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cpuset: Treat cpusets in attaching as populated

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Currently, the check for whether a partition is populated does not
account for tasks in the cpuset 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 cpuset. This new helper is used in validate_change
and partition_is_populated.

Fixes: e2d59900d9 ("cgroup/cpuset: Allow no-task partition to have empty cpuset.cpus.effective")
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Chen Ridong
2025-11-14 02:08:47 +00:00
committed by Tejun Heo
parent 318e18ed22
commit b1bcaed1e3
1 changed files with 27 additions and 8 deletions
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35
kernel/cgroup/cpuset.c
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@@ -356,6 +356,15 @@ static inline bool is_in_v2_mode(void)
(cpuset_cgrp_subsys.root->flags & CGRP_ROOT_CPUSET_V2_MODE);
}
static inline bool cpuset_is_populated(struct cpuset *cs)
{
lockdep_assert_held(&cpuset_mutex);
/* Cpusets 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
@@ -373,19 +382,29 @@ static inline bool is_in_v2_mode(void)
static inline bool partition_is_populated(struct cpuset *cs,
struct cpuset *excluded_child)
{
struct cgroup_subsys_state *css;
struct cpuset *child;
struct cpuset *cp;
struct cgroup_subsys_state *pos_css;
if (cs->css.cgroup->nr_populated_csets)
/*
* We cannot call cs_is_populated(cs) directly, as
* nr_populated_domain_children may include populated
* csets from descendants that are partitions.
*/
if (cs->css.cgroup->nr_populated_csets ||
cs->attach_in_progress)
return true;
rcu_read_lock();
cpuset_for_each_child(child, css, cs) {
if (child == excluded_child)
cpuset_for_each_descendant_pre(cp, pos_css, cs) {
if (cp == cs || cp == excluded_child)
continue;
if (is_partition_valid(child))
if (is_partition_valid(cp)) {
pos_css = css_rightmost_descendant(pos_css);
continue;
if (cgroup_is_populated(child->css.cgroup)) {
}
if (cpuset_is_populated(cp)) {
rcu_read_unlock();
return true;
}
@@ -670,7 +689,7 @@ 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 (cpuset_is_populated(cur)) {
if (!cpumask_empty(cur->cpus_allowed) &&
cpumask_empty(trial->cpus_allowed))
goto out;