Commit 0e40cf2a authored by Kinsey Ho's avatar Kinsey Ho Committed by Andrew Morton
Browse files

cgroup: clarify css sibling linkage is protected by cgroup_mutex or RCU 

Patch series "Improve mem_cgroup_iter()", v4.

Incremental cgroup iteration is being used again [1]. This patchset
improves the reliability of mem_cgroup_iter(). It also improves
simplicity and code readability.

[1] https://lore.kernel.org/20240514202641.2821494-1-hannes@cmpxchg.org/


This patch (of 5):

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 it 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.

Link: https://lkml.kernel.org/r/20240905003058.1859929-1-kinseyho@google.com
Link: https://lkml.kernel.org/r/20240905003058.1859929-2-kinseyho@google.com


Suggested-by: default avatarYosry Ahmed <yosryahmed@google.com>
Reviewed-by: default avatarMichal Koutný <mkoutny@suse.com>
Signed-off-by: default avatarKinsey Ho <kinseyho@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: T.J. Mercier <tjmercier@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 6e94da94

include/linux/cgroup-defs.h

+5 −1
Original line number Diff line number Diff line
@@ -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;

kernel/cgroup/cgroup.c

+9 −7
Original line number Diff line number Diff line
@@ -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