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

Let me look at this more and see what is going on now.

>> 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.

In any case, it seem like some more analysis is needed based on the
latest data.

Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

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