linux-kernel - Re: [PATCH v3 2/3] sched: Provide copy_user_cpus

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Date:   Mon, 15 Aug 2022 09:37:02 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, 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>,
        Tejun Heo <tj@...nel.org>, 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 v3 2/3] sched: Provide copy_user_cpus_mask() to copy out
 user mask

On 8/15/22 04:58, Peter Zijlstra wrote:
> On Fri, Aug 12, 2022 at 04:39:28PM -0400, Waiman Long wrote:
>> Since accessing the content of the user_cpus_ptr requires lock protection
>> to ensure its validity, provide a helper function copy_user_cpus_mask()
>> to facilitate its reading.
> Sure, but this is atrocious.
>
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2619,6 +2619,24 @@ void release_user_cpus_ptr(struct task_struct *p)
>>   	kfree(clear_user_cpus_ptr(p));
>>   }
>>   
>> +/*
>> + * Return the copied mask pointer or NULL if user mask not available.
>> + */
>> +struct cpumask *copy_user_cpus_mask(struct task_struct *p,
>> +				    struct cpumask *user_mask)
>> +{
>> +	struct rq_flags rf;
>> +	struct rq *rq = task_rq_lock(p, &rf);
>> +	struct cpumask *mask = NULL;
>> +
>> +	if (p->user_cpus_ptr) {
>> +		cpumask_copy(user_mask, p->user_cpus_ptr);
>> +		mask = user_mask;
>> +	}
>> +	task_rq_unlock(rq, p, &rf);
>> +	return mask;
>> +}
> For reading the mask you only need one of those locks, and I would
> suggest p->pi_lock is much less contended than rq->lock.
>
Right. pi_lock should be enough for read access. Will make the change.

Thanks,
Longman