cgroup: make css_for_each_descendant() and friends include the origin…

… css in the iteration

Previously, all css descendant iterators didn't include the origin
(root of subtree) css in the iteration.  The reasons were maintaining
consistency with css_for_each_child() and that at the time of
introduction more use cases needed skipping the origin anyway;
however, given that css_is_descendant() considers self to be a
descendant, omitting the origin css has become more confusing and
looking at the accumulated use cases rather clearly indicates that
including origin would result in simpler code overall.

While this is a change which can easily lead to subtle bugs, cgroup
API including the iterators has recently gone through major
restructuring and no out-of-tree changes will be applicable without
adjustments making this a relatively acceptable opportunity for this
type of change.

The conversions are mostly straight-forward.  If the iteration block
had explicit origin handling before or after, it's moved inside the
iteration.  If not, if (pos == origin) continue; is added.  Some
conversions add extra reference get/put around origin handling by
consolidating origin handling and the rest.  While the extra ref
operations aren't strictly necessary, this shouldn't cause any
noticeable difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Aristeu Rozanski <aris@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>

block/blk-cgroup.c

@@ -615,12 +615,10 @@ u64 blkg_stat_recursive_sum(struct blkg_policy_data *pd, int off)
 	struct blkcg_policy *pol = blkcg_policy[pd->plid];
 	struct blkcg_gq *pos_blkg;
 	struct cgroup_subsys_state *pos_css;
-	u64 sum;
+	u64 sum = 0;
 
 	lockdep_assert_held(pd->blkg->q->queue_lock);
 
-	sum = blkg_stat_read((void *)pd + off);
-
 	rcu_read_lock();
 	blkg_for_each_descendant_pre(pos_blkg, pos_css, pd_to_blkg(pd)) {
 		struct blkg_policy_data *pos_pd = blkg_to_pd(pos_blkg, pol);
@@ -650,13 +648,11 @@ struct blkg_rwstat blkg_rwstat_recursive_sum(struct blkg_policy_data *pd,
 	struct blkcg_policy *pol = blkcg_policy[pd->plid];
 	struct blkcg_gq *pos_blkg;
 	struct cgroup_subsys_state *pos_css;
-	struct blkg_rwstat sum;
+	struct blkg_rwstat sum = { };
 	int i;
 
 	lockdep_assert_held(pd->blkg->q->queue_lock);
 
-	sum = blkg_rwstat_read((void *)pd + off);
-
 	rcu_read_lock();
 	blkg_for_each_descendant_pre(pos_blkg, pos_css, pd_to_blkg(pd)) {
 		struct blkg_policy_data *pos_pd = blkg_to_pd(pos_blkg, pol);

block/blk-cgroup.h

@@ -291,6 +291,7 @@ struct blkcg_gq *__blkg_lookup(struct blkcg *blkcg, struct request_queue *q,
  * read locked.  If called under either blkcg or queue lock, the iteration
  * is guaranteed to include all and only online blkgs.  The caller may
  * update @pos_css by calling css_rightmost_descendant() to skip subtree.
+ * @p_blkg is included in the iteration and the first node to be visited.
  */
 #define blkg_for_each_descendant_pre(d_blkg, pos_css, p_blkg)		\
 	css_for_each_descendant_pre((pos_css), &(p_blkg)->blkcg->css)	\
@@ -304,7 +305,8 @@ struct blkcg_gq *__blkg_lookup(struct blkcg *blkcg, struct request_queue *q,
  * @p_blkg: target blkg to walk descendants of
  *
  * Similar to blkg_for_each_descendant_pre() but performs post-order
- * traversal instead.  Synchronization rules are the same.
+ * traversal instead.  Synchronization rules are the same.  @p_blkg is
+ * included in the iteration and the last node to be visited.
  */
 #define blkg_for_each_descendant_post(d_blkg, pos_css, p_blkg)		\
 	css_for_each_descendant_post((pos_css), &(p_blkg)->blkcg->css)	\

block/blk-throttle.c

@@ -1379,7 +1379,6 @@ static int tg_set_conf(struct cgroup_subsys_state *css, struct cftype *cft,
 	 * restrictions in the whole hierarchy and allows them to bypass
 	 * blk-throttle.
 	 */
-	tg_update_has_rules(tg);
 	blkg_for_each_descendant_pre(blkg, pos_css, ctx.blkg)
 		tg_update_has_rules(blkg_to_tg(blkg));
 
@@ -1639,8 +1638,6 @@ void blk_throtl_drain(struct request_queue *q)
 	blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg)
 		tg_drain_bios(&blkg_to_tg(blkg)->service_queue);
 
-	tg_drain_bios(&td_root_tg(td)->service_queue);
-
 	/* finally, transfer bios from top-level tg's into the td */
 	tg_drain_bios(&td->service_queue);
 

include/linux/cgroup.h

@@ -798,7 +798,8 @@ css_rightmost_descendant(struct cgroup_subsys_state *pos);
  * @pos: the css * to use as the loop cursor
  * @root: css whose descendants to walk
  *
- * Walk @root's descendants.  Must be called under rcu_read_lock().  A
+ * Walk @root's descendants.  @root is included in the iteration and the
+ * first node to be visited.  Must be called under rcu_read_lock().  A
  * descendant css which hasn't finished ->css_online() or already has
  * finished ->css_offline() may show up during traversal and it's each
  * subsystem's responsibility to verify that each @pos is alive.
@@ -820,13 +821,12 @@ css_rightmost_descendant(struct cgroup_subsys_state *pos);
  *
  * my_update_state(@css)
  * {
- *	Lock @css;
- *	Update @css's state;
- *	Unlock @css;
- *
  *	css_for_each_descendant_pre(@pos, @css) {
  *		Lock @pos;
- *		Verify @pos is alive and inherit state from @pos's parent;
+ *		if (@pos == @css)
+ *			Update @css's state;
+ *		else
+ *			Verify @pos is alive and inherit state from its parent;
  *		Unlock @pos;
  *	}
  * }
@@ -864,8 +864,9 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
  * @css: css whose descendants to walk
  *
  * Similar to css_for_each_descendant_pre() but performs post-order
- * traversal instead.  Note that the walk visibility guarantee described in
- * pre-order walk doesn't apply the same to post-order walks.
+ * traversal instead.  @root is included in the iteration and the last
+ * node to be visited.  Note that the walk visibility guarantee described
+ * in pre-order walk doesn't apply the same to post-order walks.
  */
 #define css_for_each_descendant_post(pos, css)				\
 	for ((pos) = css_next_descendant_post(NULL, (css)); (pos);	\

kernel/cgroup.c

@@ -2868,17 +2868,6 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
 
 	mutex_unlock(&cgroup_mutex);
 
-	/* @root always needs to be updated */
-	inode = root->dentry->d_inode;
-	mutex_lock(&inode->i_mutex);
-	mutex_lock(&cgroup_mutex);
-	ret = cgroup_addrm_files(root, cfts, is_add);
-	mutex_unlock(&cgroup_mutex);
-	mutex_unlock(&inode->i_mutex);
-
-	if (ret)
-		goto out_deact;
-
 	/* add/rm files for all cgroups created before */
 	rcu_read_lock();
 	css_for_each_descendant_pre(css, cgroup_css(root, ss->subsys_id)) {
@@ -2907,7 +2896,6 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
 	}
 	rcu_read_unlock();
 	dput(prev);
-out_deact:
 	deactivate_super(sb);
 	return ret;
 }
@@ -3099,7 +3087,8 @@ EXPORT_SYMBOL_GPL(css_next_child);
  * @root: css whose descendants to walk
  *
  * To be used by css_for_each_descendant_pre().  Find the next descendant
- * to visit for pre-order traversal of @root's descendants.
+ * to visit for pre-order traversal of @root's descendants.  @root is
+ * included in the iteration and the first node to be visited.
  *
  * While this function requires RCU read locking, it doesn't require the
  * whole traversal to be contained in a single RCU critical section.  This
@@ -3114,9 +3103,9 @@ css_next_descendant_pre(struct cgroup_subsys_state *pos,
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
 
-	/* if first iteration, pretend we just visited @root */
+	/* if first iteration, visit @root */
 	if (!pos)
-		pos = root;
+		return root;
 
 	/* visit the first child if exists */
 	next = css_next_child(NULL, pos);
@@ -3186,7 +3175,8 @@ css_leftmost_descendant(struct cgroup_subsys_state *pos)
  * @root: css whose descendants to walk
  *
  * To be used by css_for_each_descendant_post().  Find the next descendant
- * to visit for post-order traversal of @root's descendants.
+ * to visit for post-order traversal of @root's descendants.  @root is
+ * included in the iteration and the last node to be visited.
  *
  * While this function requires RCU read locking, it doesn't require the
  * whole traversal to be contained in a single RCU critical section.  This
@@ -3207,14 +3197,17 @@ css_next_descendant_post(struct cgroup_subsys_state *pos,
 		return next != root ? next : NULL;
 	}
 
+	/* if we visited @root, we're done */
+	if (pos == root)
+		return NULL;
+
 	/* if there's an unvisited sibling, visit its leftmost descendant */
 	next = css_next_child(pos, css_parent(pos));
 	if (next)
 		return css_leftmost_descendant(next);
 
 	/* no sibling left, visit parent */
-	next = css_parent(pos);
-	return next != root ? next : NULL;
+	return css_parent(pos);
 }
 EXPORT_SYMBOL_GPL(css_next_descendant_post);
 

kernel/cgroup_freezer.c

@@ -311,7 +311,6 @@ static int freezer_read(struct cgroup_subsys_state *css, struct cftype *cft,
 	/* update states bottom-up */
 	css_for_each_descendant_post(pos, css)
 		update_if_frozen(pos);
-	update_if_frozen(css);
 
 	rcu_read_unlock();
 
@@ -391,11 +390,6 @@ static void freezer_change_state(struct freezer *freezer, bool freeze)
 {
 	struct cgroup_subsys_state *pos;
 
-	/* update @freezer */
-	spin_lock_irq(&freezer->lock);
-	freezer_apply_state(freezer, freeze, CGROUP_FREEZING_SELF);
-	spin_unlock_irq(&freezer->lock);
-
 	/*
 	 * Update all its descendants in pre-order traversal.  Each
 	 * descendant will try to inherit its parent's FREEZING state as
@@ -406,14 +400,23 @@ static void freezer_change_state(struct freezer *freezer, bool freeze)
 		struct freezer *pos_f = css_freezer(pos);
 		struct freezer *parent = parent_freezer(pos_f);
 
-		/*
-		 * Our update to @parent->state is already visible which is
-		 * all we need.  No need to lock @parent.  For more info on
-		 * synchronization, see freezer_post_create().
-		 */
 		spin_lock_irq(&pos_f->lock);
-		freezer_apply_state(pos_f, parent->state & CGROUP_FREEZING,
-				    CGROUP_FREEZING_PARENT);
+
+		if (pos_f == freezer) {
+			freezer_apply_state(pos_f, freeze,
+					    CGROUP_FREEZING_SELF);
+		} else {
+			/*
+			 * Our update to @parent->state is already visible
+			 * which is all we need.  No need to lock @parent.
+			 * For more info on synchronization, see
+			 * freezer_post_create().
+			 */
+			freezer_apply_state(pos_f,
+					    parent->state & CGROUP_FREEZING,
+					    CGROUP_FREEZING_PARENT);
+		}
+
 		spin_unlock_irq(&pos_f->lock);
 	}
 	rcu_read_unlock();

kernel/cpuset.c

@@ -222,7 +222,8 @@ static struct cpuset top_cpuset = {
  *
  * Walk @des_cs through the online descendants of @root_cs.  Must be used
  * with RCU read locked.  The caller may modify @pos_css by calling
- * css_rightmost_descendant() to skip subtree.
+ * css_rightmost_descendant() to skip subtree.  @root_cs is included in the
+ * iteration and the first node to be visited.
  */
 #define cpuset_for_each_descendant_pre(des_cs, pos_css, root_cs)	\
 	css_for_each_descendant_pre((pos_css), &(root_cs)->css)		\
@@ -506,6 +507,9 @@ static void update_domain_attr_tree(struct sched_domain_attr *dattr,
 
 	rcu_read_lock();
 	cpuset_for_each_descendant_pre(cp, pos_css, root_cs) {
+		if (cp == root_cs)
+			continue;
+
 		/* skip the whole subtree if @cp doesn't have any CPU */
 		if (cpumask_empty(cp->cpus_allowed)) {
 			pos_css = css_rightmost_descendant(pos_css);
@@ -613,6 +617,8 @@ static int generate_sched_domains(cpumask_var_t **domains,
 
 	rcu_read_lock();
 	cpuset_for_each_descendant_pre(cp, pos_css, &top_cpuset) {
+		if (cp == &top_cpuset)
+			continue;
 		/*
 		 * Continue traversing beyond @cp iff @cp has some CPUs and
 		 * isn't load balancing.  The former is obvious.  The
@@ -875,15 +881,17 @@ static void update_tasks_cpumask_hier(struct cpuset *root_cs,
 	struct cpuset *cp;
 	struct cgroup_subsys_state *pos_css;
 
-	if (update_root)
-		update_tasks_cpumask(root_cs, heap);
-
 	rcu_read_lock();
 	cpuset_for_each_descendant_pre(cp, pos_css, root_cs) {
-		/* skip the whole subtree if @cp have some CPU */
-		if (!cpumask_empty(cp->cpus_allowed)) {
-			pos_css = css_rightmost_descendant(pos_css);
-			continue;
+		if (cp == root_cs) {
+			if (!update_root)
+				continue;
+		} else {
+			/* skip the whole subtree if @cp have some CPU */
+			if (!cpumask_empty(cp->cpus_allowed)) {
+				pos_css = css_rightmost_descendant(pos_css);
+				continue;
+			}
 		}
 		if (!css_tryget(&cp->css))
 			continue;
@@ -1130,15 +1138,17 @@ static void update_tasks_nodemask_hier(struct cpuset *root_cs,
 	struct cpuset *cp;
 	struct cgroup_subsys_state *pos_css;
 
-	if (update_root)
-		update_tasks_nodemask(root_cs, heap);
-
 	rcu_read_lock();
 	cpuset_for_each_descendant_pre(cp, pos_css, root_cs) {
-		/* skip the whole subtree if @cp have some CPU */
-		if (!nodes_empty(cp->mems_allowed)) {
-			pos_css = css_rightmost_descendant(pos_css);
-			continue;
+		if (cp == root_cs) {
+			if (!update_root)
+				continue;
+		} else {
+			/* skip the whole subtree if @cp have some CPU */
+			if (!nodes_empty(cp->mems_allowed)) {
+				pos_css = css_rightmost_descendant(pos_css);
+				continue;
+			}
 		}
 		if (!css_tryget(&cp->css))
 			continue;
@@ -2237,7 +2247,7 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
 
 		rcu_read_lock();
 		cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
-			if (!css_tryget(&cs->css))
+			if (cs == &top_cpuset || !css_tryget(&cs->css))
 				continue;
 			rcu_read_unlock();
 

mm/memcontrol.c

@@ -1079,14 +1079,7 @@ static struct mem_cgroup *__mem_cgroup_iter_next(struct mem_cgroup *root,
 {
 	struct cgroup_subsys_state *prev_css, *next_css;
 
-	/*
-	 * Root is not visited by cgroup iterators so it needs an
-	 * explicit visit.
-	 */
-	if (!last_visited)
-		return root;
-
-	prev_css = (last_visited == root) ? NULL : &last_visited->css;
+	prev_css = last_visited ? &last_visited->css : NULL;
 skip_node:
 	next_css = css_next_descendant_pre(prev_css, &root->css);
 

security/device_cgroup.c

@@ -456,7 +456,7 @@ static int propagate_exception(struct dev_cgroup *devcg_root,
 		 * methods), and online ones are safe to access outside RCU
 		 * read lock without bumping refcnt.
 		 */
-		if (!is_devcg_online(devcg))
+		if (pos == &devcg_root->css || !is_devcg_online(devcg))
 			continue;
 
 		rcu_read_unlock();