Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers | Date | Mon, 07 Dec 2015 02:28:43 +0100 |
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On Saturday, December 05, 2015 09:40:42 AM Viresh Kumar wrote: > On 05-12-15, 03:14, Rafael J. Wysocki wrote: > > Well, almost, but not quite yet, because now the question is what prevents > > gov_cancel_work() from racing with dbs_work_handler(). > > > > If you can guarantee that they'll never run in parallel with each other, > > They can run in parallel and that's how we fix it now: > - raising skip_work to 2 makes sure that no new timer-handler can > queue a new work.
What about if that happens in parallel with the decrementation in dbs_work_handler()?
Is there anything preventing that from happening?
> - After raising the value of skip_work to 2, we do cancel_work_sync(). > Which will make sure that the work-handler has finished after > cancel_work_sync() has returned. > - At this point of time we are sure that the works and their handlers > are completely killed. > - All that is left is to kill all timer-handler (which might have > gotten queued from the work handler, before it finished). > - And we do that with gov_cancel_timers(). > - And then we are in safe state, where we are guaranteed that there > are no leftovers.
Yes, that part will work.
> > you probably don't need the whole counter dance. Otherwise, dbs_work_handler() > > should decrement the counter under the spinlock after all I suppose. > > Its not required because we don't have any race around that decrement > operation.
As I said, if you can guarantee that the decrementation of the counter in dbs_work_handler() cannot happen at the same time as the incrementation of it in gov_cancel_work(), all is fine, but can you actually guarantee that?
That aside, I think you could avoid using the spinlock altogether if the counter was atomic (and which would make the above irrelevant too).
Say, skip_work is atomic the the relevant code in dbs_timer_handler() is written as
atomic_inc(&shared->skip_work); smp_mb__after_atomic(); if (atomic_read(&shared->skip_work) > 1) atomic_dec(&shared->skip_work); else queue_work(system_wq, &shared->work);
and the remaining incrementation and decrementation of skip_work are replaced with the corresponding atomic operations, it still should work, no?
Thanks, Rafael
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