Re: [PATCH v3 2/3] sched: Provide copy_user_cpus_mask() to copy out user mask

Waiman Long
2022-08-15T13:37:02Z

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