Messages in this thread | | | Date | Wed, 23 Nov 2022 12:20:00 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr() |
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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 :-(
> > + /* > + * 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().
> 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); }
?
> raw_spin_unlock_irqrestore(&src->pi_lock, flags); > + > + if (unlikely(user_mask)) > + kfree(user_mask); > + > return 0; > } > > -- > 2.31.1 >
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