[PATCH 2/2] cgroup/cpuset: Fix invalid local partition recovery

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

From: Guopeng Zhang <zhangguopeng@kylinos.cn>

An invalid local partition may fail to recover after its exclusive CPU
list is changed to a valid one.

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 0-7 > A/cpuset.cpus
    echo 0-7 > A/cpuset.cpus.exclusive
    echo root > A/cpuset.cpus.partition
    cat A/cpuset.cpus.partition
    echo 1 > A/cpuset.cpus.exclusive
    cat A/cpuset.cpus.partition

The first read reports root invalid because all CPUs were requested.
After the mask is changed to CPU 1, the partition should become valid,
but it remains invalid with PERR_NOCPUS.

update_exclusive_cpumask() calls partition_cpus_change() before updating
cs->exclusive_cpus. update_parent_effective_cpumask() then gets the
old 0-7 mask from user_xcpus() and sets PERR_NOCPUS even though the new
mask leaves CPUs for the parent.

Move the exclusive_cpus update before partition_cpus_change() so that
the partition update sees the new mask.

Fixes: 46c521bac592 ("cgroup/cpuset: Enable invalid to valid local partition transition")
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
---
 kernel/cgroup/cpuset.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index de7553459adc..e96bc66ffa07 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2546,10 +2546,14 @@ static int update_exclusive_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 		return -ENOMEM;
 
 	trialcs->prs_err = PERR_NONE;
-	partition_cpus_change(cs, trialcs, &tmp);
 
 	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