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SubjectRe: [PATCH] hotplug: Optimize {get,put}_online_cpus()
And somehow I didn't notice that cpuhp_set_state() doesn't look right,

On 09/19, Peter Zijlstra wrote:
> void cpu_hotplug_begin(void)
> {
> - cpu_hotplug.active_writer = current;
> + lockdep_assert_held(&cpu_add_remove_lock);
>
> - for (;;) {
> - mutex_lock(&cpu_hotplug.lock);
> - if (likely(!cpu_hotplug.refcount))
> - break;
> - __set_current_state(TASK_UNINTERRUPTIBLE);
> - mutex_unlock(&cpu_hotplug.lock);
> - schedule();
> - }
> + __cpuhp_writer = current;
> +
> + /* After this everybody will observe _writer and take the slow path. */
> + synchronize_sched();
> +
> + /* Wait for no readers -- reader preference */
> + cpuhp_wait_refcount();
> +
> + /* Stop new readers. */
> + cpuhp_set_state(1);

But this stops all readers, not only new. Even if cpuhp_wait_refcount()
was correct, a new reader can come right before cpuhp_set_state(1) and
then it can call another recursive get_online_cpus() right after.

Oleg.



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