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Commit 4dcabece authored by Roman Gushchin's avatar Roman Gushchin Committed by Tejun Heo
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cgroup: protect cgroup->nr_(dying_)descendants by css_set_lock 



The number of descendant cgroups and the number of dying
descendant cgroups are currently synchronized using the cgroup_mutex.

The number of descendant cgroups will be required by the cgroup v2
freezer, which will use it to determine if a cgroup is frozen
(depending on total number of descendants and number of frozen
descendants). It's not always acceptable to grab the cgroup_mutex,
especially from quite hot paths (e.g. exit()).

To avoid this, let's additionally synchronize these counters using
the css_set_lock.

So, it's safe to read these counters with either cgroup_mutex or
css_set_lock locked, and for changing both locks should be acquired.

Signed-off-by: default avatarRoman Gushchin <guro@fb.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com
parent aade7f9e

include/linux/cgroup-defs.h

+5 −0
Original line number Diff line number Diff line
@@ -349,6 +349,11 @@ struct cgroup { 
	 * Dying cgroups are cgroups which were deleted by a user,

	 * but are still existing because someone else is holding a reference.

	 * max_descendants is a maximum allowed number of descent cgroups.

	 *

	 * nr_descendants and nr_dying_descendants are protected

	 * by cgroup_mutex and css_set_lock. It's fine to read them holding

	 * any of cgroup_mutex and css_set_lock; for writing both locks

	 * should be held.

	 */

	int nr_descendants;

	int nr_dying_descendants;

kernel/cgroup/cgroup.c

+6 −0
Original line number Diff line number Diff line
@@ -4811,9 +4811,11 @@ static void css_release_work_fn(struct work_struct *work) 
		if (cgroup_on_dfl(cgrp))

			cgroup_rstat_flush(cgrp);



		spin_lock_irq(&css_set_lock);

		for (tcgrp = cgroup_parent(cgrp); tcgrp;

		     tcgrp = cgroup_parent(tcgrp))

			tcgrp->nr_dying_descendants--;

		spin_unlock_irq(&css_set_lock);



		cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);

		cgrp->id = -1;
@@ -5031,12 +5033,14 @@ static struct cgroup *cgroup_create(struct cgroup *parent) 
	if (ret)

		goto out_psi_free;



	spin_lock_irq(&css_set_lock);

	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {

		cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;



		if (tcgrp != cgrp)

			tcgrp->nr_descendants++;

	}

	spin_unlock_irq(&css_set_lock);



	if (notify_on_release(parent))

		set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
@@ -5321,10 +5325,12 @@ static int cgroup_destroy_locked(struct cgroup *cgrp) 
	if (parent && cgroup_is_threaded(cgrp))

		parent->nr_threaded_children--;



	spin_lock_irq(&css_set_lock);

	for (tcgrp = cgroup_parent(cgrp); tcgrp; tcgrp = cgroup_parent(tcgrp)) {

		tcgrp->nr_descendants--;

		tcgrp->nr_dying_descendants++;

	}

	spin_unlock_irq(&css_set_lock);



	cgroup1_check_for_release(parent);