From ffafacc1f6e403b81180aa20d0a4184ce92beef8 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sun, 10 Aug 2003 19:48:08 +0000
Subject: Repair potential deadlock created by recent changes to recycle btree
 index pages: when _bt_getbuf asks the FSM for a free index page, it is
 possible (and, in some cases, even moderately likely) that the answer will be
 the same page that _bt_split is trying to split.  _bt_getbuf already knew
 that the returned page might not be free, but it wasn't prepared for the
 possibility that even trying to lock the page could be problematic.  Fix by
 doing a conditional rather than unconditional grab of the page lock.

---
 src/backend/storage/buffer/bufmgr.c | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

(limited to 'src/backend/storage/buffer/bufmgr.c')

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 2da3b4e364d..1be30d30e2b 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.139 2003/08/04 02:40:03 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/storage/buffer/bufmgr.c,v 1.140 2003/08/10 19:48:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1936,6 +1936,37 @@ LockBuffer(Buffer buffer, int mode)
 		elog(ERROR, "unrecognized buffer lock mode: %d", mode);
 }
 
+/*
+ * Acquire the cntx_lock for the buffer, but only if we don't have to wait.
+ *
+ * This assumes the caller wants BUFFER_LOCK_EXCLUSIVE mode.
+ */
+bool
+ConditionalLockBuffer(Buffer buffer)
+{
+	BufferDesc *buf;
+
+	Assert(BufferIsValid(buffer));
+	if (BufferIsLocal(buffer))
+		return true;			/* act as though we got it */
+
+	buf = &(BufferDescriptors[buffer - 1]);
+
+	if (LWLockConditionalAcquire(buf->cntx_lock, LW_EXCLUSIVE))
+	{
+		/*
+		 * This is not the best place to set cntxDirty flag (eg indices do
+		 * not always change buffer they lock in excl mode). But please
+		 * remember that it's critical to set cntxDirty *before* logging
+		 * changes with XLogInsert() - see comments in BufferSync().
+		 */
+		buf->cntxDirty = true;
+
+		return true;
+	}
+	return false;
+}
+
 /*
  * LockBufferForCleanup - lock a buffer in preparation for deleting items
  *
-- 
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