On Sun, Mar 30, 2025 at 05:52:39PM -0400, Waiman Long wrote:
> There is a possible race between removing a cgroup diectory that is
> a partition root and the creation of a new partition. The partition
> to be removed can be dying but still online, it doesn't not currently
> participate in checking for exclusive CPUs conflict, but the exclusive
> CPUs are still there in subpartitions_cpus and isolated_cpus. These
> two cpumasks are global states that affect the operation of cpuset
> partitions. The exclusive CPUs in dying cpusets will only be removed
> when cpuset_css_offline() function is called after an RCU delay.
>
> As a result, it is possible that a new partition can be created with
> exclusive CPUs that overlap with those of a dying one. When that dying
> partition is finally offlined, it removes those overlapping exclusive
> CPUs from subpartitions_cpus and maybe isolated_cpus resulting in an
> incorrect CPU configuration.
>
> This bug was found when a warning was triggered in
> remote_partition_disable() during testing because the subpartitions_cpus
> mask was empty.
>
> One possible way to fix this is to iterate the dying cpusets as well and
> avoid using the exclusive CPUs in those dying cpusets. However, this
> can still cause random partition creation failures or other anomalies
> due to racing. A better way to fix this race is to reset the partition
> state at the moment when a cpuset is being killed.
>
> Introduce a new css_killed() CSS function pointer and call it, if
> defined, before setting CSS_DYING flag in kill_css(). Also update the
> css_is_dying() helper to use the CSS_DYING flag introduced by commit
> 33c35aa48178 ("cgroup: Prevent kill_css() from being called more than
> once") for proper synchronization.
>
> Add a new cpuset_css_killed() function to reset the partition state of
> a valid partition root if it is being killed.
>
> Fixes: ee8dde0cd2ce ("cpuset: Add new v2 cpuset.sched.partition flag")
> Signed-off-by: Waiman Long <long...@redhat.com>
Applied to cgroup/for-6.15-fixes.
Thanks.
--
tejun
