From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Dave Jones <davej@codemonkey.org.uk>,
Linux Kernel <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Stephane Eranian <eranian@gmail.com>,
Andi Kleen <andi@firstfloor.org>
Subject: Re: perf related lockdep bug
Date: Wed, 4 Nov 2015 15:20:58 +0100 [thread overview]
Message-ID: <20151104142058.GX3604@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20151104134838.GR29027@linux.vnet.ibm.com>
On Wed, Nov 04, 2015 at 05:48:38AM -0800, Paul E. McKenney wrote:
> Ouch!!! Thank you for the analysis, though I am very surprised that
> my testing did not find this.
Yeah, not sure how that ended up not triggering earlier.
I'm thinking of adding a might_wake(), much like we have might_fault()
and add that to printk().
> But pulling all printk()s out from under
> rnp->lock is going to re-introduce some stall-warning bugs.
figures :/
> So what other options do I have?
Kill printk() :-) Its unreliable garbage anyway ;-)
> o I could do raise_softirq(), then report the quiescent state in
> the core RCU code, but I bet that raise_softirq()'s wakeup gets
> me into just as much trouble.
Yep..
> o Ditto for workqueues, I suspect.
Yep..
> o I cannot send an IPI because interrupts are disabled, and that
> would be rather annoying from a real-time perspective in any
> case.
Indeed.
> So this hit the code in perf_lock_task_context() that disables preemption
> across an RCU read-side critical section, which previously sufficed to
> prevent this scenario. What happened this time is as follows:
>
> o CPU 0 entered perf_lock_task_context(), disabled preemption,
> and entered its RCU read-side critical section. Of course,
> the whole point of disabling preemption is to prevent the
> matching rcu_read_unlock() from grabbing locks.
>
> o CPU 1 started an expedited grace period. It checked CPU
> state, saw that CPU 0 was running in the kernel, and therefore
> IPIed it.
>
> o The IPI handler running on CPU 0 saw that there was an
> RCU read-side critical section in effect, so it set the
> ->exp_need_qs flag.
>
> o When the matching rcu_read_unlock() executes, it notes that
> ->exp_need_qs is set, and therefore grabs the locks that it
> shouldn't, hence lockdep's complaints about deadlock.
>
> This problem is caused by the IPI handler interrupting the RCU read-side
> critical section. One way to prevent the IPI from doing this is to
> disable interrupts across the RCU read-side critical section instead
> of merely disabling preemption. This is a reasonable approach given
> that acquiring the scheduler locks is going to disable interrupts
> in any case.
>
> The (untested) patch below takes this approach.
>
> Thoughts?
Yes, this should work, but now I worry I need to go audit all of perf
and sched for this :/
next prev parent reply other threads:[~2015-11-04 14:21 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-04 5:17 perf related lockdep bug Dave Jones
2015-11-04 10:21 ` Peter Zijlstra
2015-11-04 10:28 ` Peter Zijlstra
2015-11-04 10:50 ` Peter Zijlstra
2015-11-04 13:48 ` Paul E. McKenney
2015-11-04 14:20 ` Peter Zijlstra [this message]
2015-11-04 15:34 ` Paul E. McKenney
2015-11-04 15:36 ` Peter Zijlstra
2015-11-04 15:51 ` Paul E. McKenney
2015-11-04 20:58 ` Andi Kleen
2015-11-05 0:55 ` Paul E. McKenney
2015-11-05 1:59 ` Paul E. McKenney
2015-11-05 2:46 ` Andi Kleen
2015-11-05 14:04 ` Paul E. McKenney
2015-11-11 13:29 ` Paul E. McKenney
2015-11-10 6:39 ` [tip:perf/urgent] perf: Disable IRQs across RCU RS CS that acquires scheduler lock tip-bot for Paul E. McKenney
2015-11-04 14:01 ` perf related lockdep bug Paul E. McKenney
2015-11-04 14:34 ` Peter Zijlstra
2015-11-05 1:57 ` Paul E. McKenney
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