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

Peter Zijlstra
2020-05-20T13:06:01Z

On Wed, May 20, 2020 at 03:22:15PM +0300, Konstantin Khlebnikov wrote:
> On 20/05/2020 00.45, Ahmed S. Darwish wrote:
> > Commit eef1a429f234 ("mm/swap.c: piggyback lru_add_drain_all() calls")
> > implemented an optimization mechanism to exit the to-be-started LRU
> > drain operation (name it A) if another drain operation *started and
> > finished* while (A) was blocked on the LRU draining mutex.

That commit is horrible...

> Well, I thought it fits perfectly =)
>
> Maybe it's worth to add helpers with appropriate semantics?
> This is pretty common pattern.

Where's more sites?

> > @@ -725,21 +735,48 @@ void lru_add_drain_all(void)
> > if (WARN_ON(!mm_percpu_wq))
> > return;
> > - seq = raw_read_seqcount_latch(&seqcount);
> > mutex_lock(&lock);
> > /*
> > - * Piggyback on drain started and finished while we waited for lock:
> > - * all pages pended at the time of our enter were drained from vectors.
> > */
> > - if (__read_seqcount_retry(&seqcount, seq))
> > goto done;

Since there is no ordering in raw_read_seqcount_latch(), and
mutex_lock() is an ACQUIRE, there's no guarantee the read actually
happens before the mutex is acquired.

> > - raw_write_seqcount_latch(&seqcount);
> > cpumask_clear(&has_work);