Re: perf related lockdep bug

From: Peter Zijlstra
Date: Wed Nov 04 2015 - 09:21:15 EST

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.


> o Ditto for workqueues, I suspect.


> o I cannot send an IPI because interrupts are disabled, and that
> would be rather annoying from a real-time perspective in any
> case.


> 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 :/

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at