Revert "Get rid of WALBufMappingLock"

akorotkov · akorotkov · commit c13070a27b63 · 2025-08-22T19:26:38.000+03:00
This reverts commit bc22dc0. It appears that conditional variables are not suitable for use inside critical sections. If WaitLatch()/WaitEventSetWaitBlock() face postmaster death, they exit, releasing all locks instead of PANIC. In certain situations, this leads to data corruption. Reported-by: Andrey Borodin <x4mmm@yandex-team.ru> Discussion: https://postgr.es/m/B3C69B86-7F82-4111-B97F-0005497BB745%40yandex-team.ru Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru> Reviewed-by: Aleksander Alekseev <aleksander@tigerdata.com> Reviewed-by: Kirill Reshke <reshkekirill@gmail.com> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Reviewed-by: Thomas Munro <thomas.munro@gmail.com> Reviewed-by: Tomas Vondra <tomas@vondra.me> Reviewed-by: Andres Freund <andres@anarazel.de> Reviewed-by: Yura Sokolov <y.sokolov@postgrespro.ru> Reviewed-by: Michael Paquier <michael@paquier.xyz> Backpatch-through: 18
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
@@ -303,6 +303,11 @@ static bool doPageWrites;
  * so it's a plain spinlock.  The other locks are held longer (potentially
  * over I/O operations), so we use LWLocks for them.  These locks are:
  *
+ * WALBufMappingLock: must be held to replace a page in the WAL buffer cache.
+ * It is only held while initializing and changing the mapping.  If the
+ * contents of the buffer being replaced haven't been written yet, the mapping
+ * lock is released while the write is done, and reacquired afterwards.
+ *
  * WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
  * XLogFlush).
  *
@@ -468,37 +473,21 @@ typedef struct XLogCtlData
 	pg_atomic_uint64 logFlushResult;	/* last byte + 1 flushed */
 
 	/*
-	 * First initialized page in the cache (first byte position).
-	 */
-	XLogRecPtr	InitializedFrom;
-
-	/*
-	 * Latest reserved for initialization page in the cache (last byte
-	 * position + 1).
+	 * Latest initialized page in the cache (last byte position + 1).
 	 *
-	 * To change the identity of a buffer, you need to advance
-	 * InitializeReserved first.  To change the identity of a buffer that's
+	 * To change the identity of a buffer (and InitializedUpTo), you need to
+	 * hold WALBufMappingLock.  To change the identity of a buffer that's
 	 * still dirty, the old page needs to be written out first, and for that
 	 * you need WALWriteLock, and you need to ensure that there are no
 	 * in-progress insertions to the page by calling
 	 * WaitXLogInsertionsToFinish().
 	 */
-	pg_atomic_uint64 InitializeReserved;
-
-	/*
-	 * Latest initialized page in the cache (last byte position + 1).
-	 *
-	 * InitializedUpTo is updated after the buffer initialization.  After
-	 * update, waiters got notification using InitializedUpToCondVar.
-	 */
-	pg_atomic_uint64 InitializedUpTo;
-	ConditionVariable InitializedUpToCondVar;
+	XLogRecPtr	InitializedUpTo;
 
 	/*
 	 * These values do not change after startup, although the pointed-to pages
-	 * and xlblocks values certainly do.  xlblocks values are changed
-	 * lock-free according to the check for the xlog write position and are
-	 * accompanied by changes of InitializeReserved and InitializedUpTo.
+	 * and xlblocks values certainly do.  xlblocks values are protected by
+	 * WALBufMappingLock.
 	 */
 	char	   *pages;			/* buffers for unwritten XLOG pages */
 	pg_atomic_uint64 *xlblocks; /* 1st byte ptr-s + XLOG_BLCKSZ */
@@ -821,9 +810,9 @@ XLogInsertRecord(XLogRecData *rdata,
 	 * fullPageWrites from changing until the insertion is finished.
 	 *
 	 * Step 2 can usually be done completely in parallel. If the required WAL
-	 * page is not initialized yet, you have to go through AdvanceXLInsertBuffer,
-	 * which will ensure it is initialized. But the WAL writer tries to do that
-	 * ahead of insertions to avoid that from happening in the critical path.
+	 * page is not initialized yet, you have to grab WALBufMappingLock to
+	 * initialize it, but the WAL writer tries to do that ahead of insertions
+	 * to avoid that from happening in the critical path.
 	 *
 	 *----------
 	 */
@@ -2005,79 +1994,32 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 	XLogRecPtr	NewPageEndPtr = InvalidXLogRecPtr;
 	XLogRecPtr	NewPageBeginPtr;
 	XLogPageHeader NewPage;
-	XLogRecPtr	ReservedPtr;
 	int			npages pg_attribute_unused() = 0;
 
-	/*
-	 * We must run the loop below inside the critical section as we expect
-	 * XLogCtl->InitializedUpTo to eventually keep up.  The most of callers
-	 * already run inside the critical section. Except for WAL writer, which
-	 * passed 'opportunistic == true', and therefore we don't perform
-	 * operations that could error out.
-	 *
-	 * Start an explicit critical section anyway though.
-	 */
-	Assert(CritSectionCount > 0 || opportunistic);
-	START_CRIT_SECTION();
+	LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
 
-	/*--
-	 * Loop till we get all the pages in WAL buffer before 'upto' reserved for
-	 * initialization.  Multiple process can initialize different buffers with
-	 * this loop in parallel as following.
-	 *
-	 * 1. Reserve page for initialization using XLogCtl->InitializeReserved.
-	 * 2. Initialize the reserved page.
-	 * 3. Attempt to advance XLogCtl->InitializedUpTo,
+	/*
+	 * Now that we have the lock, check if someone initialized the page
+	 * already.
 	 */
-	ReservedPtr = pg_atomic_read_u64(&XLogCtl->InitializeReserved);
-	while (upto >= ReservedPtr || opportunistic)
+	while (upto >= XLogCtl->InitializedUpTo || opportunistic)
 	{
-		Assert(ReservedPtr % XLOG_BLCKSZ == 0);
+		nextidx = XLogRecPtrToBufIdx(XLogCtl->InitializedUpTo);
 
 		/*
-		 * Get ending-offset of the buffer page we need to replace.
-		 *
-		 * We don't lookup into xlblocks, but rather calculate position we
-		 * must wait to be written. If it was written, xlblocks will have this
-		 * position (or uninitialized)
+		 * Get ending-offset of the buffer page we need to replace (this may
+		 * be zero if the buffer hasn't been used yet).  Fall through if it's
+		 * already written out.
 		 */
-		if (ReservedPtr + XLOG_BLCKSZ > XLogCtl->InitializedFrom + XLOG_BLCKSZ * XLOGbuffers)
-			OldPageRqstPtr = ReservedPtr + XLOG_BLCKSZ - (XLogRecPtr) XLOG_BLCKSZ * XLOGbuffers;
-		else
-			OldPageRqstPtr = InvalidXLogRecPtr;
-
-		if (LogwrtResult.Write < OldPageRqstPtr && opportunistic)
+		OldPageRqstPtr = pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]);
+		if (LogwrtResult.Write < OldPageRqstPtr)
 		{
 			/*
-			 * If we just want to pre-initialize as much as we can without
-			 * flushing, give up now.
+			 * Nope, got work to do. If we just want to pre-initialize as much
+			 * as we can without flushing, give up now.
 			 */
-			upto = ReservedPtr - 1;
-			break;
-		}
-
-		/*
-		 * Attempt to reserve the page for initialization.  Failure means that
-		 * this page got reserved by another process.
-		 */
-		if (!pg_atomic_compare_exchange_u64(&XLogCtl->InitializeReserved,
-											&ReservedPtr,
-											ReservedPtr + XLOG_BLCKSZ))
-			continue;
-
-		/*
-		 * Wait till page gets correctly initialized up to OldPageRqstPtr.
-		 */
-		nextidx = XLogRecPtrToBufIdx(ReservedPtr);
-		while (pg_atomic_read_u64(&XLogCtl->InitializedUpTo) < OldPageRqstPtr)
-			ConditionVariableSleep(&XLogCtl->InitializedUpToCondVar, WAIT_EVENT_WAL_BUFFER_INIT);
-		ConditionVariableCancelSleep();
-		Assert(pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]) == OldPageRqstPtr);
-
-		/* Fall through if it's already written out. */
-		if (LogwrtResult.Write < OldPageRqstPtr)
-		{
-			/* Nope, got work to do. */
+			if (opportunistic)
+				break;
 
 			/* Advance shared memory write request position */
 			SpinLockAcquire(&XLogCtl->info_lck);
@@ -2092,6 +2034,14 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 			RefreshXLogWriteResult(LogwrtResult);
 			if (LogwrtResult.Write < OldPageRqstPtr)
 			{
+				/*
+				 * Must acquire write lock. Release WALBufMappingLock first,
+				 * to make sure that all insertions that we need to wait for
+				 * can finish (up to this same position). Otherwise we risk
+				 * deadlock.
+				 */
+				LWLockRelease(WALBufMappingLock);
+
 				WaitXLogInsertionsToFinish(OldPageRqstPtr);
 
 				LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
@@ -2119,16 +2069,21 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 					 */
 					pgstat_report_fixed = true;
 				}
+				/* Re-acquire WALBufMappingLock and retry */
+				LWLockAcquire(WALBufMappingLock, LW_EXCLUSIVE);
+				continue;
 			}
 		}
 
 		/*
 		 * Now the next buffer slot is free and we can set it up to be the
 		 * next output page.
 		 */
-		NewPageBeginPtr = ReservedPtr;
+		NewPageBeginPtr = XLogCtl->InitializedUpTo;
 		NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;
 
+		Assert(XLogRecPtrToBufIdx(NewPageBeginPtr) == nextidx);
+
 		NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);
 
 		/*
@@ -2192,100 +2147,12 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 		 */
 		pg_write_barrier();
 
-		/*-----
-		 * Update the value of XLogCtl->xlblocks[nextidx] and try to advance
-		 * XLogCtl->InitializedUpTo in a lock-less manner.
-		 *
-		 * First, let's provide a formal proof of the algorithm.  Let it be 'n'
-		 * process with the following variables in shared memory:
-		 *	f - an array of 'n' boolean flags,
-		 *	v - atomic integer variable.
-		 *
-		 * Also, let
-		 *	i - a number of a process,
-		 *	j - local integer variable,
-		 * CAS(var, oldval, newval) - compare-and-swap atomic operation
-		 *							  returning true on success,
-		 * write_barrier()/read_barrier() - memory barriers.
-		 *
-		 * The pseudocode for each process is the following.
-		 *
-		 *	j := i
-		 *	f[i] := true
-		 *	write_barrier()
-		 *	while CAS(v, j, j + 1):
-		 *		j := j + 1
-		 *		read_barrier()
-		 *		if not f[j]:
-		 *			break
-		 *
-		 * Let's prove that v eventually reaches the value of n.
-		 * 1. Prove by contradiction.  Assume v doesn't reach n and stucks
-		 *	  on k, where k < n.
-		 * 2. Process k attempts CAS(v, k, k + 1).  1). If, as we assumed, v
-		 *	  gets stuck at k, then this CAS operation must fail.  Therefore,
-		 *    v < k when process k attempts CAS(v, k, k + 1).
-		 * 3. If, as we assumed, v gets stuck at k, then the value k of v
-		 *	  must be achieved by some process m, where m < k.  The process
-		 *	  m must observe f[k] == false.  Otherwise, it will later attempt
-		 *	  CAS(v, k, k + 1) with success.
-		 * 4. Therefore, corresponding read_barrier() (while j == k) on
-		 *	  process m reached before write_barrier() of process k.  But then
-		 *	  process k attempts CAS(v, k, k + 1) after process m successfully
-		 *	  incremented v to k, and that CAS operation must succeed.
-		 *	  That leads to a contradiction.  So, there is no such k (k < n)
-		 *    where v gets stuck.  Q.E.D.
-		 *
-		 * To apply this proof to the code below, we assume
-		 * XLogCtl->InitializedUpTo will play the role of v with XLOG_BLCKSZ
-		 * granularity.  We also assume setting XLogCtl->xlblocks[nextidx] to
-		 * NewPageEndPtr to play the role of setting f[i] to true.  Also, note
-		 * that processes can't concurrently map different xlog locations to
-		 * the same nextidx because we previously requested that
-		 * XLogCtl->InitializedUpTo >= OldPageRqstPtr.  So, a xlog buffer can
-		 * be taken for initialization only once the previous initialization
-		 * takes effect on XLogCtl->InitializedUpTo.
-		 */
-
 		pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx], NewPageEndPtr);
-
-		pg_write_barrier();
-
-		while (pg_atomic_compare_exchange_u64(&XLogCtl->InitializedUpTo, &NewPageBeginPtr, NewPageEndPtr))
-		{
-			NewPageBeginPtr = NewPageEndPtr;
-			NewPageEndPtr = NewPageBeginPtr + XLOG_BLCKSZ;
-			nextidx = XLogRecPtrToBufIdx(NewPageBeginPtr);
-
-			pg_read_barrier();
-
-			if (pg_atomic_read_u64(&XLogCtl->xlblocks[nextidx]) != NewPageEndPtr)
-			{
-				/*
-				 * Page at nextidx wasn't initialized yet, so we can't move
-				 * InitializedUpto further. It will be moved by backend which
-				 * will initialize nextidx.
-				 */
-				ConditionVariableBroadcast(&XLogCtl->InitializedUpToCondVar);
-				break;
-			}
-		}
+		XLogCtl->InitializedUpTo = NewPageEndPtr;
 
 		npages++;
 	}
-
-	END_CRIT_SECTION();
-
-	/*
-	 * All the pages in WAL buffer before 'upto' were reserved for
-	 * initialization.  However, some pages might be reserved by concurrent
-	 * processes.  Wait till they finish initialization.
-	 */
-	while (upto >= pg_atomic_read_u64(&XLogCtl->InitializedUpTo))
-		ConditionVariableSleep(&XLogCtl->InitializedUpToCondVar, WAIT_EVENT_WAL_BUFFER_INIT);
-	ConditionVariableCancelSleep();
-
-	pg_read_barrier();
+	LWLockRelease(WALBufMappingLock);
 
 #ifdef WAL_DEBUG
 	if (XLOG_DEBUG && npages > 0)
@@ -5178,10 +5045,6 @@ XLOGShmemInit(void)
 	pg_atomic_init_u64(&XLogCtl->logWriteResult, InvalidXLogRecPtr);
 	pg_atomic_init_u64(&XLogCtl->logFlushResult, InvalidXLogRecPtr);
 	pg_atomic_init_u64(&XLogCtl->unloggedLSN, InvalidXLogRecPtr);
-
-	pg_atomic_init_u64(&XLogCtl->InitializeReserved, InvalidXLogRecPtr);
-	pg_atomic_init_u64(&XLogCtl->InitializedUpTo, InvalidXLogRecPtr);
-	ConditionVariableInit(&XLogCtl->InitializedUpToCondVar);
 }
 
 /*
@@ -6205,8 +6068,7 @@ StartupXLOG(void)
 		memset(page + len, 0, XLOG_BLCKSZ - len);
 
 		pg_atomic_write_u64(&XLogCtl->xlblocks[firstIdx], endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ);
-		pg_atomic_write_u64(&XLogCtl->InitializedUpTo, endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ);
-		XLogCtl->InitializedFrom = endOfRecoveryInfo->lastPageBeginPtr;
+		XLogCtl->InitializedUpTo = endOfRecoveryInfo->lastPageBeginPtr + XLOG_BLCKSZ;
 	}
 	else
 	{
@@ -6215,10 +6077,8 @@ StartupXLOG(void)
 		 * let the first attempt to insert a log record to initialize the next
 		 * buffer.
 		 */
-		pg_atomic_write_u64(&XLogCtl->InitializedUpTo, EndOfLog);
-		XLogCtl->InitializedFrom = EndOfLog;
+		XLogCtl->InitializedUpTo = EndOfLog;
 	}
-	pg_atomic_write_u64(&XLogCtl->InitializeReserved, pg_atomic_read_u64(&XLogCtl->InitializedUpTo));
 
 	/*
 	 * Update local and shared status.  This is OK to do without any locks
diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt
@@ -156,7 +156,6 @@ REPLICATION_SLOT_DROP	"Waiting for a replication slot to become inactive so it c
 RESTORE_COMMAND	"Waiting for <xref linkend="guc-restore-command"/> to complete."
 SAFE_SNAPSHOT	"Waiting to obtain a valid snapshot for a <literal>READ ONLY DEFERRABLE</literal> transaction."
 SYNC_REP	"Waiting for confirmation from a remote server during synchronous replication."
-WAL_BUFFER_INIT	"Waiting on WAL buffer to be initialized."
 WAL_RECEIVER_EXIT	"Waiting for the WAL receiver to exit."
 WAL_RECEIVER_WAIT_START	"Waiting for startup process to send initial data for streaming replication."
 WAL_SUMMARY_READY	"Waiting for a new WAL summary to be generated."
@@ -316,6 +315,7 @@ XidGen	"Waiting to allocate a new transaction ID."
 ProcArray	"Waiting to access the shared per-process data structures (typically, to get a snapshot or report a session's transaction ID)."
 SInvalRead	"Waiting to retrieve messages from the shared catalog invalidation queue."
 SInvalWrite	"Waiting to add a message to the shared catalog invalidation queue."
+WALBufMapping	"Waiting to replace a page in WAL buffers."
 WALWrite	"Waiting for WAL buffers to be written to disk."
 ControlFile	"Waiting to read or update the <filename>pg_control</filename> file or create a new WAL file."
 MultiXactGen	"Waiting to read or update shared multixact state."
diff --git a/src/include/storage/lwlocklist.h b/src/include/storage/lwlocklist.h
@@ -38,7 +38,7 @@ PG_LWLOCK(3, XidGen)
 PG_LWLOCK(4, ProcArray)
 PG_LWLOCK(5, SInvalRead)
 PG_LWLOCK(6, SInvalWrite)
-/* 7 was WALBufMapping */
+PG_LWLOCK(7, WALBufMapping)
 PG_LWLOCK(8, WALWrite)
 PG_LWLOCK(9, ControlFile)
 /* 10 was CheckpointLock */
