On Wed 07-09-22 08:13:40, Keith Busch wrote: > On Wed, Sep 07, 2022 at 12:23:18PM +0200, Jan Kara wrote: > > On Tue 06-09-22 15:27:51, Keith Busch wrote: > > > On Wed, Aug 03, 2022 at 08:15:04PM +0800, Yu Kuai wrote: > > > > wait_cnt = atomic_dec_return(&ws->wait_cnt); > > > > - if (wait_cnt <= 0) { > > > > - int ret; > > > > + /* > > > > + * For concurrent callers of this, callers should call this function > > > > + * again to wakeup a new batch on a different 'ws'. > > > > + */ > > > > + if (wait_cnt < 0 || !waitqueue_active(&ws->wait)) > > > > + return true; > > > > > > If wait_cnt is '0', but the waitqueue_active happens to be false due to racing > > > with add_wait_queue(), this returns true so the caller will retry. > > > > Well, note that sbq_wake_ptr() called to obtain 'ws' did waitqueue_active() > > check. So !waitqueue_active() should really happen only if waiter was woken > > up by someone else or so. Not that it would matter much but I wanted to > > point it out. > > > > > The next atomic_dec will set the current waitstate wait_cnt < 0, which > > > also forces an early return true. When does the wake up happen, or > > > wait_cnt and wait_index get updated in that case? > > > > I guess your concern could be rephrased as: Who's going to ever set > > ws->wait_cnt to value > 0 if we ever exit with wait_cnt == 0 due to > > !waitqueue_active() condition? > > > > And that is a good question and I think that's a bug in this patch. I think > > we need something like: > > > > ... > > /* > > * For concurrent callers of this, callers should call this function > > * again to wakeup a new batch on a different 'ws'. > > */ > > if (wait_cnt < 0) > > return true; > > /* > > * If we decremented queue without waiters, retry to avoid lost > > * wakeups. > > */ > > if (wait_cnt > 0) > > return !waitqueue_active(&ws->wait); > > I'm not sure about this part. We've already decremented, so the freed bit is > accounted for against the batch. Returning true here may double-count the freed > bit, right? Yes, we may wake up waiters unnecessarily frequently. But that's a performance issue at worst and only if it happens frequently. So I don't think it matters in practice (famous last words ;). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR