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SubjectRe: [PATCH] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()
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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@redhat.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

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