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SubjectRe: [patch 4/5] sched: RCU sched domains
Ingo Molnar wrote:
> * Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
>
>>>At a minimum i think we need the fix+comment below.
>>
>>Well if we say "this is actually RCU", then yes. And we should
>>probably change the preempt_{dis|en}ables in other places to
>>rcu_read_lock.
>>
>>OTOH, if we say we just want all running threads to process through a
>>preemption stage, then this would just be a preempt_disable/enable
>>pair.
>>
>>In practice that makes no difference yet, but it looks like you and
>>Paul are working to distinguish these two cases in the RCU code, to
>>accomodate your low latency RCU stuff?
>
>
> it doesnt impact PREEMPT_RCU/PREEMPT_RT directly, because the scheduler
> itself always needs to be non-preemptible.
>
> those few places where we currently do preempt_disable(), which should
> thus be rcu_read_lock(), are never in codepaths that can take alot of
> time.
>
> but yes, in principle you are right, but in this particular (and
> special) case it's not a big issue. We should document the RCU read-lock
> dependencies cleanly and make all rcu-read-lock cases truly
> rcu_read_lock(), but it's not a pressing issue even considering possible
> future features like PREEMPT_RT.
>
> the only danger in this area is to PREEMPT_RT: it is a bug on PREEMPT_RT
> if kernel code has an implicit 'spinlock means preempt-off and thus
> RCU-read-lock' assumption. Most of the time these get discovered via
> PREEMPT_DEBUG. (preempt_disable() disables preemption on PREEMPT_RT too,
> so that is not a problem either.)
>

OK thanks for the good explanation. So I'll keep it as is for now,
and whatever needs cleaning up later can be worked out as it comes
up.

--
SUSE Labs, Novell Inc.


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