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

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