Re: [PATCH v2 1/2] rtmutex Real-Time Linux: Fixing kernel BUG at kernel/locking/rtmutex.c:997!

[Peter Zijlstra]

On Tue, Apr 07, 2015 at 01:47:16PM +0200, Mike Galbraith wrote:
> On Tue, 2015-04-07 at 13:23 +0200, Thomas Gleixner wrote:
> > On Mon, 6 Apr 2015, Thavatchai Makphaibulchoke wrote:
> > 
> > > This patch fixes the problem that the ownership of a mutex acquired
> > > by an interrupt handler(IH) gets incorrectly attributed to the
> > > interrupted thread.
> > 
> > An hard interrupt handler is not allowed to take a mutex. End of
> > story, nothing to fix here.
> 
> Well, the patch that started this thread..
> 
> timers-do-not-raise-softirq-unconditionally.patch

Aah, that is the problem..

@@ -1454,8 +1452,32 @@ static void run_timer_softirq(struct softirq_action *h)
  */
 void run_local_timers(void)
 {
+	struct tvec_base *base = __this_cpu_read(tvec_bases);
+
 	hrtimer_run_queues();
-	raise_softirq(TIMER_SOFTIRQ);
+	/*
+	 * We can access this lockless as we are in the timer
+	 * interrupt. If there are no timers queued, nothing to do in
+	 * the timer softirq.
+	 */
+#ifdef CONFIG_PREEMPT_RT_FULL
+	if (!spin_do_trylock(&base->lock)) {
+		raise_softirq(TIMER_SOFTIRQ);
+		return;
+	}
+#endif
+	if (!base->active_timers)
+		goto out;
+
+	/* Check whether the next pending timer has expired */
+	if (time_before_eq(base->next_timer, jiffies))
+		raise_softirq(TIMER_SOFTIRQ);
+out:
+#ifdef CONFIG_PREEMPT_RT_FULL
+	rt_spin_unlock_after_trylock_in_irq(&base->lock);
+#endif
+	/* The ; ensures that gcc won't complain in the !RT case */
+	;
 }

That smells like something we should be able to do without a lock.

If we use {READ,WRITE}_ONCE() on those two fields (->active_timers and
->next_timer) we should be able to do this without the spinlock.

Races here aren't really a problem I think, if you manage to install a
timer at the current jiffy and have already missed the tick you're in
the same boat. You get to wait for the next tick.

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