| Message ID | 20240813204716.842811-2-kinseyho@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Improve mem_cgroup_iter() | expand |
On Tue, Aug 13, 2024 at 08:47:11PM GMT, Kinsey Ho <kinseyho@google.com> wrote: > --- a/include/linux/cgroup-defs.h > +++ b/include/linux/cgroup-defs.h > @@ -172,7 +172,11 @@ struct cgroup_subsys_state { > /* reference count - access via css_[try]get() and css_put() */ > struct percpu_ref refcnt; > > - /* siblings list anchored at the parent's ->children */ > + /* > + * siblings list anchored at the parent's ->children > + * > + * linkage is protected by cgroup_mutex or RCU > + */ > struct list_head sibling; > struct list_head children; Thanks, this is good. > > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c > index 0a97cb2ef124..ece2316e2bca 100644 > --- a/kernel/cgroup/cgroup.c > +++ b/kernel/cgroup/cgroup.c > @@ -4602,8 +4602,9 @@ struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos, > * > * While this function requires cgroup_mutex or RCU read locking, it > * doesn't require the whole traversal to be contained in a single critical > - * section. This function will return the correct next descendant as long > - * as both @pos and @root are accessible and @pos is a descendant of @root. > + * section. Additionally, it isn't necessary to hold onto a reference to @pos. > + * This function will return the correct next descendant as long as both @pos > + * and @root are accessible and @pos is a descendant of @root. > * > * If a subsystem synchronizes ->css_online() and the start of iteration, a > * css which finished ->css_online() is guaranteed to be visible in the > @@ -4651,8 +4652,9 @@ EXPORT_SYMBOL_GPL(css_next_descendant_pre); > * > * While this function requires cgroup_mutex or RCU read locking, it > * doesn't require the whole traversal to be contained in a single critical > - * section. This function will return the correct rightmost descendant as > - * long as @pos is accessible. > + * section. Additionally, it isn't necessary to hold onto a reference to @pos. > + * This function will return the correct rightmost descendant as long as @pos > + * is accessible. > */ > struct cgroup_subsys_state * > css_rightmost_descendant(struct cgroup_subsys_state *pos) > @@ -4696,9 +4698,9 @@ css_leftmost_descendant(struct cgroup_subsys_state *pos) > * > * While this function requires cgroup_mutex or RCU read locking, it > * doesn't require the whole traversal to be contained in a single critical > - * section. This function will return the correct next descendant as long > - * as both @pos and @cgroup are accessible and @pos is a descendant of > - * @cgroup. > + * section. Additionally, it isn't necessary to hold onto a reference to @pos. > + * This function will return the correct next descendant as long as both @pos > + * and @cgroup are accessible and @pos is a descendant of @cgroup. > * > * If a subsystem synchronizes ->css_online() and the start of iteration, a > * css which finished ->css_online() is guaranteed to be visible in the I'd say these comments are unnecessary given the functions have cgroup_assert_mutex_or_rcu_locked() but if it helps overall understanding in broader context, why not. Reviewed-by: Michal Koutný <mkoutny@suse.com>
diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h index 7fc2d0195f56..6862243bd1c2 100644 --- a/include/linux/cgroup-defs.h +++ b/include/linux/cgroup-defs.h @@ -172,7 +172,11 @@ struct cgroup_subsys_state { /* reference count - access via css_[try]get() and css_put() */ struct percpu_ref refcnt; - /* siblings list anchored at the parent's ->children */ + /* + * siblings list anchored at the parent's ->children + * + * linkage is protected by cgroup_mutex or RCU + */ struct list_head sibling; struct list_head children; diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index 0a97cb2ef124..ece2316e2bca 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -4602,8 +4602,9 @@ struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos, * * While this function requires cgroup_mutex or RCU read locking, it * doesn't require the whole traversal to be contained in a single critical - * section. This function will return the correct next descendant as long - * as both @pos and @root are accessible and @pos is a descendant of @root. + * section. Additionally, it isn't necessary to hold onto a reference to @pos. + * This function will return the correct next descendant as long as both @pos + * and @root are accessible and @pos is a descendant of @root. * * If a subsystem synchronizes ->css_online() and the start of iteration, a * css which finished ->css_online() is guaranteed to be visible in the @@ -4651,8 +4652,9 @@ EXPORT_SYMBOL_GPL(css_next_descendant_pre); * * While this function requires cgroup_mutex or RCU read locking, it * doesn't require the whole traversal to be contained in a single critical - * section. This function will return the correct rightmost descendant as - * long as @pos is accessible. + * section. Additionally, it isn't necessary to hold onto a reference to @pos. + * This function will return the correct rightmost descendant as long as @pos + * is accessible. */ struct cgroup_subsys_state * css_rightmost_descendant(struct cgroup_subsys_state *pos) @@ -4696,9 +4698,9 @@ css_leftmost_descendant(struct cgroup_subsys_state *pos) * * While this function requires cgroup_mutex or RCU read locking, it * doesn't require the whole traversal to be contained in a single critical - * section. This function will return the correct next descendant as long - * as both @pos and @cgroup are accessible and @pos is a descendant of - * @cgroup. + * section. Additionally, it isn't necessary to hold onto a reference to @pos. + * This function will return the correct next descendant as long as both @pos + * and @cgroup are accessible and @pos is a descendant of @cgroup. * * If a subsystem synchronizes ->css_online() and the start of iteration, a * css which finished ->css_online() is guaranteed to be visible in the
Explicitly document that css sibling/descendant linkage is protected by cgroup_mutex or RCU. Also, document in css_next_descendant_pre() and similar functions that is isn't necessary to hold a ref on @pos. The following changes in this patchset rely on this clarification for simplification in memcg iteration code. Suggested-By: Yosry Ahmed <yosryahmed@google.com> Signed-Off-By: Kinsey Ho <kinseyho@google.com> --- include/linux/cgroup-defs.h | 6 +++++- kernel/cgroup/cgroup.c | 16 +++++++++------- 2 files changed, 14 insertions(+), 8 deletions(-)