linux-kernel - Re: [PATCH] cgroup/cpuset: Change nr_deadline_tasks to an atomic

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Date:   Wed, 11 Oct 2023 10:14:52 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Qais Yousef <qyousef@...alina.io>, Hao Luo <haoluo@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Xia Fukun <xiafukun@...wei.com>
Subject: Re: [PATCH] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t
 value

On 10/10/23 16:03, Waiman Long wrote:
> 
> On 10/10/23 15:44, Waiman Long wrote:
> > 
> > On 10/10/23 01:34, Juri Lelli wrote:
> > > Hi,
> > > 
> > > On 09/10/23 15:15, Waiman Long wrote:
> > > > The nr_deadline_tasks field in cpuset structure was introduced by
> > > > commit 6c24849f5515 ("sched/cpuset: Keep track of SCHED_DEADLINE task
> > > > in cpusets"). Unlike nr_migrate_dl_tasks which is only modified under
> > > > cpuset_mutex, nr_deadline_tasks can be updated in various contexts
> > > > under different locks. As a result, data races may happen that cause
> > > > incorrect value to be stored in nr_deadline_tasks leading to incorrect
> > > Could you please make an example of such data races?
> > 
> > Since update to cs->nr_deadline_tasks is not protected by a single lock,
> > it is possible that multiple CPUs may try to modify it at the same
> > time.  It is possible that nr_deadline_tasks++ and nr_deadline_tasks--
> > can be done in a single instruction like in x86 and hence atomic.
> > However, operation like "cs->nr_deadline_tasks +=
> > cs->nr_migrate_dl_tasks" is likely a RMW operation and so is subjected
> > to racing. It is mostly theoretical, but probably not impossible.
> 
> Sorry, even increment and decrement operators are not atomic.
> 
> inc_dl_tasks_cs() is only called from switched_to_dl() in deadline.c which
> is protected by the rq_lock, but there are multiple rq's. dec_dl_tasks_cs()
> is called from switched_from_dl() in deadline.c and cgroup_exit() in
> cgroup.c. The later one is protected by css_set_lock. The other place where
> nr_deadline_tasks can be changed is in cpuset_attach() protected by
> cpuset_mutex.

So, let's see. :)

switched_to_dl(), switched_from_dl() and cpuset_attach() should all be
protected (for DEADLINE tasks) by cpuset_mutex, see [1] for the former
two.

What leaves me perplexed is indeed cgroup_exit(), which seems to operate
under css_set_lock as you say. I however wonder why is that not racy
already wrt, say, cpuset_attach() which AFAIU uses css information w/o
holding css_set_lock?

Thanks,
Juri

1 - https://elixir.bootlin.com/linux/latest/source/kernel/sched/core.c#L7688