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Subject[PATCH v10 3/9] cgroup: protect cgroup->nr_(dying_)descendants by css_set_lock
Date
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: Roman Gushchin <guro@fb.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com
---
include/linux/cgroup-defs.h | 5 +++++
kernel/cgroup/cgroup.c | 6 ++++++
2 files changed, 11 insertions(+)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 1c70803e9f77..7d57890cec67 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -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;
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 3008ea684aa0..786ceef2f222 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -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);

--
2.20.1
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