On Wed, Oct 07, 2009 at 05:07:22PM +0800, Wu Fengguang wrote:
> On Wed, Oct 07, 2009 at 04:53:20PM +0800, Peter Zijlstra wrote:
> > wakeup implies a full memory barrier.
>
> If so, this smp_mb__after_clear_bit() line is also not necessary?
>
> void clear_bdi_congested(struct backing_dev_info *bdi, int sync)
> {
> //...
> clear_bit(bit, &bdi->state);
> smp_mb__after_clear_bit();
> if (waitqueue_active(wqh))
> wake_up(wqh);
Typically in these patterns you do need a barrier. You need to
ensure the load to check the waitqueue is performed after the
clear_bit.
The other side does:
add_to_waitqueue
set_current_state /* has mb() in it */
if (test_bit())
schedule()
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