linux-kernel - Re: [PATCH v8 5/8] cgroup/cpuset: convert cpuset_mutex to percpu

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Message-ID: <20190628124553.GT3419@hirez.programming.kicks-ass.net>
Date:   Fri, 28 Jun 2019 14:45:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     mingo@...hat.com, rostedt@...dmis.org, tj@...nel.org,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
        bristot@...hat.com, mathieu.poirier@...aro.org, lizefan@...wei.com,
        cgroups@...r.kernel.org
Subject: Re: [PATCH v8 5/8] cgroup/cpuset: convert cpuset_mutex to
 percpu_rwsem

On Fri, Jun 28, 2019 at 10:06:15AM +0200, Juri Lelli wrote:
> @@ -2154,7 +2154,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>  	cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
>  	cs = css_cs(css);
>  
> -	mutex_lock(&cpuset_mutex);
> +	percpu_down_read(&cpuset_rwsem);
>  
>  	/* allow moving tasks into an empty cpuset if on default hierarchy */
>  	ret = -ENOSPC;
> @@ -2178,7 +2178,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>  	cs->attach_in_progress++;
>  	ret = 0;
>  out_unlock:
> -	mutex_unlock(&cpuset_mutex);
> +	percpu_up_read(&cpuset_rwsem);
>  	return ret;
>  }
>  
> @@ -2188,9 +2188,9 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
>  
>  	cgroup_taskset_first(tset, &css);
>  
> -	mutex_lock(&cpuset_mutex);
> +	percpu_down_read(&cpuset_rwsem);
>  	css_cs(css)->attach_in_progress--;
> -	mutex_unlock(&cpuset_mutex);
> +	percpu_up_read(&cpuset_rwsem);
>  }

These are the only percpu_down_read()s introduced in this patch; are we
sure this is correct? Specifically, what serializes
->attach_in_progress?