cgroup/cpuset: Prevent recursive call of remote_cpus_update() via update_cpumasks_hier()

Guopeng reports that changing the cpumask of a remote partition root
may set the effective_cpus of the top_cpuset incorrectly. It is
due to the following call sequence:

  update_cpumask()
   -> partition_cpus_change()
    -> remote_cpus_update()
     -> update_sibling_cpumasks()
      -> update_cpumasks_hier()
       -> remote_cpus_update()

The second remote_cpus_update() call passes in the cs->effective_xcpus
instead of the trialcs->effective_xcpus of the first remote_cpus_update()
call.  As a result, the second call can revert what has been done in
the first call leading to incorrect effective_cpus of the top_cpuset
as well as subpartitions_cpus.

Fix that by adding a new remote_partition_updating flag into the cpuset
structure to indicate that a remote partition is being updated and
update_cpumasks_hier() should skip the whole subtree as update_cpumask()
or update_exclusive_cpumask() which calls partition_cpus_change()
will eventually calls update_cpumasks_hier() for that subtree anyway.

This new flag is currently set only in remote_cpus_update() to prevent
recursive call of remote_cpus_update() on the same cpuset.

A test case is added to test_cpuset_prs.sh to verify this change.

Reported-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Link: https://lore.kernel.org/lkml/20261010075947.333549-2-guopeng.zhang@linux.dev
Reviewed-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Tested-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Fixes: f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check() & make update_cpumasks_hier() handle remote partition")
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

0 files changed, 0 insertions, 0 deletions