Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers

Viresh Kumar
2015-12-08T06:56:22Z

On 07-12-15, 23:43, Rafael J. Wysocki wrote:
> On Monday, December 07, 2015 01:20:27 PM Viresh Kumar wrote:

> > At this point we might end up decrementing skip_work from
> > gov_cancel_work() and then cancel the work which we haven't queued
> > yet. And the end result will be that the work is still queued while
> > gov_cancel_work() has finished.
>
> I'm not quite sure how that can happen.

I will describe that towards the end of this email.

> There is a bug in this code snippet, but it may cause us to fail to queue
> the work at all, so the incrementation and the check need to be done
> under the spinlock.

What bug ?

> > And we have to keep the atomic operation, as well as queue_work()
> > within the lock.
>
> Putting queue_work() under the lock doesn't prevent any races from happening,

Then I am not able to think about it properly, but I will at least
present my case here :)

> because only one of the CPUs can execute that part of the function anyway.
>
> > > 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?
>
> Well, no, the above wouldn't work.
>
> But what about something like this instead:
>
> if (atomic_inc_return(&shared->skip_work) > 1)
> atomic_dec(&shared->skip_work);
> else
> queue_work(system_wq, &shared->work);
>
> (plus the changes requisite replacements in the other places)?
>
> Only one CPU can see the result of the atomic_inc_return() as 1 and this is the
> only one that will queue up the work item, unless I'm missing anything super
> subtle.

Looks like you are talking about the race between different timer
handlers, which race against queuing the work. Sorry if you are not.
But I am not talking about that thing..

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
-> cancel_work_sync(&shared->work);
-> queue_work();
-> gov_cancel_timers(shared->policy);
-> shared->skip_work = 0;
dbs_work_handler();

And according to how I understand it, we are screwed up at this point.
And its the same old bug which I fixed recently (which we hacked up by
using gov-lock earlier).

The work handler is still active after the policy-governor is stopped.

And your latest patch looks wrong for the same reason ...

--
viresh