linux-kernel - Re: [PATCH] sched: Fix NULL user_cpus_ptr check in dup_user_cpus

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Date:   Wed, 23 Nov 2022 07:50:31 -0500
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>,
        Phil Auld <pauld@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Fix NULL user_cpus_ptr check in
 dup_user_cpus_ptr()


On 11/23/22 06:20, Peter Zijlstra wrote:
> On Tue, Nov 22, 2022 at 02:06:53PM -0500, Waiman Long wrote:
>> In general, a non-null user_cpus_ptr will remain set until the task dies.
>> A possible exception to this is the fact that do_set_cpus_allowed()
>> will clear a non-null user_cpus_ptr. To allow this possible racing
>> condition, we need to check for NULL user_cpus_ptr under the pi_lock
>> before duping the user mask.
>>
>> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   kernel/sched/core.c | 23 +++++++++++++++++++++--
>>   1 file changed, 21 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 8df51b08bb38..f447a6285ea2 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2625,7 +2625,14 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>>   		      int node)
>>   {
>>   	unsigned long flags;
>> +	cpumask_t *user_mask = NULL;
> The inverse xmas tree is sad :-(
Right. The inverse xmas tree rule. Will fix that.
>
>>   
>> +	/*
>> +	 * If there is a concurrent sched_setaffinity(), we may miss the
>> +	 * newly updated user_cpus_ptr. However, a non-NULL user_cpus_ptr
>> +	 * is relatively unlikely and it is not worth the extra overhead
>> +	 * of taking the pi_lock on every fork/clone.
>> +	 */
> I think the correct argument is saying the thing is racy and loosing the
> race is a valid situation. IOW, this is the same as if the concurrent
> sched_setaffinity() happens after fork().
Good point, will update the comment.
>>   	if (!src->user_cpus_ptr)
>>   		return 0;
>>   
>> @@ -2633,10 +2640,22 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>>   	if (!dst->user_cpus_ptr)
>>   		return -ENOMEM;
>>   
>> -	/* Use pi_lock to protect content of user_cpus_ptr */
>> +	/*
>> +	 * Use pi_lock to protect content of user_cpus_ptr
>> +	 *
>> +	 * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
>> +	 * do_set_cpus_allowed().
>> +	 */
>>   	raw_spin_lock_irqsave(&src->pi_lock, flags);
>> +	if (src->user_cpus_ptr)
>> +		cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
>> +	else
>> +		swap(dst->user_cpus_ptr, user_mask);
> Uhhhh, did you mean to write:
>
> 	if (src->user_cpus_ptr) {
> 		cpumask_copy(user_mask, src->user_cpus_ptr);
> 		swap(dst->user_cpus_ptr, user_mask);
> 	}
>
> ?

Not really. The point is that dst->user_cpus_ptr has been allocated. If 
src->user_cpus_ptr turns out to be NULL, we need to clear 
dst->user_cpus_ptr which is what the swap() does and then free that 
memory after unlock. Will add a comment to make this point clear.

Cheers,
Longman