Re: [PATCH v1 02/25] mm/swap: Don't abuse the seqcount latching API

Sebastian A. Siewior
2020-05-22T15:17:13Z

On 2020-05-22 16:57:07 [+0200], Peter Zijlstra wrote:
> > @@ -725,21 +735,48 @@ void lru_add_drain_all(void)
> > if (WARN_ON(!mm_percpu_wq))
> > return;
> >
>
> > + this_gen = READ_ONCE(lru_drain_gen);
> > + smp_rmb();
>
> this_gen = smp_load_acquire(&lru_drain_gen);
> >
> > mutex_lock(&lock);
> >
> > /*
> > + * (C) Exit the draining operation if a newer generation, from another
> > + * lru_add_drain_all(), was already scheduled for draining. Check (A).
> > */
> > + if (unlikely(this_gen != lru_drain_gen))
> > goto done;
> >
>
> > + WRITE_ONCE(lru_drain_gen, lru_drain_gen + 1);
> > + smp_wmb();
>
> You can leave this smp_wmb() out and rely on the smp_mb() implied by
> queue_work_on()'s test_and_set_bit().

This is to avoid smp_store_release() ?

Sebastian