[PATCH]Add memory barrier before clear bit in unlock_buffer()

[Mingming Cao]

We are runnin SDET benchmark and saw double free issue for ext3 extended
attributes block, which complains the same xattr block already being
freed (in ext3_xattr_release_block()). The problem could also been
triggered by multiple threads loop untar/rm a kernel tree.

The race is caused by missing a memory barrier at unlock_buffer() before
the lock bit being cleared, resulting in possible concurrent
h_refcounter update. That causes a reference counter leak, then later
leads to the double free that we have seen.

Inside unlock_buffer(), there is a memory barrier is placed *after* the
lock bit is being cleared, however, there is no memory barrier *before*
the bit is cleared. On some arch the h_refcount update instruction and
the clear bit instruction could be reordered, thus leave the critical
section re-entered.

The race is like this: For example, if the h_refcount is initialized as
1,

cpu 0:                                   cpu1
--------------------------------------   -----------------------------------
lock_buffer() /* test_and_set_bit */
clear_buffer_locked(bh);             
                                        lock_buffer() /* test_and_set_bit */
h_refcount = h_refcount+1; /* = 2*/     h_refcount = h_refcount + 1; /*= 2 */
                                        clear_buffer_locked(bh);
....                                    ......

We lost a h_refcount here. We need a memory barrier before the buffer head lock
bit being cleared to force the order of the two writes.  Please apply.

Signed-Off-By: Mingming Cao <cmm@xxxxxxxxxx>


--- linux/fs/buffer.c.orig	2007-02-04 11:37:50.000000000 -0600
+++ linux/fs/buffer.c	2007-02-04 11:38:14.000000000 -0600
@@ -77,6 +77,7 @@
 
 void fastcall unlock_buffer(struct buffer_head *bh)
 {
+	smp_mb__before_clear_bit();
 	clear_buffer_locked(bh);
 	smp_mb__after_clear_bit();
 	wake_up_bit(&bh->b_state, BH_Lock);


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