From: Scott Wood <swood@redhat.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org,
"Paul E . McKenney" <paulmck@linux.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Clark Williams <williams@redhat.com>
Subject: Re: [PATCH RT v3 5/5] rcutorture: Avoid problematic critical section nesting on RT
Date: Mon, 16 Sep 2019 11:55:57 -0500 [thread overview]
Message-ID: <500cabaa80f250b974409ee4a4fca59bf2e24564.camel@redhat.com> (raw)
In-Reply-To: <20190912221706.GC150506@google.com>
On Thu, 2019-09-12 at 18:17 -0400, Joel Fernandes wrote:
> On Wed, Sep 11, 2019 at 05:57:29PM +0100, Scott Wood wrote:
> > rcutorture was generating some nesting scenarios that are not
> > reasonable. Constrain the state selection to avoid them.
> >
> > Example #1:
> >
> > 1. preempt_disable()
> > 2. local_bh_disable()
> > 3. preempt_enable()
> > 4. local_bh_enable()
> >
> > On PREEMPT_RT, BH disabling takes a local lock only when called in
> > non-atomic context. Thus, atomic context must be retained until after
> > BH
> > is re-enabled. Likewise, if BH is initially disabled in non-atomic
> > context, it cannot be re-enabled in atomic context.
> >
> > Example #2:
> >
> > 1. rcu_read_lock()
> > 2. local_irq_disable()
> > 3. rcu_read_unlock()
> > 4. local_irq_enable()
>
> If I understand correctly, these examples are not unrealistic in the real
> world unless RCU is used in the scheduler.
I hope you mean "not realistic", at least when it comes to explicit
preempt/irq disabling rather than spinlock variants that don't disable
preempt/irqs on PREEMPT_RT.
> > If the thread is preempted between steps 1 and 2,
> > rcu_read_unlock_special.b.blocked will be set, but it won't be
> > acted on in step 3 because IRQs are disabled. Thus, reporting of the
> > quiescent state will be delayed beyond the local_irq_enable().
>
> Yes, with consolidated RCU this can happen but AFAIK it has not seen to be
> a
> problem since deferred QS reporting will happen take care of it, which can
> also happen from subsequent rcu_read_unlock_special().
The defer_qs_iw_pending stuff isn't in 5.2-rt. Still, given patch 4/5 (and
special.b.deferred_qs on mainline) this shouldn't present a deadlock concern
(letting the test run a bit now to double check) so this patch could
probably be limited to the "example #1" sequence.
> > For now, these scenarios will continue to be tested on non-PREEMPT_RT
> > kernels, until debug checks are added to ensure that they are not
> > happening elsewhere.
>
> Are you seeing real issues that need this patch? It would be good to not
> complicate rcutorture if not needed.
rcutorture crashes on RT without this patch (in particular due to the
local_bh_disable misordering).
-Scott
next prev parent reply other threads:[~2019-09-16 16:56 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-11 16:57 [PATCH RT v3 0/5] RCU fixes Scott Wood
2019-09-11 16:57 ` [PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs Scott Wood
2019-09-12 22:09 ` Joel Fernandes
2019-09-17 7:44 ` Sebastian Andrzej Siewior
2019-09-17 14:06 ` Scott Wood
2019-09-17 14:42 ` Sebastian Andrzej Siewior
2019-09-17 16:12 ` Scott Wood
2019-09-23 16:41 ` Sebastian Andrzej Siewior
2019-09-11 16:57 ` [PATCH RT v3 2/5] sched: Rename sleeping_lock to rt_invol_sleep Scott Wood
2019-09-17 7:52 ` Sebastian Andrzej Siewior
2019-09-11 16:57 ` [PATCH RT v3 3/5] sched: migrate_dis/enable: Use rt_invol_sleep Scott Wood
2019-09-17 7:59 ` Sebastian Andrzej Siewior
2019-09-17 14:06 ` Scott Wood
2019-09-23 16:59 ` Scott Wood
2019-09-23 17:52 ` Sebastian Andrzej Siewior
2019-09-24 11:21 ` Sebastian Andrzej Siewior
2019-09-24 13:53 ` Scott Wood
2019-09-24 15:25 ` Sebastian Andrzej Siewior
2019-09-24 15:47 ` Scott Wood
2019-09-24 16:05 ` Sebastian Andrzej Siewior
2019-09-24 16:35 ` Scott Wood
2019-10-04 16:45 ` Sebastian Andrzej Siewior
2019-09-11 16:57 ` [PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT_RT Scott Wood
2019-09-12 21:38 ` Joel Fernandes
2019-09-12 22:19 ` Joel Fernandes
2019-09-17 9:31 ` Sebastian Andrzej Siewior
2019-09-17 14:08 ` Scott Wood
2019-09-11 16:57 ` [PATCH RT v3 5/5] rcutorture: Avoid problematic critical section nesting on RT Scott Wood
2019-09-12 22:17 ` Joel Fernandes
2019-09-16 16:55 ` Scott Wood [this message]
2019-09-17 10:07 ` Sebastian Andrzej Siewior
2019-09-17 14:36 ` Scott Wood
2019-09-17 14:50 ` Sebastian Andrzej Siewior
2019-09-17 16:32 ` Scott Wood
2019-09-23 16:25 ` Sebastian Andrzej Siewior
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