[PATCH 1/2] cgroup/cpuset: Fix cpuset.cpus update in a remote partition

[Guopeng Zhang]

From: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>

When the CPU list of a remote partition is reduced, a removed CPU may
remain unavailable to the top cpuset.

This can be reproduced on a cgroup v2 system with CPUs 0-7 online:

    cd /sys/fs/cgroup
    echo +cpuset > cgroup.subtree_control
    mkdir A
    echo 1-7 > A/cpuset.cpus
    echo 1-7 > A/cpuset.cpus.exclusive
    echo +cpuset > A/cgroup.subtree_control
    mkdir A/B
    echo 1-4 > A/B/cpuset.cpus
    echo root > A/B/cpuset.cpus.partition
    echo 1-3 > A/B/cpuset.cpus
    cat cpuset.cpus.effective

The last command should show 0,4-7, but CPU 4 is missing.

update_cpumask() calls partition_cpus_change() while cs->cpus_allowed
still contains the old mask. remote_cpus_update() first releases CPU 4,
but the hierarchy update revisits B and compute_excpus() still sees the
old mask, so CPU 4 is removed from the top cpuset again.

Move the cpus_allowed update before partition_cpus_change() so that
the hierarchy update sees the new mask when it revisits the cpuset.

Fixes: f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check() & make update_cpumasks_hier() handle remote partition")
Signed-off-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
---
 kernel/cgroup/cpuset.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 0ebba2d646c3..de7553459adc 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2478,10 +2478,13 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	 */
 	force = !cpumask_equal(cs->effective_xcpus, trialcs->effective_xcpus);
 
+	spin_lock_irq(&callback_lock);
+	cpumask_copy(cs->cpus_allowed, trialcs->cpus_allowed);
+	spin_unlock_irq(&callback_lock);
+
 	partition_cpus_change(cs, trialcs, &tmp);
 
 	spin_lock_irq(&callback_lock);
-	cpumask_copy(cs->cpus_allowed, trialcs->cpus_allowed);
 	cpumask_copy(cs->effective_xcpus, trialcs->effective_xcpus);
 	if ((old_prs > 0) && !is_partition_valid(cs))
 		reset_partition_data(cs);
-- 
2.43.0