=ReservedPtr;
2073+
NewPageBeginPtr=XLogCtl->InitializedUpTo;
21072074
NewPageEndPtr=NewPageBeginPtr+XLOG_BLCKSZ;
2108-
nextidx=XLogRecPtrToBufIdx(ReservedPtr);
21092075

2110-
#ifdefUSE_ASSERT_CHECKING
2111-
{
2112-
XLogRecPtrstoredBound=pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]);
2113-
2114-
Assert(storedBound==OldPageRqstPtr||storedBound==InvalidXLogRecPtr);
2115-
}
2116-
#endif
2076+
Assert(XLogRecPtrToBufIdx(NewPageBeginPtr)==nextidx);
21172077

21182078
NewPage= (XLogPageHeader) (XLogCtl->pages+nextidx* (Size)XLOG_BLCKSZ);
21192079

@@ -2179,50 +2139,11 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
21792139
pg_write_barrier();
21802140

21812141
pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx],NewPageEndPtr);
2182-
2183-
/*
2184-
* Try to advance XLogCtl->InitializedUpTo.
2185-
*
2186-
* If the CAS operation failed, then some of previous pages are not
2187-
* initialized yet, and this backend gives up.
2188-
*
2189-
* Since initializer of next page might give up on advancing of
2190-
* InitializedUpTo, this backend have to attempt advancing until it
2191-
* find page "in the past" or concurrent backend succeeded at
2192-
* advancing. When we finish advancing XLogCtl->InitializedUpTo, we
2193-
* notify all the waiters with XLogCtl->InitializedUpToCondVar.
2194-
*/
2195-
while (pg_atomic_compare_exchange_u64(&XLogCtl->InitializedUpTo,&NewPageBeginPtr,NewPageEndPtr))
2196-
{
2197-
NewPageBeginPtr=NewPageEndPtr;
2198-
NewPageEndPtr=NewPageBeginPtr+XLOG_BLCKSZ;
2199-
nextidx=XLogRecPtrToBufIdx(NewPageBeginPtr);
2200-
2201-
if (pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx])!=NewPageEndPtr)
2202-
{
2203-
/*
2204-
* Page at nextidx wasn't initialized yet, so we cann't move
2205-
* InitializedUpto further. It will be moved by backend which
2206-
* will initialize nextidx.
2207-
*/
2208-
ConditionVariableBroadcast(&XLogCtl->InitializedUpToCondVar);
2209-
break;
2210-
}
2211-
}
2142+
XLogCtl->InitializedUpTo=NewPageEndPtr;
22122143

22132144
npages++;
22142145
}
2215-
2216-
END_CRIT_SECTION();
2217-
2218-
/*
2219-
* All the pages in WAL buffer before 'upto' were reserved for
2220-
* initialization. However, some pages might be reserved by concurrent
2221-
* processes. Wait till they finish initialization.
2222-
*/
2223-
while (upto >=pg_atomic_read_u64(&XLogCtl->InitializedUpTo))
2224-
ConditionVariableSleep(&XLogCtl->InitializedUpToCondVar,WAIT_EVENT_WAL_BUFFER_INIT);
2225-
ConditionVariableCancelSleep();
2146+
LWLockRelease(WALBufMappingLock);
22262147

22272148
#ifdefWAL_DEBUG
22282149
if (XLOG_DEBUG&&npages>0)
@@ -5123,10 +5044,6 @@ XLOGShmemInit(void)
51235044
pg_atomic_init_u64(&XLogCtl->logWriteResult,InvalidXLogRecPtr);
51245045
pg_atomic_init_u64(&XLogCtl->logFlushResult,InvalidXLogRecPtr);
51255046
pg_atomic_init_u64(&XLogCtl->unloggedLSN,InvalidXLogRecPtr);
5126-
5127-
pg_atomic_init_u64(&XLogCtl->InitializeReserved,InvalidXLogRecPtr);
5128-
pg_atomic_init_u64(&XLogCtl->InitializedUpTo,InvalidXLogRecPtr);
5129-
ConditionVariableInit(&XLogCtl->InitializedUpToCondVar);
51305047
}
51315048

51325049
/*
@@ -6146,7 +6063,7 @@ StartupXLOG(void)
61466063
memset(page+len,0,XLOG_BLCKSZ-len);
61476064

61486065
pg_atomic_write_u64(&XLogCtl->xlblocks[firstIdx],endOfRecoveryInfo->lastPageBeginPtr+XLOG_BLCKSZ);
6149-
pg_atomic_write_u64(&XLogCtl->InitializedUpTo,endOfRecoveryInfo->lastPageBeginPtr+XLOG_BLCKSZ);
6066+
XLogCtl->InitializedUpTo=endOfRecoveryInfo->lastPageBeginPtr+XLOG_BLCKSZ;
61506067
}
61516068
else
61526069
{
@@ -6155,9 +6072,8 @@ StartupXLOG(void)
61556072
* let the first attempt to insert a log record to initialize the next
61566073
* buffer.
61576074
*/
6158-
pg_atomic_write_u64(&XLogCtl->InitializedUpTo,EndOfLog);
6075+
XLogCtl->InitializedUpTo=EndOfLog;
61596076
}
6160-
pg_atomic_write_u64(&XLogCtl->InitializeReserved,pg_atomic_read_u64(&XLogCtl->InitializedUpTo));
61616077

61626078
/*
61636079
* Update local and shared status. This is OK to do without any locks

‎src/backend/utils/activity/wait_event_names.txt

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -155,7 +155,6 @@ REPLICATION_SLOT_DROP"Waiting for a replication slot to become inactive so it c
155155
RESTORE_COMMAND"Waiting for <xref linkend="guc-restore-command"/> to complete."
156156
SAFE_SNAPSHOT"Waiting to obtain a valid snapshot for a <literal>READ ONLY DEFERRABLE</literal> transaction."
157157
SYNC_REP"Waiting for confirmation from a remote server during synchronous replication."
158-
WAL_BUFFER_INIT"Waiting on WAL buffer to be initialized."
159158
WAL_RECEIVER_EXIT"Waiting for the WAL receiver to exit."
160159
WAL_RECEIVER_WAIT_START"Waiting for startup process to send initial data for streaming replication."
161160
WAL_SUMMARY_READY"Waiting for a new WAL summary to be generated."
@@ -311,6 +310,7 @@ XidGen"Waiting to allocate a new transaction ID."
311310
ProcArray"Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID)."
312311
SInvalRead"Waiting to retrieve messages from the shared catalog invalidation queue."
313312
SInvalWrite"Waiting to add a message to the shared catalog invalidation queue."
313+
WALBufMapping"Waiting to replace a page in WAL buffers."
314314
WALWrite"Waiting for WAL buffers to be written to disk."
315315
ControlFile"Waiting to read or update the <filename>pg_control</filename> file or create a new WAL file."
316316
MultiXactGen"Waiting to read or update shared multixact state."

‎src/include/storage/lwlocklist.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -37,7 +37,7 @@ PG_LWLOCK(3, XidGen)
3737
PG_LWLOCK(4,ProcArray)
3838
PG_LWLOCK(5,SInvalRead)
3939
PG_LWLOCK(6,SInvalWrite)
40-
/* 7 wasWALBufMapping */
40+
PG_LWLOCK(7,WALBufMapping)
4141
PG_LWLOCK(8,WALWrite)
4242
PG_LWLOCK(9,ControlFile)
4343
/* 10 was CheckpointLock */

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