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

[Linus Torvalds]

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
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