From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Saravana Kannan <skannan@codeaurora.org>,
Viresh Kumar <viresh.kumar@linaro.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
Shilpa Bhat <shilpabhatppc@gmail.com>,
Juri Lelli <juri.lelli@arm.com>,
Rafael Wysocki <rjw@rjwysocki.net>,
Lists linaro-kernel <linaro-kernel@lists.linaro.org>,
"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Michael Turquette <mturquette@baylibre.com>,
Steve Muckle <steve.muckle@linaro.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
Morten Rasmussen <morten.rasmussen@arm.com>,
dietmar.eggemann@arm.com,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups
Date: Thu, 4 Feb 2016 19:18:32 +0100 [thread overview]
Message-ID: <CAJZ5v0hG2CickrbnTn8qJdFHJZ4cuZg1nxJ=8OXN8YFuw=qSFQ@mail.gmail.com> (raw)
In-Reply-To: <56B38E14.5060705@codeaurora.org>
On Thu, Feb 4, 2016 at 6:44 PM, Saravana Kannan <skannan@codeaurora.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
next prev parent reply other threads:[~2016-02-04 18:18 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-03 14:02 [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups Viresh Kumar
2016-02-03 14:02 ` [PATCH V2 1/7] cpufreq: governor: Treat min_sampling_rate as a governor-specific tunable Viresh Kumar
2016-02-05 2:31 ` Rafael J. Wysocki
2016-02-05 2:47 ` Viresh Kumar
2016-02-03 14:02 ` [PATCH V2 2/7] cpufreq: governor: New sysfs show/store callbacks for governor tunables Viresh Kumar
2016-02-03 16:17 ` Viresh Kumar
2016-02-03 14:02 ` [PATCH V2 3/7] cpufreq: governor: Drop unused macros for creating governor tunable attributes Viresh Kumar
2016-02-03 14:02 ` [PATCH V2 4/7] Revert "cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT" Viresh Kumar
2016-02-03 14:02 ` [PATCH V2 5/7] cpufreq: Merge cpufreq_offline_prepare/finish routines Viresh Kumar
2016-02-03 20:21 ` Saravana Kannan
2016-02-04 1:49 ` Viresh Kumar
2016-02-03 14:02 ` [PATCH V2 6/7] cpufreq: Call __cpufreq_governor() with policy->rwsem held Viresh Kumar
2016-02-03 14:02 ` [PATCH V2 7/7] cpufreq: Remove cpufreq_governor_lock Viresh Kumar
2016-02-04 6:43 ` Viresh Kumar
2016-02-03 15:54 ` [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups Juri Lelli
2016-02-03 16:10 ` Viresh Kumar
2016-02-03 17:20 ` Juri Lelli
2016-02-03 17:20 ` Rafael J. Wysocki
2016-02-03 23:31 ` Shilpa Bhat
2016-02-03 23:50 ` Rafael J. Wysocki
2016-02-04 5:51 ` Viresh Kumar
2016-02-04 11:09 ` Viresh Kumar
2016-02-04 17:43 ` Saravana Kannan
2016-02-04 17:44 ` Saravana Kannan
2016-02-04 18:18 ` Rafael J. Wysocki [this message]
2016-02-05 2:44 ` Viresh Kumar
2016-02-05 3:54 ` Rafael J. Wysocki
2016-02-05 9:49 ` Viresh Kumar
2016-02-08 2:20 ` Rafael J. Wysocki
2016-02-06 2:22 ` Saravana Kannan
2016-02-08 2:28 ` Rafael J. Wysocki
2016-02-09 21:02 ` Saravana Kannan
2016-02-04 6:24 ` Viresh Kumar
2016-02-04 12:17 ` Viresh Kumar
2016-02-04 20:50 ` Shilpasri G Bhat
2016-02-05 2:49 ` Viresh Kumar
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