Messages in this thread | | | Date | Mon, 23 Sep 2013 19:50:52 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() |
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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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