From: Viresh Kumar <viresh.kumar@linaro.org>
To: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: linaro-kernel@lists.linaro.org, linux-pm@vger.kernel.org,
ashwin.chaugule@linaro.org,
"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers
Date: Tue, 8 Dec 2015 12:26:22 +0530 [thread overview]
Message-ID: <20151208065622.GZ3294@ubuntu> (raw)
In-Reply-To: <3999637.u4UiK7zxOR@vostro.rjw.lan>
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
next prev parent reply other threads:[~2015-12-08 6:56 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <cover.1449115453.git.viresh.kumar@linaro.org>
2015-12-03 4:07 ` [PATCH V2 1/6] cpufreq: ondemand: Update sampling rate only for concerned policies Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 2/6] cpufreq: ondemand: Work is guaranteed to be pending Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 3/6] cpufreq: governor: Pass policy as argument to ->gov_dbs_timer() Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 4/6] cpufreq: governor: initialize/destroy timer_mutex with 'shared' Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers Viresh Kumar
2015-12-04 1:18 ` Rafael J. Wysocki
2015-12-04 6:11 ` Viresh Kumar
2015-12-05 2:14 ` Rafael J. Wysocki
2015-12-05 4:10 ` Viresh Kumar
2015-12-07 1:28 ` Rafael J. Wysocki
2015-12-07 7:50 ` Viresh Kumar
2015-12-07 22:43 ` Rafael J. Wysocki
2015-12-07 23:17 ` Rafael J. Wysocki
2015-12-08 0:39 ` [PATCH][experimantal] cpufreq: governor: Use an atomic variable for synchronization Rafael J. Wysocki
2015-12-08 6:59 ` Viresh Kumar
2015-12-08 13:30 ` Rafael J. Wysocki
2015-12-08 13:36 ` Viresh Kumar
2015-12-08 14:19 ` Rafael J. Wysocki
2015-12-08 13:55 ` Viresh Kumar
2015-12-08 14:30 ` Rafael J. Wysocki
2015-12-08 14:56 ` Viresh Kumar
2015-12-08 16:42 ` Rafael J. Wysocki
2015-12-08 16:34 ` Viresh Kumar
2015-12-08 6:46 ` [PATCH V2 5/6] cpufreq: governor: replace per-cpu delayed work with timers Viresh Kumar
2015-12-08 6:56 ` Viresh Kumar [this message]
2015-12-08 13:18 ` Rafael J. Wysocki
2015-12-08 13:30 ` Viresh Kumar
2015-12-08 14:04 ` Rafael J. Wysocki
2015-12-04 6:13 ` [PATCH V3 " Viresh Kumar
2015-12-09 2:04 ` [PATCH V4 " Viresh Kumar
2015-12-09 22:06 ` Rafael J. Wysocki
2015-12-10 2:36 ` Viresh Kumar
2015-12-10 22:17 ` Rafael J. Wysocki
2015-12-11 1:42 ` Viresh Kumar
2015-12-03 4:07 ` [PATCH V2 6/6] cpufreq: ondemand: update update_sampling_rate() to make it more efficient Viresh Kumar
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