Re: [RFC][PATCH] stop_machine: Fix deadlock between multiple stop_two_cpus()

[Rik van Riel <riel@redhat.com>]

On 06/05/2015 11:30 AM, Peter Zijlstra wrote:
> 
> Jiri reported a machine stuck in multi_cpu_stop() with
> migrate_swap_stop() as function and with the following src,dst cpu
> pairs: {11,  4} {13, 11} { 4, 13}
> 
>                         4       11      13
> 
> cpuM: queue(4 ,13)
>                         *Ma
> cpuN: queue(13,11)
>                                 *N      Na
>                         *M              Mb
> cpuO: queue(11, 4)
>                         *O      Oa
>                                 *Nb
>                         *Ob
> 
> Where *X denotes the cpu running the queueing of cpu-X and X[ab] denotes
> the first/second queued work.
> 
> You'll observe the top of the workqueue for each cpu: 4,11,13 to be work
> from cpus: M, O, N resp. IOW. deadlock.
> 
> Do away with the queueing trickery and introduce lg_double_lock() to
> lock both CPUs and fully serialize the stop_two_cpus() callers instead
> of the partial (and buggy) serialization we have now.
> 
> Completely untested..
> 
> Reported-by: Jiri Olsa <jolsa@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Certainly looks like it would work.

I suspect we should probably apply this patch after some basic
testing, since the race is so incredibly hard to reproduce.

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