[patch] fix race in __block_prepare_write (again)

[Nick Piggin]

... I somehow didn't send it to Andrew last time.

-- 
SUSE Labs, Novell Inc.

Fix a race where __block_prepare_write can leak out an in-flight
read against a bh if get_block returns an error. This can lead to
the page becoming unlocked while the buffer is locked and the read
still in flight. __mpage_writepage BUGs on this condition.

BUG sighted on a 2-way Itanium2 system with 16K PAGE_SIZE running

	fsstress -v -d $DIR/tmp -n 1000 -p 1000 -l 2
	
where $DIR is a new ext2 filesystem with 4K blocks that is quite
small (causing get_block to fail often with -ENOSPC).

Signed-off-by: Nick Piggin <nickpiggin@xxxxxxxxxxxx>

Index: linux-2.6/fs/buffer.c
===================================================================
--- linux-2.6.orig/fs/buffer.c	2005-04-21 11:55:17.549614278 +1000
+++ linux-2.6/fs/buffer.c	2005-04-21 15:55:41.483826075 +1000
@@ -1988,6 +1988,7 @@
 			*wait_bh++=bh;
 		}
 	}
+out:
 	/*
 	 * If we issued read requests - let them complete.
 	 */
@@ -1996,8 +1997,9 @@
 		if (!buffer_uptodate(*wait_bh))
 			return -EIO;
 	}
-	return 0;
-out:
+	if (!err)
+		return err;
+
 	/*
 	 * Zero out any newly allocated blocks to avoid exposing stale
 	 * data.  If BH_New is set, we know that the block was newly