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

Peter Zijlstra
2020-05-22T14:57:33Z

On Tue, May 19, 2020 at 11:45:24PM +0200, Ahmed S. Darwish wrote:
> @@ -713,10 +713,20 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
> */
> void lru_add_drain_all(void)
> {

> + static unsigned int lru_drain_gen;
> static struct cpumask has_work;
> + static DEFINE_MUTEX(lock);
> + int cpu, this_gen;
>
> /*
> * Make sure nobody triggers this path before mm_percpu_wq is fully
> @@ -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().

> cpumask_clear(&has_work);
> -
> for_each_online_cpu(cpu) {
> struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
>

While you're here, do:

s/cpumask_set_cpu/__&/

> @@ -766,7 +803,7 @@ void lru_add_drain_all(void)
> {
> lru_add_drain();
> }
> -#endif
> +#endif /* CONFIG_SMP */
>
> /**
> * release_pages - batched put_page()