linux-kernel - Re: [PATCH v3] cpufreq: fix race on cpufreq online

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Message-ID: <CAJZ5v0i_qpcaUwdZY=KR_e=HC6MZUw2aDGL=hxgo866dF1zwOA@mail.gmail.com>
Date:   Tue, 24 May 2022 13:22:50 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Schspa Shi <schspa@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v3] cpufreq: fix race on cpufreq online

On Tue, May 24, 2022 at 1:15 PM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 13-05-22, 09:57, Viresh Kumar wrote:
> > On 12-05-22, 12:49, Rafael J. Wysocki wrote:
> > > > > Moreover, I'm not sure why the locking dance in store() is necessary.
> > > >
> > > > commit fdd320da84c6 ("cpufreq: Lock CPU online/offline in cpufreq_register_driver()")
> > >
> > > I get that, but I'm wondering if locking CPU hotplug from store() is
> > > needed at all.  I mean, if we are in store(), we are holding an active
> > > reference to the policy kobject, so the policy cannot go away until we
> > > are done anyway.  Thus it should be sufficient to use the policy rwsem
> > > for synchronization.
> >
> > I think after the current patchset is applied and we have the inactive
> > policy check in store(), we won't required the dance after all.
>
> I was writing a patch for this and then thought maybe look at mails
> around this time, when you sent the patch, and found the reason why we
> need the locking dance :)
>
> https://lore.kernel.org/lkml/20150729091136.GN7557@n2100.arm.linux.org.uk/
>
> I will add a comment for this now.

Well, again, if we are in store(), we are holding a reference to the
policy kobject, so this is not initialization time.