Re: [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization

Viresh Kumar
2015-12-08T13:36:36Z

On 08-12-15, 14:30, Rafael J. Wysocki wrote:
> OK, but instead of relying on the spinlock to wait for the already running

That's the purpose of the spinlock, not a side-effect.

> dbs_timer_handler() in gov_cancel_work() (which is really easy to overlook
> and should at least be mentioned in a comment) we can wait for it explicitly.

I agree, and I will add explicit comment about it.

> That is, if the relevant code in gov_cancel_work() is like this:
>
>
> atomic_inc(&shared->skip_work);
> gov_cancel_timers(shared->policy);
> cancel_work_sync(&shared->work);
> gov_cancel_timers(shared->policy);

Apart from it being *really* ugly (we should know exactly what should
be done, it rather looks like hit and try), it is still racy.

> atomic_set(&shared->skip_work, 0);
>
> then the work item should not be leaked behind the cancel_work_sync() any more
> AFAICS.

Suppose queue_work() isn't done within the spin lock.

CPU0 CPU1

cpufreq_governor_stop() dbs_timer_handler()
-> gov_cancel_work() -> lock
-> shared->skip_work++, as skip_work was 0. //skip_work=1
-> unlock
-> lock
-> shared->skip_work++; //skip_work=2
-> unlock
-> queue_work();
-> gov_cancel_timers(shared->policy);
dbs_work_handler();
-> queue-timers again (as we aren't checking skip_work here)
-> cancel_work_sync(&shared->work);
dbs_timer_handler()
-> lock
-> shared->skip_work++, as skip_work was 0. //skip_work=1
-> unlock
->queue_work()
-> gov_cancel_timers(shared->policy);
-> shared->skip_work = 0;

And we have the same situation again. I have thought of all this
before I wrote the initial patch, and really tried the ugly double
timer-cancel thing. But the current approach is really the right thing
to do.

I will send a patch adding the comment.

--
viresh