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Date:   Thu, 6 Sep 2018 16:13:03 +0200
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Patrick Bellasi <patrick.bellasi@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tejun Heo <tj@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Paul Turner <pjt@google.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Todd Kjos <tkjos@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Steve Muckle <smuckle@google.com>,
        Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values
 into CPU's clamp groups
Message-ID: <20180906141303.GE27626@localhost.localdomain>
References: <20180828135324.21976-1-patrick.bellasi@arm.com>
 <20180828135324.21976-3-patrick.bellasi@arm.com>
 <20180905104545.GB20267@localhost.localdomain>
 <20180906134846.GB25636@e110439-lin>
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On 06/09/18 14:48, Patrick Bellasi wrote:
> Hi Juri!
> 
> On 05-Sep 12:45, Juri Lelli wrote:
> > Hi,
> > 
> > On 28/08/18 14:53, Patrick Bellasi wrote:
> > 
> > [...]
> > 
> > >  static inline int __setscheduler_uclamp(struct task_struct *p,
> > >  					const struct sched_attr *attr)
> > >  {
> > > -	if (attr->sched_util_min > attr->sched_util_max)
> > > -		return -EINVAL;
> > > -	if (attr->sched_util_max > SCHED_CAPACITY_SCALE)
> > > -		return -EINVAL;
> > > +	int group_id[UCLAMP_CNT] = { UCLAMP_NOT_VALID };
> > > +	int lower_bound, upper_bound;
> > > +	struct uclamp_se *uc_se;
> > > +	int result = 0;
> > >  
> > > -	p->uclamp[UCLAMP_MIN] = attr->sched_util_min;
> > > -	p->uclamp[UCLAMP_MAX] = attr->sched_util_max;
> > > +	mutex_lock(&uclamp_mutex);
> > 
> > This is going to get called from an rcu_read_lock() section, which is a
> > no-go for using mutexes:
> > 
> >  sys_sched_setattr ->
> >    rcu_read_lock()
> >    ...
> >    sched_setattr() ->
> >      __sched_setscheduler() ->
> >        ...
> >        __setscheduler_uclamp() ->
> >          ...
> > 	 mutex_lock()
> 
> Rightm, great catch, thanks!
> 
> > Guess you could fix the issue by getting the task struct after find_
> > process_by_pid() in sys_sched_attr() and then calling sched_setattr()
> > after rcu_read_lock() (putting the task struct at the end). Peter
> > actually suggested this mod to solve a different issue.
> 
> I guess you mean something like this ?
> 
> ---8<---
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5792,10 +5792,15 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
>         rcu_read_lock();
>         retval = -ESRCH;
>         p = find_process_by_pid(pid);
> -       if (p != NULL)
> -               retval = sched_setattr(p, &attr);
> +       if (likely(p))
> +               get_task_struct(p);
>         rcu_read_unlock();
>  
> +       if (likely(p)) {
> +               retval = sched_setattr(p, &attr);
> +               put_task_struct(p);
> +       }
> +
>         return retval;
>  }
> ---8<---

This should do the job yes.