linux-kernel - Re: [PATCH v2] cpufreq: Avoid attempts to create duplicate symbolic links

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Date:	Wed, 29 Jul 2015 10:11:36 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] cpufreq: Avoid attempts to create duplicate symbolic
 links

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.

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