Re: [PATCH v2] cpufreq: Fix the race condition while updating the transition_task of policy

[Rafael J. Wysocki]

On Tue, Aug 29, 2023 at 9:20 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> 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@xxxxxxxxxx/
> > Fixes: ca654dc3a93d ("cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end")
> > Signed-off-by: Liao Chang <liaochang1@xxxxxxxxxx>
> > ---
> >  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@xxxxxxxxxx>

Applied as 6.6-rc material, thanks!