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cgroup: protect cgroup->nr_(dying_)descendants by css_set_lock

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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>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: kernel-team@fb.com
This commit is contained in:
Roman Gushchin 2019-04-19 10:03:03 -07:00 • committed by Tejun Heo
parent aade7f9efb
commit 4dcabece4c
2 changed files with 11 additions and 0 deletions
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5
include/linux/cgroup-defs.h
View file

@ -349,6 +349,11 @@ struct cgroup {
* Dying cgroups are cgroups which were deleted by a user, * Dying cgroups are cgroups which were deleted by a user,
* but are still existing because someone else is holding a reference. * but are still existing because someone else is holding a reference.
* max_descendants is a maximum allowed number of descent cgroups. * 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_descendants;
int nr_dying_descendants; int nr_dying_descendants;

6
kernel/cgroup/cgroup.c
View file

@ -4811,9 +4811,11 @@ static void css_release_work_fn(struct work_struct *work)
if (cgroup_on_dfl(cgrp)) if (cgroup_on_dfl(cgrp))
cgroup_rstat_flush(cgrp); cgroup_rstat_flush(cgrp);
spin_lock_irq(&css_set_lock);
for (tcgrp = cgroup_parent(cgrp); tcgrp; for (tcgrp = cgroup_parent(cgrp); tcgrp;
tcgrp = cgroup_parent(tcgrp)) tcgrp = cgroup_parent(tcgrp))
tcgrp->nr_dying_descendants--; tcgrp->nr_dying_descendants--;
spin_unlock_irq(&css_set_lock);
cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id); cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
cgrp->id = -1; cgrp->id = -1;
@ -5031,12 +5033,14 @@ static struct cgroup *cgroup_create(struct cgroup *parent)
if (ret) if (ret)
goto out_psi_free; goto out_psi_free;
spin_lock_irq(&css_set_lock);
for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) { for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
cgrp->ancestor_ids[tcgrp->level] = tcgrp->id; cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;
if (tcgrp != cgrp) if (tcgrp != cgrp)
tcgrp->nr_descendants++; tcgrp->nr_descendants++;
} }
spin_unlock_irq(&css_set_lock);
if (notify_on_release(parent)) if (notify_on_release(parent))
set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags); 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)) if (parent && cgroup_is_threaded(cgrp))
parent->nr_threaded_children--; parent->nr_threaded_children--;
spin_lock_irq(&css_set_lock);
for (tcgrp = cgroup_parent(cgrp); tcgrp; tcgrp = cgroup_parent(tcgrp)) { for (tcgrp = cgroup_parent(cgrp); tcgrp; tcgrp = cgroup_parent(tcgrp)) {
tcgrp->nr_descendants--; tcgrp->nr_descendants--;
tcgrp->nr_dying_descendants++; tcgrp->nr_dying_descendants++;
} }
spin_unlock_irq(&css_set_lock);
cgroup1_check_for_release(parent); cgroup1_check_for_release(parent);