Commit b1bcaed1 authored by Chen Ridong's avatar Chen Ridong Committed by Tejun Heo
Browse files

cpuset: Treat cpusets in attaching as populated 



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: e2d59900 ("cgroup/cpuset: Allow no-task partition to have empty cpuset.cpus.effective")
Signed-off-by: default avatarChen Ridong <chenridong@huawei.com>
Reviewed-by: default avatarWaiman Long <longman@redhat.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 318e18ed

kernel/cgroup/cpuset.c

+27 −8
Original line number Diff line number Diff line
@@ -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;