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

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Message-ID: <7208072.lZriMRM1aD@vostro.rjw.lan>
Date:	Tue, 08 Dec 2015 14:18:54 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	ashwin.chaugule@...aro.org,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers

On Tuesday, December 08, 2015 12:26:22 PM Viresh Kumar wrote:
> 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 ?

Well, if the timer function runs on all CPUs at the same time, they all
can see skip_work > 1 and none of them will queue the work.

> > > 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).

You are right, I've overlooked that race (but then it is rather easy to
overlook).

Thanks,
Rafael

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