[PATCH v4 7/7] cgroup/cpuset: Publish exclusive_cpus before partition updates

[Guopeng Zhang <guopeng.zhang@linux.dev>]

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

update_exclusive_cpumask() calls partition_cpus_change() before publishing
the new exclusive_cpus mask. When an invalid partition is recovered,
update_parent_effective_cpumask() therefore gets the old mask from
user_xcpus(). If that mask contains only offline CPUs and the partition is
populated, the valid replacement is rejected with PERR_NOCPUS.

This can be reproduced on a cgroup v2 system with CPUs 1-4 online:

    cd /sys/fs/cgroup
    echo +cpuset > cgroup.subtree_control
    mkdir exclusive-repro
    echo 1-4 > exclusive-repro/cpuset.cpus
    echo isolated > exclusive-repro/cpuset.cpus.partition
    echo +cpuset > exclusive-repro/cgroup.subtree_control
    mkdir exclusive-repro/child
    echo 4 > exclusive-repro/child/cpuset.cpus.exclusive
    echo isolated > exclusive-repro/child/cpuset.cpus.partition
    sleep 1000 & task=$!
    echo $task > exclusive-repro/child/cgroup.procs
    echo 0 > /sys/devices/system/cpu/cpu4/online
    echo 3 > exclusive-repro/child/cpuset.cpus.exclusive
    cat exclusive-repro/child/cpuset.cpus.partition
    echo 1 > /sys/devices/system/cpu/cpu4/online
    kill $task

Without this fix, the child remains invalid after CPU 3 is written because
the recovery path still validates the old CPU 4 mask.

Publish exclusive_cpus after all fallible preparation and before
partition_cpus_change(), as update_cpumask() already does for cpus_allowed.
Keep effective_xcpus unchanged until the partition update has computed the
old-to-new difference.

Fixes: c49b5e89c45f ("cpuset: use partition_cpus_change for setting exclusive cpus")
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 kernel/cgroup/cpuset.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 80a709bfa4b7..94c56ec8d4f8 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2690,10 +2690,18 @@ static int update_exclusive_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 		return -ENOMEM;
 
 	trialcs->prs_err = PERR_NONE;
-	partition_cpus_change(cs, trialcs, &tmp);
 
+	/*
+	 * partition_cpus_change() may inspect user_xcpus(cs). Publish the
+	 * configured mask that was used to compute trialcs->effective_xcpus.
+	 */
 	spin_lock_irq(&callback_lock);
 	cpumask_copy(cs->exclusive_cpus, trialcs->exclusive_cpus);
+	spin_unlock_irq(&callback_lock);
+
+	partition_cpus_change(cs, trialcs, &tmp);
+
+	spin_lock_irq(&callback_lock);
 	cpumask_copy(cs->effective_xcpus, trialcs->effective_xcpus);
 	if ((old_prs > 0) && !is_partition_valid(cs))
 		reset_partition_data(cs);
-- 
2.43.0