linux-kernel - Re: [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups

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Date:	Thu, 4 Feb 2016 19:18:32 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Saravana Kannan <skannan@...eaurora.org>,
	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Shilpa Bhat <shilpabhatppc@...il.com>,
	Juri Lelli <juri.lelli@....com>,
	Rafael Wysocki <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Turquette <mturquette@...libre.com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	dietmar.eggemann@....com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups

On Thu, Feb 4, 2016 at 6:44 PM, Saravana Kannan <skannan@...eaurora.org> wrote:
> On 02/04/2016 09:43 AM, Saravana Kannan wrote:
>>
>> On 02/04/2016 03:09 AM, Viresh Kumar wrote:
>>>
>>> On 04-02-16, 00:50, Rafael J. Wysocki wrote:
>>>>
>>>> This is exactly right.  We've avoided one deadlock only to trip into
>>>> another one.
>>>>
>>>> This happens because update_sampling_rate() acquires
>>>> od_dbs_cdata.mutex which is held around cpufreq_governor_exit() by
>>>> cpufreq_governor_dbs().
>>>>
>>>> Worse yet, a deadlock can still happen without (the new)
>>>> dbs_data->mutex, just between s_active and od_dbs_cdata.mutex if
>>>> update_sampling_rate() runs in parallel with
>>>> cpufreq_governor_dbs()->cpufreq_governor_exit() and the latter wins
>>>> the race.
>>>>
>>>> It looks like we need to drop the governor mutex before putting the
>>>> kobject in cpufreq_governor_exit().
>>>

[cut]

>>
>> No no no no! Let's not open up this can of worms of queuing up the work
>> to handle a write to a sysfs file. It *MIGHT* work for this specific
>> tunable (I haven't bothered to analyze), but this makes it impossible to
>> return a useful/proper error value.
>
>
> Sent too soon. Not only that, but it can also cause the writes to the sysfs
> files to get processed in a different order and I don't know what other
> issues/races THAT will open up.

Well, I don't like this too.

I actually do have an idea about how to fix these deadlocks, but it is
on top of my cleanup series.

I'll write more about it later today.

Thanks,
Rafael