Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline

From: Paul E. McKenney
Date: Wed Jun 21 2017 - 16:42:35 EST

On Wed, Jun 21, 2017 at 08:39:45AM -0600, Jeffrey Hugo wrote:
> On 6/20/2017 5:46 PM, Paul E. McKenney wrote:
> >On Mon, Mar 27, 2017 at 11:17:11AM -0700, Paul E. McKenney wrote:
> >>On Mon, Mar 27, 2017 at 12:02:27PM -0600, Jeffrey Hugo wrote:
> >>>Hi Paul.
> >>>
> >>>Thanks for the quick reply.
> >>>
> >>>On 3/26/2017 5:28 PM, Paul E. McKenney wrote:
> >>>>On Sun, Mar 26, 2017 at 05:10:40PM -0600, Jeffrey Hugo wrote:
> >>>
> >>>>>It is a race between this work running, and the cpu offline processing.
> >>>>
> >>>>One quick way to test this assumption is to build a kernel with Kconfig
> >>>>options CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y. This will
> >>>>cause call_rcu_sched() to queue the work to a kthread, which can migrate
> >>>>to some other CPU. If your analysis is correct, this should avoid
> >>>>the deadlock. (Note that the deadlock should be fixed in any case,
> >>>>just a diagnostic assumption-check procedure.)
> >>>
> >>>CONFIG_RCU_NOCB_CPU_ALL=y in my build. I've only had time so far to
> >>>do one test run however the issue reproduced, but it took a fair bit
> >>>longer to do so. An initial look at the data indicates that the
> >>>work is still not running. An odd observation, the two threads are
> >>>no longer blocked on the same queue, but different ones.
> >>
> >>I was afraid of that...
> >>
> >>>Let me look at this more and see what is going on now.
> >>
> >>Another thing to try would be to affinity the "rcuo" kthreads to
> >>some CPU that is never taken offline, just in case that kthread is
> >>sometimes somehow getting stuck during the CPU-hotplug operation.
> >>
> >>>>>What is the opinion of the domain experts?
> >>>>
> >>>>I do hope that we can come up with a better fix. No offense intended,
> >>>>as coming up with -any- fix in the CPU-hotplug domain is not to be
> >>>>denigrated, but this looks to be at vest quite fragile.
> >>>>
> >>>> Thanx, Paul
> >>>>
> >>>
> >>>None taken. I'm not particularly attached to the current fix. I
> >>>agree, it does appear to be quite fragile.
> >>>
> >>>I'm still not sure what a better solution would be though. Maybe
> >>>the RCU framework flushes the work somehow during cpu offline? It
> >>>would need to ensure further work is not queued after that point,
> >>>which seems like it might be tricky to synchronize. I don't know
> >>>enough about the working of RCU to even attempt to implement that.
> >>
> >>There are some ways that RCU might be able to shrink the window during
> >>which the outgoing CPU's callbacks are in limbo, but they are not free
> >>of risk, so we really need to compleetly understand what is going on
> >>before making any possibly ill-conceived changes. ;-)
> >>
> >>>In any case, it seem like some more analysis is needed based on the
> >>>latest data.
> >>
> >>Looking forward to hearing about you find!
> >
> >Hearing nothing, I eventually took unilateral action (I am a citizen of
> >USA, after all!) and produced the lightly tested patch shown below.
> >
> >Does it help?
> >
> > Thanx, Paul
> >
> Wow, has it been 3 months already? I am extremely sorry, I've been
> preempted multiple times, and this has sat on my todo list where I
> keep thinking I need to find time to come back to it but apparently
> not doing enough to make that happen.
> Thank you for not forgetting about this. I promise I will somehow
> clear my schedule to test this next week.

No worries, and I am very much looking forward to seeing the results of
your testing.

Thanx, Paul