From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Viresh Kumar <viresh.kumar@linaro.org>,
"Rafael J. Wysocki" <rafael@kernel.org>,
Linux PM list <linux-pm@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] cpufreq: Avoid attempts to create duplicate symbolic links
Date: Wed, 29 Jul 2015 10:11:36 +0100 [thread overview]
Message-ID: <20150729091136.GN7557@n2100.arm.linux.org.uk> (raw)
In-Reply-To: <2112385.YuDJ7h1x56@vostro.rjw.lan>
On Wed, Jul 29, 2015 at 03:38:03AM +0200, Rafael J. Wysocki wrote:
> On Monday, July 27, 2015 08:09:35 PM Viresh Kumar wrote:
> > On 27-07-15, 15:45, Rafael J. Wysocki wrote:
> > > Say the subsys add callback runs for a CPU and it doesn't have a policy.
> > > If it is offline, we ignore it and the add callback won't be executed
> > > for it again.
> > >
> > > In turn, if it is online, we create a policy for it and we should (right
> > > away) link the policy to all of the CPUs that were offline when the subsys add
> > > callback was called for them. That's what we do today.
> > >
> > > Is there anything missing in that?
> >
> > So the code is working properly after your patch, but I was talking
> > on the lines of what Russell suggested.
> >
> > We should play with the links only when we receive add-dev/remove-dev
> > from subsys callbacks. The exception to that will be the offline CPUs
> > for which add-dev is called before their policy existed.
>
> The rule is supposed to be "all of the present CPUs which do not own
> a policy should point to one, unless it doesn't exist". The right
> approach is then to create links from them to a policy object as soon
> as we create one for them. Waiting for something else to happen is just
> pointless and this approach covers both the offline and online CPUs, so
> I don't think that changing it would improve things really.
I'm not sure we disagree with that. It's more about when the symlinks
are created, and when you define that a CPU exists.
If you're attaching to subsystem in sysfs, then the point that the
subsystem interface gets to know about a sysfs node existing is when
it's add_dev method is called - it should not assume that a node exists
prior to that point, otherwise things are racy.
Consider a policy initialisation in parallel with an update of the
CPU present map and adding a CPU to sysfs. The CPU present map will be
updated first, and then it will be added to sysfs. If you're initialising
a cpufreq policy in the middle of that and creating symlinks for all
present CPUs, there's a window where the CPU present map indicates that
a CPU is present, but there is no sysfs directory for you to create a
symlink in.
--
FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
according to speedtest.net.
next prev parent reply other threads:[~2015-07-29 9:11 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-23 21:14 [PATCH] cpufreq: Avoid attempts to create duplicate symbolic links Rafael J. Wysocki
2015-07-24 14:09 ` Viresh Kumar
2015-07-24 20:20 ` Rafael J. Wysocki
2015-07-24 22:17 ` [PATCH v2] " Rafael J. Wysocki
2015-07-25 13:00 ` Viresh Kumar
2015-07-25 22:46 ` Rafael J. Wysocki
2015-07-26 0:28 ` [PATCH v3] " Rafael J. Wysocki
2015-07-27 2:29 ` Viresh Kumar
2015-07-27 12:39 ` Rafael J. Wysocki
2015-07-27 2:27 ` [PATCH v2] " Viresh Kumar
2015-07-27 13:45 ` Rafael J. Wysocki
2015-07-27 14:39 ` Viresh Kumar
2015-07-29 1:38 ` Rafael J. Wysocki
2015-07-29 5:45 ` Viresh Kumar
2015-07-29 9:11 ` Russell King - ARM Linux [this message]
2015-07-29 13:57 ` Rafael J. Wysocki
2015-07-29 14:21 ` Viresh Kumar
2015-07-29 20:32 ` Rafael J. Wysocki
2015-07-30 9:00 ` Viresh Kumar
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