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Date:   Tue, 10 Oct 2023 15:44:11 -0400
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Subject: Re: [PATCH] cgroup/cpuset: Change nr_deadline_tasks to an atomic_t
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To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Qais Yousef <qyousef@layalina.io>, Hao Luo <haoluo@google.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Xia Fukun <xiafukun@huawei.com>
References: <20231009191515.3262292-1-longman@redhat.com>
 <ZSTiULEnD7SF9n7y@localhost.localdomain>
From:   Waiman Long <longman@redhat.com>
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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.

Cheers,
Longman