Re: perf related lockdep bug

[Peter Zijlstra]

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 :/


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