From mboxrd@z Thu Jan  1 00:00:00 1970
From: Patrick Bellasi <patrick.bellasi@arm.com>
Subject: Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values
 into CPU's clamp groups
Date: Thu, 6 Sep 2018 14:48:46 +0100
Message-ID: <20180906134846.GB25636@e110439-lin>
References: <20180828135324.21976-1-patrick.bellasi@arm.com>
 <20180828135324.21976-3-patrick.bellasi@arm.com>
 <20180905104545.GB20267@localhost.localdomain>
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To: Juri Lelli <juri.lelli@redhat.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>
List-Id: linux-pm@vger.kernel.org

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<---

Cheers,
Patrick

-- 
#include <best/regards.h>

Patrick Bellasi