Re: [PATCH v5 08/20] kthread: Allow to cancel kthread work

On Fri, Feb 26, 2016 at 06:00:56PM +0100, Petr Mladek wrote:

On Fri 2016-02-26 17:25:52, Peter Zijlstra wrote:

quoted

On Fri, Feb 26, 2016 at 04:38:18PM +0100, Petr Mladek wrote:

quoted

On Thu 2016-02-25 13:59:32, Peter Zijlstra wrote:

quoted

On Wed, Feb 24, 2016 at 05:18:05PM +0100, Petr Mladek wrote:

quoted

@@ -770,7 +782,22 @@ void delayed_kthread_work_timer_fn(unsigned long __data)
 	if (WARN_ON_ONCE(!worker))
 		return;
 
-	spin_lock(&worker->lock);
+	/*
+	 * We might be unable to take the lock if someone is trying to
+	 * cancel this work and calls del_timer_sync() when this callback
+	 * has already been removed from the timer list.
+	 */
+	while (!spin_trylock(&worker->lock)) {
+		/*
+		 * Busy wait with spin_is_locked() to avoid cache bouncing.
+		 * Break when canceling is set to avoid a deadlock.
+		 */
+		do {
+			if (work->canceling)
+				return;
+			cpu_relax();
+		} while (spin_is_locked(&worker->lock));
+	}
 	/* Work must not be used with more workers, see queue_kthread_work(). */
 	WARN_ON_ONCE(work->worker != worker);

quoted

And since you do add_timer() while holding the spinlock, this should all
work out, no?

Interesting idea. Yes, it should work. But is this really easier? The
try_again/relock/recheck code is not trivial either.

The trylock loop has very bad worst case behaviour, it really is best to
avoid such patterns if at all possible.