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SubjectRe: [Lse-tech] [RFC, PATCH] 5/5 rcu lock update: Hierarchical rcu_cpu_mask bitmap
Paul E. McKenney wrote:

>I don't understand how the following helps, given that the CPU being
>offlined is supposed to be completely dead by the time we get here.
>Can't see anything that it really hurts other than a bit of delay in
>an extremely infrequent operation, but...
>
>
>
>>+ spin_unlock_wait(&rcu_state.mutex);
>> cpu_quiet(cpu, 1);
>>
>>

I was concerned about a race of cpu_quiet for the dead cpu (last cpu in
a group) with a concurrent cpu_quiet for another cpu in another cpu
group. A stale bit in either rcu_state.outstanding or
rcu_groups[].outstanding would lock up the rcu subsystem. I've thought
about it again and I agree with you - there is no race.

I agree with most of your other points - the patch is a proof of
concept, it definitively needs a big cleanup before is should be
considered for merging [+ a system that needs it]

--
Manfred
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