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); - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html