[PATCH] Do not access buffers after dropping reference count

[Lachlan McIlroy <lachlan@xxxxxxx>]

We should not access a buffer after dropping it's reference count otherwise
we could race with another thread that releases the final reference count
and frees the buffer causing us to access potentially unmapped memory.

Lachlan

--- fs/xfs/linux-2.6/xfs_buf.c_1.257    2008-07-16 17:38:00.000000000 +1000
+++ fs/xfs/linux-2.6/xfs_buf.c  2008-07-22 12:05:50.000000000 +1000
@@ -839,6 +839,7 @@ xfs_buf_rele(
                return;
        }

+       ASSERT(atomic_read(&bp->b_hold) > 0);
        if (atomic_dec_and_lock(&bp->b_hold, &hash->bh_lock)) {
                if (bp->b_relse) {
                        atomic_inc(&bp->b_hold);
@@ -852,11 +853,6 @@ xfs_buf_rele(
                        spin_unlock(&hash->bh_lock);
                        xfs_buf_free(bp);
                }
-       } else {
-               /*
-                * Catch reference count leaks
-                */
-               ASSERT(atomic_read(&bp->b_hold) >= 0);
        }
}