cgroup/cpuset: Fix partition root's cpuset.cpus update bug

It was found that commit 7a2127e6 ("cpuset: Call
set_cpus_allowed_ptr() with appropriate mask for task") introduced a bug
that corrupted "cpuset.cpus" of a partition root when it was updated.

It is because the tmp->new_cpus field of the passed tmp parameter
of update_parent_subparts_cpumask() should not be used at all as
it contains important cpumask data that should not be overwritten.
Fix it by using tmp->addmask instead.

Also update update_cpumask() to make sure that trialcs->cpu_allowed
will not be corrupted until it is no longer needed.

Fixes: 7a2127e6 ("cpuset: Call set_cpus_allowed_ptr() with appropriate mask for task")
Signed-off-by: Waiman Long <longman@redhat.com>
Cc: stable@vger.kernel.org # v6.2+
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cpuset.c

@@ -1513,7 +1513,7 @@ static int update_parent_subparts_cpumask(struct cpuset *cs, int cmd,
 	spin_unlock_irq(&callback_lock);
 
 	if (adding || deleting)
-		update_tasks_cpumask(parent, tmp->new_cpus);
+		update_tasks_cpumask(parent, tmp->addmask);
 
 	/*
 	 * Set or clear CS_SCHED_LOAD_BALANCE when partcmd_update, if necessary.
@@ -1770,10 +1770,13 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	/*
 	 * Use the cpumasks in trialcs for tmpmasks when they are pointers
 	 * to allocated cpumasks.
+	 *
+	 * Note that update_parent_subparts_cpumask() uses only addmask &
+	 * delmask, but not new_cpus.
 	 */
 	tmp.addmask  = trialcs->subparts_cpus;
 	tmp.delmask  = trialcs->effective_cpus;
-	tmp.new_cpus = trialcs->cpus_allowed;
+	tmp.new_cpus = NULL;
 #endif
 
 	retval = validate_change(cs, trialcs);
@@ -1838,6 +1841,11 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	}
 	spin_unlock_irq(&callback_lock);
 
+#ifdef CONFIG_CPUMASK_OFFSTACK
+	/* Now trialcs->cpus_allowed is available */
+	tmp.new_cpus = trialcs->cpus_allowed;
+#endif
+
 	/* effective_cpus will be updated here */
 	update_cpumasks_hier(cs, &tmp, false);