linux-kernel - Re: [PATCH v5 3/3] cgroup/cpuset: Keep user set cpus affinity

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Message-ID: <Yvv66EWygCwHUCqy@slm.duckdns.org>
Date:   Tue, 16 Aug 2022 10:15:36 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Will Deacon <will@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 3/3] cgroup/cpuset: Keep user set cpus affinity

On Tue, Aug 16, 2022 at 03:27:34PM -0400, Waiman Long wrote:
> +static int cpuset_set_cpus_allowed_ptr(struct task_struct *p,
> +				       const struct cpumask *mask)
> +{
> +	cpumask_var_t new_mask;
> +	int ret;
> +
> +	if (!READ_ONCE(p->user_cpus_ptr)) {
> +		ret = set_cpus_allowed_ptr(p, mask);
> +		/*
> +		 * If user_cpus_ptr becomes set now, we are racing with
> +		 * a concurrent sched_setaffinity(). So use the newly
> +		 * set user_cpus_ptr and retry again.
> +		 *
> +		 * TODO: We cannot detect change in the cpumask pointed to
> +		 * by user_cpus_ptr. We will have to add a sequence number
> +		 * if such a race needs to be addressed.
> +		 */

This is too ugly and obviously broken. Let's please do it properly.

Thanks.

-- 
tejun