From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Thavatchai Makphaibulchoke <tmac@hp.com>,
LKML <linux-kernel@vger.kernel.org>,
mingo@redhat.com, linux-rt-users <linux-rt-users@vger.kernel.org>
Subject: Re: [PATCH v2 1/2] rtmutex Real-Time Linux: Fixing kernel BUG at kernel/locking/rtmutex.c:997!
Date: Tue, 7 Apr 2015 08:41:54 -0400 [thread overview]
Message-ID: <20150407084154.74c01a6c@gandalf.local.home> (raw)
In-Reply-To: <20150407120403.GN21418@twins.programming.kicks-ass.net>
On Tue, 7 Apr 2015 14:04:03 +0200
Peter Zijlstra <peterz@infradead.org> wrote:
> 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..
>
Yep, all this nonsense came from that patch and trying to get
NO_HZ_FULL working with -rt. It's a bit ironic that the push to get
NO_HZ_FULL into mainline came from our RT mini summit, but its
implementation is broken on -rt :-p
Ideally, we don't want to take mutexes in hard interrupt context.
> @@ -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.
I'll take a deeper look at this code too. If we can get rid of this
hack, then we don't need the mutex-in-irq hack either.
-- Steve
next prev parent reply other threads:[~2015-04-07 12:42 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-20 1:31 [PATCH 3.14.25-rt22 0/2] rtmutex Real-Time Linux: fix kernel BUG at kernel/locking/rtmutex.c:997! and some optimization Thavatchai Makphaibulchoke
2015-02-20 1:31 ` [PATCH 3.14.25-rt22 1/2] rtmutex Real-Time Linux: Fixing kernel BUG at kernel/locking/rtmutex.c:997! Thavatchai Makphaibulchoke
2015-02-20 4:53 ` Steven Rostedt
2015-02-20 18:54 ` Thavatchai Makphaibulchoke
2015-02-21 1:49 ` Steven Rostedt
2015-02-23 18:37 ` Steven Rostedt
2015-02-24 0:16 ` Thavatchai Makphaibulchoke
2015-02-24 0:57 ` Steven Rostedt
2015-02-26 13:56 ` Sebastian Andrzej Siewior
2015-02-26 14:06 ` Steven Rostedt
2015-03-06 12:19 ` Sebastian Andrzej Siewior
2015-03-09 16:36 ` Steven Rostedt
2015-03-09 16:49 ` Sebastian Andrzej Siewior
2015-02-20 1:31 ` [PATCH 3.14.25-rt22 2/2] kernel/locking/rtmutex.c: some code optimization Thavatchai Makphaibulchoke
2015-04-07 1:26 ` [PATCH v2 0/2] rtmutex Real-Time Linux: fix BUG at kernel/locking/rtmutex.c:997! Thavatchai Makphaibulchoke
2015-04-07 1:26 ` [PATCH v2 1/2] rtmutex Real-Time Linux: Fixing kernel " Thavatchai Makphaibulchoke
2015-04-07 1:59 ` Steven Rostedt
2015-04-07 5:09 ` Mike Galbraith
2015-04-07 10:29 ` Peter Zijlstra
2015-04-07 10:49 ` Mike Galbraith
2015-04-07 10:56 ` Peter Zijlstra
2015-04-07 11:01 ` Mike Galbraith
2015-04-08 0:55 ` Thavatchai Makphaibulchoke
2015-04-08 8:50 ` Thomas Gleixner
2015-04-09 22:56 ` Thavatchai Makphaibulchoke
2015-04-07 11:23 ` Thomas Gleixner
2015-04-07 11:47 ` Mike Galbraith
2015-04-07 12:04 ` Peter Zijlstra
2015-04-07 12:07 ` Peter Zijlstra
2015-04-07 12:41 ` Steven Rostedt [this message]
2015-04-07 12:54 ` Peter Zijlstra
2015-04-07 13:58 ` Steven Rostedt
2015-04-07 18:12 ` Jason Low
2015-04-07 19:17 ` Thomas Gleixner
2015-04-07 19:57 ` Jason Low
2015-04-07 21:38 ` Thomas Gleixner
2015-04-07 1:26 ` [PATCH v2 2/2] kernel/locking/rtmutex.c: some code optimization Thavatchai Makphaibulchoke
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