[PATCH v3 3/7] cgroup: protect cgroup->nr_(dying_)descendants by css_set_lock

[Roman Gushchin]

Now the number of descendant cgroups and the number of dying
descendant cgroups are 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.

Signed-off-by: Roman Gushchin <guro@xxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: kernel-team@xxxxxx
---
 include/linux/cgroup-defs.h |  3 +++
 kernel/cgroup/cgroup.c      | 20 ++++++++++++++++----
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 22254c1fe1c5..9e77559c7f49 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -346,6 +346,9 @@ 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 css_set_lock.
 	 */
 	int nr_descendants;
 	int nr_dying_descendants;
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index ef3442555b32..2241cb1d1238 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3409,11 +3409,15 @@ static int cgroup_events_show(struct seq_file *seq, void *v)
 static int cgroup_stat_show(struct seq_file *seq, void *v)
 {
 	struct cgroup *cgroup = seq_css(seq)->cgroup;
+	int nr_descendants, nr_dying_descendants;
 
-	seq_printf(seq, "nr_descendants %d\n",
-		   cgroup->nr_descendants);
-	seq_printf(seq, "nr_dying_descendants %d\n",
-		   cgroup->nr_dying_descendants);
+	spin_lock_irq(&css_set_lock);
+	nr_descendants = cgroup->nr_descendants;
+	nr_dying_descendants = cgroup->nr_dying_descendants;
+	spin_unlock_irq(&css_set_lock);
+
+	seq_printf(seq, "nr_descendants %d\n", nr_descendants);
+	seq_printf(seq, "nr_dying_descendants %d\n", nr_dying_descendants);
 
 	return 0;
 }
@@ -4684,9 +4688,11 @@ 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;
@@ -4899,12 +4905,14 @@ static struct cgroup *cgroup_create(struct cgroup *parent)
 	if (ret)
 		goto out_idr_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);
@@ -4956,6 +4964,7 @@ static bool cgroup_check_hierarchy_limits(struct cgroup *parent)
 
 	lockdep_assert_held(&cgroup_mutex);
 
+	spin_lock_irq(&css_set_lock);
 	for (cgroup = parent; cgroup; cgroup = cgroup_parent(cgroup)) {
 		if (cgroup->nr_descendants >= cgroup->max_descendants)
 			goto fail;
@@ -4968,6 +4977,7 @@ static bool cgroup_check_hierarchy_limits(struct cgroup *parent)
 
 	ret = true;
 fail:
+	spin_unlock_irq(&css_set_lock);
 	return ret;
 }
 
@@ -5187,10 +5197,12 @@ 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);
 
-- 
2.17.2