Re: [PATCH v6 4/5] sched: Handle set_cpus_allowed_ptr() & sched_setaffinity() race

[Peter Zijlstra]

On Thu, Aug 25, 2022 at 09:01:18PM -0400, Waiman Long wrote:
> Racing is possible between set_cpus_allowed_ptr() and sched_setaffinity()
> or between multiple sched_setaffinity() calls from different CPUs. To
> resolve these race conditions, we need to update both user_cpus_ptr
> and cpus_mask in a single lock critical section instead of separated
> ones. This requires moving the user_cpus_ptr update to
> affine_move_task() before doing task_rq_unlock().
> 
> A new argument puser_mask is added to affine_move_task(),
> __set_cpus_allowed_ptr_locked() and __set_cpus_allowed_ptr() to do that.
> 
> Ideally, user_cpus_ptr should only be updated if the sched_setaffinity()
> is successful. However, this patch will update user_cpus_ptr when the
> first call to __set_cpus_allowed_ptr() is successful. However, if there
> is racing between sched_setaffinity() and cpuset update, the subsequent
> calls to __set_cpus_allowed_ptr() may fail but the user_cpus_ptr will
> still be updated in this corner case.

Urgh, this is a bit weird, to have a fix for a patch in the same series.