On Sat, 24 Sep 2022, Hillf Danton wrote: > > I think the lockup can be avoided by > a) either advancing wake_index as early as I can [1], > b) or doing wakeup in case of zero wait_cnt to kill all cases of waitqueue_active(). > > Only for thoughts now. Thanks Hillf: I gave your __sbq_wake_up() patch below several tries, and as far as I could tell, it works just as well as my one-liner. But I don't think it's what we would want to do: doesn't it increment wake_index on every call to __sbq_wake_up()? whereas I thought it was intended to be incremented only after wake_batch calls (thinking in terms of nr 1). I'll not be surprised if your advance-wake_index-earlier idea ends up as a part of the solution: but mainly I agree with Jan that the whole code needs a serious redesign (or perhaps the whole design needs a serious recode). So I didn't give your version more thought. Hugh > > Hillf > > [1] https://lore.kernel.org/lkml/afe5b403-4e37-80fd-643d-79e0876a7047@xxxxxxxxxxxxxxxxx/ > > +++ b/lib/sbitmap.c > @@ -613,6 +613,16 @@ static bool __sbq_wake_up(struct sbitmap > if (!ws) > return false; > > + do { > + /* open code sbq_index_atomic_inc(&sbq->wake_index) to avoid race */ > + int old = atomic_read(&sbq->wake_index); > + int new = sbq_index_inc(old); > + > + /* try another ws if someone else takes care of this one */ > + if (old != atomic_cmpxchg(&sbq->wake_index, old, new)) > + return true; > + } while (0); > + > cur = atomic_read(&ws->wait_cnt); > do { > /* > @@ -620,7 +630,7 @@ static bool __sbq_wake_up(struct sbitmap > * function again to wakeup a new batch on a different 'ws'. > */ > if (cur == 0) > - return true; > + goto out; > sub = min(*nr, cur); > wait_cnt = cur - sub; > } while (!atomic_try_cmpxchg(&ws->wait_cnt, &cur, wait_cnt)); > @@ -634,6 +644,7 @@ static bool __sbq_wake_up(struct sbitmap > > *nr -= sub; > > +out: > /* > * When wait_cnt == 0, we have to be particularly careful as we are > * responsible to reset wait_cnt regardless whether we've actually > @@ -661,12 +672,6 @@ static bool __sbq_wake_up(struct sbitmap > */ > smp_mb__before_atomic(); > > - /* > - * Increase wake_index before updating wait_cnt, otherwise concurrent > - * callers can see valid wait_cnt in old waitqueue, which can cause > - * invalid wakeup on the old waitqueue. > - */ > - sbq_index_atomic_inc(&sbq->wake_index); > atomic_set(&ws->wait_cnt, wake_batch); > > return ret || *nr;
