From: Viresh Kumar <viresh.kumar@linaro.org>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Rafael Wysocki <rjw@rjwysocki.net>,
Juri Lelli <juri.lelli@arm.com>,
Lists linaro-kernel <linaro-kernel@lists.linaro.org>,
"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
Saravana Kannan <skannan@codeaurora.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 4/5] cpufreq: Don't drop rwsem before calling CPUFREQ_GOV_POLICY_EXIT
Date: Wed, 3 Feb 2016 11:21:26 +0530 [thread overview]
Message-ID: <20160203055126.GR31828@vireshk> (raw)
In-Reply-To: <CAJZ5v0hMp7XLP2gnAY=9qkj_yE9hDUwyzJ+4H1H6eadZuYDokw@mail.gmail.com>
On 02-02-16, 22:53, Rafael J. Wysocki wrote:
> First of all, this is effectively reverting commit 955ef4833574, so
> the subject should be "Revert commit 955ef4833574 (cpufreq: Drop rwsem
> lock around CPUFREQ_GOV_POLICY_EXIT)".
>
> There should be a Fixes: tag pointing to commit 955ef4833574 and a
> Reported-by: for Juri.
>
> If there is a link to a bug report related to this, it should be
> pointed to by a Link: tag.
>
> The changelog should say why the original commit was there and why the
> way it attempted to solve the problem was incorrect. It also should
> say that the original problem was addressed by a previous commit, so
> this one can be reverted without consequences.
How about this:
Revert "cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT"
Earlier, when the struct freq-attr was used to represent governor
attributes, the standard cpufreq show/store sysfs attribute callbacks
were applied to the governor tunable attributes and they always acquire
the policy->rwsem lock before carrying out the operation. That could
have resulted in an ABBA deadlock if governor tunable attributes are
removed under policy->rwsem while one of them is being accessed
concurrently (if sysfs attributes removal wins the race, it will wait
for the access to complete with policy->rwsem held while the attribute
callback will block on policy->rwsem indefinitely).
We attempted to address this issue by dropping policy->rwsem around
governor tunable attributes removal (that is, around invocations of the
->governor callback with the event arg equal to CPUFREQ_GOV_POLICY_EXIT)
in cpufreq_set_policy(), but that opened up race conditions that had not
been possible with policy->rwsem held all the time.
The previous commit, "cpufreq: governor: New sysfs show/store callbacks
for governor tunables", fixed the original ABBA deadlock by adding new
governor specific show/store callbacks.
We don't have to drop rwsem around invocations of governor event
CPUFREQ_GOV_POLICY_EXIT anymore, and original fix can be reverted now.
Fixes: 955ef4833574 ("cpufreq: Drop rwsem lock around CPUFREQ_GOV_POLICY_EXIT")
Reported-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> But I'm not going to write that changelog. I actually am not going to
> write any changelogs for you any more, because I'm seriously tired of
> doing that. Moreover, if I see a patch from you with a changelog
> that's not acceptable to me, it will immediately go to the "not
> applicable" trash bin no matter what the changes below look like. You
> *have* *to* write useful changelogs. This isn't optional or best
> effort. This is mandatory and important.
Will try to improve, sorry about that (again).
> Now, I'm not really sure if the ordering of this patchset is right.
> Maybe we should just revert upfront with the "we'll address the
> original problem in the following commits" statement in the changelog
> and fix it in a different way?
Wouldn't that break things like 'git bisect'? People running kernels
after the reverted commits may start hitting lockdeps.
> It looks like patches [1-3/5] fix a
> problem that isn't there even, but would appear after the [4/5] if
> they were not applied previously, which doesn't sound really
> straightforward to me.
I am going to fight hard for it, if you feel 4/5 should be the first
patch here, I will do that.
--
viresh
next prev parent reply other threads:[~2016-02-03 5:51 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-02 10:57 [PATCH 0/5] cpufreq: governors: Solve the ABBA lockups Viresh Kumar
2016-02-02 10:57 ` [PATCH 1/5] cpufreq: governor: Kill declare_show_sampling_rate_min() Viresh Kumar
2016-02-02 20:23 ` Rafael J. Wysocki
2016-02-03 2:29 ` Viresh Kumar
2016-02-02 10:57 ` [PATCH 2/5] cpufreq: governor: Create separate sysfs-ops Viresh Kumar
2016-02-02 15:47 ` Juri Lelli
2016-02-02 16:35 ` Rafael J. Wysocki
2016-02-02 17:01 ` Juri Lelli
2016-02-02 19:40 ` Rafael J. Wysocki
2016-02-02 22:21 ` Saravana Kannan
2016-02-02 23:42 ` Rafael J. Wysocki
2016-02-03 1:07 ` Rafael J. Wysocki
2016-02-03 1:32 ` Saravana Kannan
2016-02-03 1:52 ` Rafael J. Wysocki
2016-02-03 4:03 ` Saravana Kannan
2016-02-03 6:57 ` Viresh Kumar
2016-02-03 20:07 ` Saravana Kannan
2016-02-03 6:54 ` Viresh Kumar
2016-02-03 10:51 ` Juri Lelli
2016-02-03 10:55 ` Viresh Kumar
2016-02-03 20:14 ` Saravana Kannan
2016-02-03 6:51 ` Viresh Kumar
2016-02-03 6:33 ` Viresh Kumar
2016-02-02 21:23 ` Rafael J. Wysocki
2016-02-03 6:58 ` Viresh Kumar
2016-02-03 12:42 ` Rafael J. Wysocki
2016-02-03 13:21 ` Viresh Kumar
2016-02-03 13:38 ` Rafael J. Wysocki
2016-02-02 10:57 ` [PATCH 3/5] cpufreq: governor: Remove unused sysfs attribute macros Viresh Kumar
2016-02-02 21:34 ` Rafael J. Wysocki
2016-02-02 10:57 ` [PATCH 4/5] cpufreq: Don't drop rwsem before calling CPUFREQ_GOV_POLICY_EXIT Viresh Kumar
2016-02-02 21:53 ` Rafael J. Wysocki
2016-02-03 5:51 ` Viresh Kumar [this message]
2016-02-03 12:24 ` Rafael J. Wysocki
2016-02-03 13:09 ` Viresh Kumar
2016-02-02 10:57 ` [PATCH 5/5] cpufreq: Get rid of ->governor_enabled and its lock Viresh Kumar
2016-02-02 16:49 ` Juri Lelli
2016-02-03 6:05 ` Viresh Kumar
2016-02-03 11:05 ` Juri Lelli
2016-02-03 11:08 ` Viresh Kumar
2016-02-02 21:57 ` Rafael J. Wysocki
2016-02-02 11:25 ` [PATCH 0/5] cpufreq: governors: Solve the ABBA lockups Juri Lelli
2016-02-02 20:04 ` Rafael J. Wysocki
2016-02-03 2:22 ` Viresh Kumar
2016-02-03 11:37 ` Viresh Kumar
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