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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
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@@ -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;