On 09/24, Peter Zijlstra wrote:
>
> +void __get_online_cpus(void)
> {
> - if (cpu_hotplug.active_writer == current)
> + /* Support reader-in-writer recursion */
> + if (__cpuhp_writer == current)
> return;
> - mutex_lock(&cpu_hotplug.lock);
>
> - if (WARN_ON(!cpu_hotplug.refcount))
> - cpu_hotplug.refcount++; /* try to fix things up */
> + atomic_inc(&cpuhp_waitcount);
>
> - if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
> - wake_up_process(cpu_hotplug.active_writer);
> - mutex_unlock(&cpu_hotplug.lock);
> + /*
> + * We either call schedule() in the wait, or we'll fall through
> + * and reschedule on the preempt_enable() in get_online_cpus().
> + */
> + preempt_enable_no_resched();
> + wait_event(cpuhp_readers, !__cpuhp_writer);
> + preempt_disable();
> +
> + if (atomic_dec_and_test(&cpuhp_waitcount))
> + wake_up_all(&cpuhp_writer);
Yes, this should fix the races with the exiting writer, but still this
doesn't look right afaics.
In particular let me repeat,
> void cpu_hotplug_begin(void)
> {
> - cpu_hotplug.active_writer = current;
> + unsigned int count = 0;
> + int cpu;
> +
> + 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();
synchronize_sched() is slow. The new readers will likely notice
__cpuhp_writer != NULL much earlier and they will be blocked in
__get_online_cpus() while the writer sleeps before it actually
enters the critical section.
Or I completely misunderstood this all?
Oleg.
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