Re: [PATCH] Fix race with shared tag queue maps

Linus Torvalds
2007-09-13T15:16:52Z

On Thu, 13 Sep 2007, Jens Axboe wrote:
> +
> + /*
> + * Ensure ordering with tag section
> + */
> + smp_mb__before_clear_bit();
> +
> + if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {

You don't need the "smp_mb__before_clear_bit()" there.

The regular "clear_bit()" needs it, but the "test_and_xxx()" versions are
architecturally defined to be memory barriers, exactly because they are
regularly used for locking.

This is even documented - see Documentation/atomic_ops.txt.

Linus