Do all accesses to shared buffer headers through volatile-qualified

pointers, to ensure that compilers won't rearrange accesses to occur
while we're not holding the buffer header spinlock.  It's probably
not necessary to mark volatile in every single place in bufmgr.c,
but better safe than sorry.  Per trouble report from Kevin Grittner.

contrib/pg_buffercache/pg_buffercache_pages.c

@@ -3,7 +3,7 @@
  * pg_buffercache_pages.c
  *    display some contents of the buffer cache
  *
- *	  $PostgreSQL: pgsql/contrib/pg_buffercache/pg_buffercache_pages.c,v 1.4 2005/05/31 00:07:47 tgl Exp $
+ *	  $PostgreSQL: pgsql/contrib/pg_buffercache/pg_buffercache_pages.c,v 1.5 2005/10/12 16:45:13 tgl Exp $
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
@@ -72,10 +72,8 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
 
 	if (SRF_IS_FIRSTCALL())
 	{
-		RelFileNode	rnode;
 		uint32		i;
-		BufferDesc	*bufHdr;
-
+		volatile BufferDesc	*bufHdr;
 
 		funcctx = SRF_FIRSTCALL_INIT();
 
@@ -136,35 +134,24 @@ pg_buffercache_pages(PG_FUNCTION_ARGS)
 			/* Lock each buffer header before inspecting. */
 			LockBufHdr(bufHdr);
 
-			rnode = bufHdr->tag.rnode;
-
 			fctx->record[i].bufferid = BufferDescriptorGetBuffer(bufHdr);
-			fctx->record[i].relfilenode = rnode.relNode;
-			fctx->record[i].reltablespace = rnode.spcNode;
-			fctx->record[i].reldatabase = rnode.dbNode;
+			fctx->record[i].relfilenode = bufHdr->tag.rnode.relNode;
+			fctx->record[i].reltablespace = bufHdr->tag.rnode.spcNode;
+			fctx->record[i].reldatabase = bufHdr->tag.rnode.dbNode;
 			fctx->record[i].blocknum = bufHdr->tag.blockNum;
 
-			if ( bufHdr->flags & BM_DIRTY) 
-			{
+			if (bufHdr->flags & BM_DIRTY) 
 				fctx->record[i].isdirty = true;
-			}
 			else
-			{
 				fctx->record[i].isdirty = false;
-			}
 
 			/* Note if the buffer is valid, and has storage created */
-			if ( (bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_TAG_VALID))
-			{
+			if ((bufHdr->flags & BM_VALID) && (bufHdr->flags & BM_TAG_VALID))
 				fctx->record[i].isvalid = true;
-			}
 			else
-			{
 				fctx->record[i].isvalid = false;
-			}
 
 			UnlockBufHdr(bufHdr);
-
 		}
 
 		/* Release Buffer map. */

src/backend/storage/buffer/bufmgr.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.195 2005/08/12 23:13:54 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.196 2005/10/12 16:45:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -76,24 +76,24 @@ long		NDirectFileWrite;	/* e.g., I/O in psort and hashjoin. */
 
 
 /* local state for StartBufferIO and related functions */
-static BufferDesc *InProgressBuf = NULL;
+static volatile BufferDesc *InProgressBuf = NULL;
 static bool IsForInput;
 /* local state for LockBufferForCleanup */
-static BufferDesc *PinCountWaitBuf = NULL;
+static volatile BufferDesc *PinCountWaitBuf = NULL;
 
 
-static bool PinBuffer(BufferDesc *buf);
-static void PinBuffer_Locked(BufferDesc *buf);
-static void UnpinBuffer(BufferDesc *buf, bool fixOwner, bool trashOK);
+static bool PinBuffer(volatile BufferDesc *buf);
+static void PinBuffer_Locked(volatile BufferDesc *buf);
+static void UnpinBuffer(volatile BufferDesc *buf, bool fixOwner, bool trashOK);
 static bool SyncOneBuffer(int buf_id, bool skip_pinned);
-static void WaitIO(BufferDesc *buf);
-static bool StartBufferIO(BufferDesc *buf, bool forInput);
-static void TerminateBufferIO(BufferDesc *buf, bool clear_dirty,
+static void WaitIO(volatile BufferDesc *buf);
+static bool StartBufferIO(volatile BufferDesc *buf, bool forInput);
+static void TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
 							  int set_flag_bits);
 static void buffer_write_error_callback(void *arg);
-static BufferDesc *BufferAlloc(Relation reln, BlockNumber blockNum,
+static volatile BufferDesc *BufferAlloc(Relation reln, BlockNumber blockNum,
 			bool *foundPtr);
-static void FlushBuffer(BufferDesc *buf, SMgrRelation reln);
+static void FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln);
 static void AtProcExit_Buffers(int code, Datum arg);
 static void write_buffer(Buffer buffer, bool unpin);
 
@@ -116,7 +116,7 @@ static void write_buffer(Buffer buffer, bool unpin);
 Buffer
 ReadBuffer(Relation reln, BlockNumber blockNum)
 {
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 	Block		bufBlock;
 	bool		found;
 	bool		isExtend;
@@ -255,7 +255,7 @@ ReadBuffer(Relation reln, BlockNumber blockNum)
  *
  * No locks are held either at entry or exit.
  */
-static BufferDesc *
+static volatile BufferDesc *
 BufferAlloc(Relation reln,
 			BlockNumber blockNum,
 			bool *foundPtr)
@@ -264,7 +264,7 @@ BufferAlloc(Relation reln,
 	BufferTag	oldTag;
 	BufFlags	oldFlags;
 	int			buf_id;
-	BufferDesc *buf;
+	volatile BufferDesc *buf;
 	bool		valid;
 
 	/* create a tag so we can lookup the buffer */
@@ -512,7 +512,7 @@ BufferAlloc(Relation reln,
  * to acquire the necessary locks; if so, don't mess it up.
  */
 static void
-InvalidateBuffer(BufferDesc *buf)
+InvalidateBuffer(volatile BufferDesc *buf)
 {
 	BufferTag	oldTag;
 	BufFlags	oldFlags;
@@ -602,7 +602,7 @@ retry:
 static void
 write_buffer(Buffer buffer, bool unpin)
 {
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer id: %d", buffer);
@@ -679,7 +679,7 @@ ReleaseAndReadBuffer(Buffer buffer,
 					 Relation relation,
 					 BlockNumber blockNum)
 {
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	if (BufferIsValid(buffer))
 	{
@@ -722,7 +722,7 @@ ReleaseAndReadBuffer(Buffer buffer,
  * some callers to avoid an extra spinlock cycle.
  */
 static bool
-PinBuffer(BufferDesc *buf)
+PinBuffer(volatile BufferDesc *buf)
 {
 	int			b = buf->buf_id;
 	bool		result;
@@ -760,7 +760,7 @@ PinBuffer(BufferDesc *buf)
  * its state can change under us.
  */
 static void
-PinBuffer_Locked(BufferDesc *buf)
+PinBuffer_Locked(volatile BufferDesc *buf)
 {
 	int			b = buf->buf_id;
 
@@ -787,7 +787,7 @@ PinBuffer_Locked(BufferDesc *buf)
  * used recently, and trashOK is true, send the buffer to the freelist.
  */
 static void
-UnpinBuffer(BufferDesc *buf, bool fixOwner, bool trashOK)
+UnpinBuffer(volatile BufferDesc *buf, bool fixOwner, bool trashOK)
 {
 	int			b = buf->buf_id;
 
@@ -967,7 +967,7 @@ BgBufferSync(void)
 static bool
 SyncOneBuffer(int buf_id, bool skip_pinned)
 {
-	BufferDesc *bufHdr = &BufferDescriptors[buf_id];
+	volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
 
 	/*
 	 * Check whether buffer needs writing.
@@ -1114,7 +1114,7 @@ AtProcExit_Buffers(int code, Datum arg)
 	{
 		if (PrivateRefCount[i] != 0)
 		{
-			BufferDesc *buf = &(BufferDescriptors[i]);
+			volatile BufferDesc *buf = &(BufferDescriptors[i]);
 
 			/*
 			 * We don't worry about updating ResourceOwner; if we even got
@@ -1136,7 +1136,7 @@ AtProcExit_Buffers(int code, Datum arg)
 void
 PrintBufferLeakWarning(Buffer buffer)
 {
-	BufferDesc *buf;
+	volatile BufferDesc *buf;
 	int32		loccount;
 
 	Assert(BufferIsValid(buffer));
@@ -1199,7 +1199,7 @@ BufmgrCommit(void)
 BlockNumber
 BufferGetBlockNumber(Buffer buffer)
 {
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	Assert(BufferIsPinned(buffer));
 
@@ -1222,7 +1222,7 @@ BufferGetBlockNumber(Buffer buffer)
 RelFileNode
 BufferGetFileNode(Buffer buffer)
 {
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	if (BufferIsLocal(buffer))
 		bufHdr = &(LocalBufferDescriptors[-buffer - 1]);
@@ -1252,7 +1252,7 @@ BufferGetFileNode(Buffer buffer)
  * as the second parameter.  If not, pass NULL.
  */
 static void
-FlushBuffer(BufferDesc *buf, SMgrRelation reln)
+FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln)
 {
 	XLogRecPtr	recptr;
 	ErrorContextCallback errcontext;
@@ -1267,7 +1267,7 @@ FlushBuffer(BufferDesc *buf, SMgrRelation reln)
 
 	/* Setup error traceback support for ereport() */
 	errcontext.callback = buffer_write_error_callback;
-	errcontext.arg = buf;
+	errcontext.arg = (void *) buf;
 	errcontext.previous = error_context_stack;
 	error_context_stack = &errcontext;
 
@@ -1375,7 +1375,7 @@ DropRelFileNodeBuffers(RelFileNode rnode, bool istemp,
 					   BlockNumber firstDelBlock)
 {
 	int			i;
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	if (istemp)
 	{
@@ -1427,7 +1427,7 @@ void
 DropBuffers(Oid dbid)
 {
 	int			i;
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	/*
 	 * We needn't consider local buffers, since by assumption the target
@@ -1457,7 +1457,7 @@ void
 PrintBufferDescs(void)
 {
 	int			i;
-	BufferDesc *buf = BufferDescriptors;
+	volatile BufferDesc *buf = BufferDescriptors;
 
 	for (i = 0; i < NBuffers; ++i, ++buf)
 	{
@@ -1479,7 +1479,7 @@ void
 PrintPinnedBufs(void)
 {
 	int			i;
-	BufferDesc *buf = BufferDescriptors;
+	volatile BufferDesc *buf = BufferDescriptors;
 
 	for (i = 0; i < NBuffers; ++i, ++buf)
 	{
@@ -1522,7 +1522,7 @@ void
 FlushRelationBuffers(Relation rel)
 {
 	int			i;
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	/* Open rel at the smgr level if not already done */
 	RelationOpenSmgr(rel);
@@ -1539,7 +1539,7 @@ FlushRelationBuffers(Relation rel)
 
 				/* Setup error traceback support for ereport() */
 				errcontext.callback = buffer_write_error_callback;
-				errcontext.arg = bufHdr;
+				errcontext.arg = (void *) bufHdr;
 				errcontext.previous = error_context_stack;
 				error_context_stack = &errcontext;
 
@@ -1586,7 +1586,7 @@ FlushRelationBuffers(Relation rel)
 void
 ReleaseBuffer(Buffer buffer)
 {
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer id: %d", buffer);
@@ -1657,7 +1657,7 @@ IncrBufferRefCount(Buffer buffer)
 void
 SetBufferCommitInfoNeedsSave(Buffer buffer)
 {
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	if (!BufferIsValid(buffer))
 		elog(ERROR, "bad buffer id: %d", buffer);
@@ -1694,7 +1694,7 @@ SetBufferCommitInfoNeedsSave(Buffer buffer)
 void
 UnlockBuffers(void)
 {
-	BufferDesc *buf = PinCountWaitBuf;
+	volatile BufferDesc *buf = PinCountWaitBuf;
 
 	if (buf)
 	{
@@ -1727,7 +1727,7 @@ UnlockBuffers(void)
 void
 LockBuffer(Buffer buffer, int mode)
 {
-	BufferDesc *buf;
+	volatile BufferDesc *buf;
 
 	Assert(BufferIsValid(buffer));
 	if (BufferIsLocal(buffer))
@@ -1765,7 +1765,7 @@ LockBuffer(Buffer buffer, int mode)
 bool
 ConditionalLockBuffer(Buffer buffer)
 {
-	BufferDesc *buf;
+	volatile BufferDesc *buf;
 
 	Assert(BufferIsValid(buffer));
 	if (BufferIsLocal(buffer))
@@ -1809,7 +1809,7 @@ ConditionalLockBuffer(Buffer buffer)
 void
 LockBufferForCleanup(Buffer buffer)
 {
-	BufferDesc *bufHdr;
+	volatile BufferDesc *bufHdr;
 
 	Assert(BufferIsValid(buffer));
 	Assert(PinCountWaitBuf == NULL);
@@ -1875,7 +1875,7 @@ LockBufferForCleanup(Buffer buffer)
  * WaitIO -- Block until the IO_IN_PROGRESS flag on 'buf' is cleared.
  */
 static void
-WaitIO(BufferDesc *buf)
+WaitIO(volatile BufferDesc *buf)
 {
 	/*
 	 * Changed to wait until there's no IO - Inoue 01/13/2000
@@ -1922,7 +1922,7 @@ WaitIO(BufferDesc *buf)
  * FALSE if someone else already did the work.
  */
 static bool
-StartBufferIO(BufferDesc *buf, bool forInput)
+StartBufferIO(volatile BufferDesc *buf, bool forInput)
 {
 	Assert(!InProgressBuf);
 
@@ -1989,7 +1989,8 @@ StartBufferIO(BufferDesc *buf, bool forInput)
  * be 0, or BM_VALID if we just finished reading in the page.
  */
 static void
-TerminateBufferIO(BufferDesc *buf, bool clear_dirty, int set_flag_bits)
+TerminateBufferIO(volatile BufferDesc *buf, bool clear_dirty,
+				  int set_flag_bits)
 {
 	Assert(buf == InProgressBuf);
 
@@ -2021,7 +2022,7 @@ TerminateBufferIO(BufferDesc *buf, bool clear_dirty, int set_flag_bits)
 void
 AbortBufferIO(void)
 {
-	BufferDesc *buf = InProgressBuf;
+	volatile BufferDesc *buf = InProgressBuf;
 
 	if (buf)
 	{
@@ -2075,7 +2076,7 @@ AbortBufferIO(void)
 static void
 buffer_write_error_callback(void *arg)
 {
-	BufferDesc *bufHdr = (BufferDesc *) arg;
+	volatile BufferDesc *bufHdr = (volatile BufferDesc *) arg;
 
 	/* Buffer is pinned, so we can read the tag without locking the spinlock */
 	if (bufHdr != NULL)

src/backend/storage/buffer/freelist.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/buffer/freelist.c,v 1.52 2005/08/20 23:26:17 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/buffer/freelist.c,v 1.53 2005/10/12 16:45:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -55,10 +55,10 @@ bool		strategy_hint_vacuum = false;
  *	means that we return with the BufFreelistLock still held, as well;
  *	the caller must release that lock once the spinlock is dropped.
  */
-BufferDesc *
+volatile BufferDesc *
 StrategyGetBuffer(void)
 {
-	BufferDesc *buf;
+	volatile BufferDesc *buf;
 	int			trycounter;
 
 	LWLockAcquire(BufFreelistLock, LW_EXCLUSIVE);
@@ -138,7 +138,7 @@ StrategyGetBuffer(void)
  * quickly the buffer is reused.
  */
 void
-StrategyFreeBuffer(BufferDesc *buf, bool at_head)
+StrategyFreeBuffer(volatile BufferDesc *buf, bool at_head)
 {
 	LWLockAcquire(BufFreelistLock, LW_EXCLUSIVE);
 

src/include/storage/buf_internals.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/buf_internals.h,v 1.79 2005/08/20 23:26:33 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/storage/buf_internals.h,v 1.80 2005/10/12 16:45:14 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -145,6 +145,11 @@ typedef struct sbufdesc
  * NoHoldoff cases may be used when we know that we hold some LWLock
  * and therefore interrupts are already held off.  Do not apply these
  * to local buffers!
+ *
+ * Note: as a general coding rule, if you are using these then you probably
+ * want to be using a volatile-qualified pointer to the buffer header, to
+ * ensure that the compiler doesn't rearrange accesses to the header to
+ * occur before or after the spinlock is acquired/released.
  */
 #define LockBufHdr(bufHdr)  \
 	SpinLockAcquire(&(bufHdr)->buf_hdr_lock)
@@ -179,8 +184,8 @@ extern long int LocalBufferFlushCount;
  */
 
 /* freelist.c */
-extern BufferDesc *StrategyGetBuffer(void);
-extern void StrategyFreeBuffer(BufferDesc *buf, bool at_head);
+extern volatile BufferDesc *StrategyGetBuffer(void);
+extern void StrategyFreeBuffer(volatile BufferDesc *buf, bool at_head);
 extern int	StrategySyncStart(void);
 extern Size StrategyShmemSize(void);
 extern void StrategyInitialize(bool init);