Re: perf related lockdep bug

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Nov 04, 2015 at 03:20:58PM +0100, Peter Zijlstra wrote:
> 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().

The idea being that might_wake() complains if a scheduler lock is held?
Sounds like a good idea to me.

> > 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 :/

Could lockdep be convinced to do the auditing for you?

							Thanx, Paul