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Message-ID: <CAJZ5v0jbeYvCV0ZxEPVAk=1Lvbd3wxSrq72jUxBLqvHm1mTStQ@mail.gmail.com>
Date: Tue, 29 Aug 2023 20:20:53 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Viresh Kumar <viresh.kumar@...aro.org>,
Liao Chang <liaochang1@...wei.com>
Cc: srivatsa.bhat@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] cpufreq: Fix the race condition while updating the
transition_task of policy
On Tue, Aug 29, 2023 at 9:20 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 29-08-23, 07:03, Liao Chang wrote:
> > The field 'transition_task' of policy structure is used to track the
> > task which is performing the frequency transition. Using this field to
> > print a warning once detect a case where the same task is calling
> > _begin() again before completing the preivous frequency transition via
> > the _end().
> >
> > However, there is a potential race condition in _end() and _begin() APIs
> > while updating the field 'transition_task' of policy, the scenario is
> > depicted below:
> >
> > Task A Task B
> >
> > /* 1st freq transition */
> > Invoke _begin() {
> > ...
> > ...
> > }
> > /* 2nd freq transition */
> > Invoke _begin() {
> > ... //waiting for A to
> > ... //clear
> > ... //transition_ongoing
> > ... //in _end() for
> > ... //the 1st transition
> > |
> > Change the frequency |
> > |
> > Invoke _end() { |
> > ... |
> > ... |
> > transition_ongoing = false; V
> > transition_ongoing = true;
> > transition_task = current;
> > transition_task = NULL;
> > ... //A overwrites the task
> > ... //performing the transition
> > ... //result in error warning.
> > }
> >
> > To fix this race condition, the transition_lock of policy structure is
> > now acquired before updating policy structure in _end() API. Which ensure
> > that only one task can update the 'transition_task' field at a time.
> >
> > Link: https://lore.kernel.org/all/b3c61d8a-d52d-3136-fbf0-d1de9f1ba411@huawei.com/
> > Fixes: ca654dc3a93d ("cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end")
> > Signed-off-by: Liao Chang <liaochang1@...wei.com>
> > ---
> > drivers/cpufreq/cpufreq.c | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index 26fa99d46684..5c09a4936699 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -455,8 +455,10 @@ void cpufreq_freq_transition_end(struct cpufreq_policy *policy,
> > policy->cur,
> > policy->cpuinfo.max_freq);
> >
> > + spin_lock(&policy->transition_lock);
> > policy->transition_ongoing = false;
> > policy->transition_task = NULL;
> > + spin_unlock(&policy->transition_lock);
> >
> > wake_up(&policy->transition_wait);
> > }
>
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
Applied as 6.6-rc material, thanks!
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