On Thu, Sep 26, 2013 at 06:14:26PM +0200, Oleg Nesterov wrote:

On 09/26, Peter Zijlstra wrote:

quoted

On Thu, Sep 26, 2013 at 05:53:21PM +0200, Oleg Nesterov wrote:

quoted

On 09/26, Peter Zijlstra wrote:

quoted

 void cpu_hotplug_done(void)
 {
-	cpu_hotplug.active_writer = NULL;
-	mutex_unlock(&cpu_hotplug.lock);
+	/* Signal the writer is done, no fast path yet. */
+	__cpuhp_state = readers_slow;
+	wake_up_all(&cpuhp_readers);
+
+	/*
+	 * The wait_event()/wake_up_all() prevents the race where the readers
+	 * are delayed between fetching __cpuhp_state and blocking.
+	 */
+
+	/* See percpu_up_write(); readers will no longer attempt to block. */
+	synchronize_sched();

Shouldn't you move wake_up_all(&cpuhp_readers) down after
synchronize_sched() (or add another one) ? To ensure that a reader can't
see state = BLOCK after wakeup().

Well, if they are blocked, the wake_up_all() will do an actual
try_to_wake_up() which issues a MB as per smp_mb__before_spinlock().

Yes. Everything is fine with the already blocked readers.

I meant the new reader which still can see state = BLOCK after we
do wakeup(), but I didn't notice it should do __wait_event() which
takes the lock unconditionally, it must see the change after that.

Ah, because both __wake_up() and __wait_event()->prepare_to_wait() take
q->lock. Thereby matching the __wake_up() RELEASE to the __wait_event()
ACQUIRE, creating the full barrier.

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