[PATCH next] cpuset: Treat tasks in attaching process as populated

[Chen Ridong]

From: Chen Ridong <chenridong@xxxxxxxxxx>

Currently, the check for whether a partition is populated does not
account for tasks in the process of attaching. This is a corner case
that can leave a task stuck in a partition with no effective CPUs.

The race condition occurs as follows:

cpu0				cpu1
				//cpuset A  with cpu N
migrate task p to A
cpuset_can_attach
// with effective cpus
// check ok

// cpuset_mutex is not held	// clear cpuset.cpus.exclusive
				// making effective cpus empty
				update_exclusive_cpumask
				// tasks_nocpu_error check ok
				// empty effective cpus, partition valid
cpuset_attach
...
// task p stays in A, with non-effective cpus.

To fix this issue, this patch introduces cs_is_populated, which considers
tasks in the attaching process. This new helper is used in validate_change
and partition_is_populated.

Fixes: e2d59900d936 ("cgroup/cpuset: Allow no-task partition to have empty cpuset.cpus.effective")
Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
---
 kernel/cgroup/cpuset.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index daf813386260..3ffa0c02a020 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -356,6 +356,13 @@ static inline bool is_in_v2_mode(void)
 	      (cpuset_cgrp_subsys.root->flags & CGRP_ROOT_CPUSET_V2_MODE);
 }
 
+static inline bool cs_is_populated(struct cpuset *cs)
+{
+	/* Tasks in the process of attaching should be considered as populated */
+	return cgroup_is_populated(cs->css.cgroup) ||
+		cs->attach_in_progress;
+}
+
 /**
  * partition_is_populated - check if partition has tasks
  * @cs: partition root to be checked
@@ -376,7 +383,8 @@ static inline bool partition_is_populated(struct cpuset *cs,
 	struct cgroup_subsys_state *css;
 	struct cpuset *child;
 
-	if (cs->css.cgroup->nr_populated_csets)
+	if (cs->css.cgroup->nr_populated_csets ||
+	    cs->attach_in_progress)
 		return true;
 
 	rcu_read_lock();
@@ -385,7 +393,7 @@ static inline bool partition_is_populated(struct cpuset *cs,
 			continue;
 		if (is_partition_valid(child))
 			continue;
-		if (cgroup_is_populated(child->css.cgroup)) {
+		if (cs_is_populated(child)) {
 			rcu_read_unlock();
 			return true;
 		}
@@ -670,7 +678,7 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
 	 * be changed to have empty cpus_allowed or mems_allowed.
 	 */
 	ret = -ENOSPC;
-	if ((cgroup_is_populated(cur->css.cgroup) || cur->attach_in_progress)) {
+	if (cs_is_populated(cur)) {
 		if (!cpumask_empty(cur->cpus_allowed) &&
 		    cpumask_empty(trial->cpus_allowed))
 			goto out;
-- 
2.34.1