[PATCH] Add a read memory barrier to wait_on_buffer

[Mikulas Patocka]

Let's have a look at this piece of code in __bread_slow:
	get_bh(bh);
	bh->b_end_io = end_buffer_read_sync;
	submit_bh(REQ_OP_READ, 0, bh);
	wait_on_buffer(bh);
	if (buffer_uptodate(bh))
		return bh;
Neither wait_on_buffer nor buffer_uptodate contain a memory barrier.
Consequently, if someone calls sb_bread and then reads the buffer data,
the read of buffer data may be speculatively executed before
wait_on_buffer(bh) and it may return invalid data.

Also, there is this pattern present several times:
	wait_on_buffer(bh);
	if (!buffer_uptodate(bh))
		err = -EIO;
It may be possible that buffer_uptodate is executed before wait_on_buffer
and it may return spurious error.

Fix these bugs by adding a read memory barrier to wait_on_buffer().

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

Index: linux-2.6/include/linux/buffer_head.h
===================================================================
--- linux-2.6.orig/include/linux/buffer_head.h
+++ linux-2.6/include/linux/buffer_head.h
@@ -353,6 +353,11 @@ static inline void wait_on_buffer(struct
 	might_sleep();
 	if (buffer_locked(bh))
 		__wait_on_buffer(bh);
+	/*
+	 * Make sure that the following accesses to buffer state or buffer data
+	 * are not reordered with buffer_locked(bh).
+	 */
+	smp_rmb();
 }
 
 static inline int trylock_buffer(struct buffer_head *bh)