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Commitbab9599

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Fix race condition that lead to WALInsertLock deadlock with commit_delay.
If a call to WaitForXLogInsertionsToFinish() returned a value in the middleof a page, and another backend then started to insert a record to the samepage, and then you called WaitXLogInsertionsToFinish() again, the secondcall might return a smaller value than the first call. The problem was inGetXLogBuffer(), which always updated the insertingAt value to thebeginning of the requested page, not the actual requested location. Becauseof that, the second call might return a xlog pointer to the beginning ofthe page, while the first one returned a later position on the same page.XLogFlush() performs two calls to WaitXLogInsertionsToFinish() insuccession, and holds WALWriteLock on the second call, which can deadlockif the second call to WaitXLogInsertionsToFinish() blocks.Reported by Spiros Ioannou. Backpatch to 9.4, where the more scalableWALInsertLock mechanism, and this bug, was introduced.
1 parente39a3b2 commitbab9599

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  • src/backend/access/transam

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‎src/backend/access/transam/xlog.c

Lines changed: 24 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1839,11 +1839,32 @@ GetXLogBuffer(XLogRecPtr ptr)
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endptr=XLogCtl->xlblocks[idx];
18401840
if (expectedEndPtr!=endptr)
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{
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XLogRecPtrinitializedUpto;
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/*
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* Let others know that we're finished inserting the record up to the
1844-
* page boundary.
1845+
* Before calling AdvanceXLInsertBuffer(), which can block, let others
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* know how far we're finished with inserting the record.
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*
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* NB: If 'ptr' points to just after the page header, advertise a
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* position at the beginning of the page rather than 'ptr' itself. If
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* there are no other insertions running, someone might try to flush
1851+
* up to our advertised location. If we advertised a position after
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* the page header, someone might try to flush the page header, even
1853+
* though page might actually not be initialized yet. As the first
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* inserter on the page, we are effectively responsible for making
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* sure that it's initialized, before we let insertingAt to move past
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* the page header.
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*/
1846-
WALInsertLockUpdateInsertingAt(expectedEndPtr-XLOG_BLCKSZ);
1858+
if (ptr %XLOG_BLCKSZ==SizeOfXLogShortPHD&&
1859+
ptr %XLOG_SEG_SIZE>XLOG_BLCKSZ)
1860+
initializedUpto=ptr-SizeOfXLogShortPHD;
1861+
elseif (ptr %XLOG_BLCKSZ==SizeOfXLogLongPHD&&
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ptr %XLOG_SEG_SIZE<XLOG_BLCKSZ)
1863+
initializedUpto=ptr-SizeOfXLogLongPHD;
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else
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initializedUpto=ptr;
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WALInsertLockUpdateInsertingAt(initializedUpto);
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AdvanceXLInsertBuffer(ptr, false);
18491870
endptr=XLogCtl->xlblocks[idx];

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