linux-kernel - Re: [PATCH] cpufreq: Avoid double addition/removal of sysfs links

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Date:	Wed, 22 Jul 2015 01:15:01 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: Avoid double addition/removal of sysfs links

Hi VIresh,

On Mon, Jul 20, 2015 at 11:47 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Consider a dual core (0/1) system with two CPUs:
> - sharing clock/voltage rails and hence cpufreq-policy
> - CPU1 is offline while the cpufreq driver is registered
>
> - cpufreq_add_dev() is called from subsys callback for CPU0 and we
>   create the policy for the CPUs and create link for CPU1.
> - cpufreq_add_dev() is called from subsys callback for CPU1, we find
>   that the cpu is offline and we try to create a sysfs link for CPU1.

So the problem is that the cpu_is_offline(cpu) check in
cpufreq_add_dev() matches two distinct cases: (1) the CPU was not
present before and it is just being hot-added and (2) the CPU is
initially offline, but present, and this is the first time its device
is registered.  In the first case we can expect that the CPU will
become online shortly (although that is not guaranteed too), but in
the second case that very well may not happen.

We need to be able to distinguish between those two cases and your
patch does that, but I'm not sure if this really is the most
straightforward way to do it.

I'm also unsure why you're changing the removal code paths.  Is there
any particular failure scenario you're concerned about?

Thanks,
Rafael
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