cgroup: make hierarchy iterators deal with cgroup_subsys_state instead of cgroup

cgroup is currently in the process of transitioning to using css
(cgroup_subsys_state) as the primary handle instead of cgroup in
subsystem API.  For hierarchy iterators, this is beneficial because

* In most cases, css is the only thing subsystems care about anyway.

* On the planned unified hierarchy, iterations for different
  subsystems will need to skip over different subtrees of the
  hierarchy depending on which subsystems are enabled on each cgroup.
  Passing around css makes it unnecessary to explicitly specify the
  subsystem in question as css is intersection between cgroup and
  subsystem

* For the planned unified hierarchy, css's would need to be created
  and destroyed dynamically independent from cgroup hierarchy.  Having
  cgroup core manage css iteration makes enforcing deref rules a lot
  easier.

Most subsystem conversions are straight-forward.  Noteworthy changes
are

* blkio: cgroup_to_blkcg() is no longer used.  Removed.

* freezer: cgroup_freezer() is no longer used.  Removed.

* devices: cgroup_to_devcgroup() is no longer used.  Removed.

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

1 files changed, 73 insertions, 58 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2b7354faaca7..91eac33fac86 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2814,8 +2814,8 @@ static void cgroup_cfts_prepare(void)
 	/*
 	 * Thanks to the entanglement with vfs inode locking, we can't walk
 	 * the existing cgroups under cgroup_mutex and create files.
-	 * Instead, we use cgroup_for_each_descendant_pre() and drop RCU
-	 * read lock before calling cgroup_addrm_files().
+	 * Instead, we use css_for_each_descendant_pre() and drop RCU read
+	 * lock before calling cgroup_addrm_files().
 	 */
 	mutex_lock(&cgroup_mutex);
 }
@@ -2825,10 +2825,11 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
 {
 	LIST_HEAD(pending);
 	struct cgroup_subsys *ss = cfts[0].ss;
-	struct cgroup *cgrp, *root = &ss->root->top_cgroup;
+	struct cgroup *root = &ss->root->top_cgroup;
 	struct super_block *sb = ss->root->sb;
 	struct dentry *prev = NULL;
 	struct inode *inode;
+	struct cgroup_subsys_state *css;
 	u64 update_before;
 	int ret = 0;
 
@@ -2861,7 +2862,9 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
 
 	/* add/rm files for all cgroups created before */
 	rcu_read_lock();
-	cgroup_for_each_descendant_pre(cgrp, root) {
+	css_for_each_descendant_pre(css, cgroup_css(root, ss->subsys_id)) {
+		struct cgroup *cgrp = css->cgroup;
+
 		if (cgroup_is_dead(cgrp))
 			continue;
 
@@ -3037,17 +3040,21 @@ static void cgroup_enable_task_cg_lists(void)
 }
 
 /**
- * cgroup_next_child - find the next child of a given cgroup
- * @pos: the current position (%NULL to initiate traversal)
- * @cgrp: cgroup whose descendants to walk
+ * css_next_child - find the next child of a given css
+ * @pos_css: the current position (%NULL to initiate traversal)
+ * @parent_css: css whose children to walk
  *
- * This function returns the next child of @cgrp and should be called under
- * RCU read lock.  The only requirement is that @cgrp and @pos are
- * accessible.  The next sibling is guaranteed to be returned regardless of
- * their states.
+ * This function returns the next child of @parent_css and should be called
+ * under RCU read lock.  The only requirement is that @parent_css and
+ * @pos_css are accessible.  The next sibling is guaranteed to be returned
+ * regardless of their states.
  */
-struct cgroup *cgroup_next_child(struct cgroup *pos, struct cgroup *cgrp)
+struct cgroup_subsys_state *
+css_next_child(struct cgroup_subsys_state *pos_css,
+	       struct cgroup_subsys_state *parent_css)
 {
+	struct cgroup *pos = pos_css ? pos_css->cgroup : NULL;
+	struct cgroup *cgrp = parent_css->cgroup;
 	struct cgroup *next;
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
@@ -3081,59 +3088,64 @@ struct cgroup *cgroup_next_child(struct cgroup *pos, struct cgroup *cgrp)
 				break;
 	}
 
-	if (&next->sibling != &cgrp->children)
-		return next;
-	return NULL;
+	if (&next->sibling == &cgrp->children)
+		return NULL;
+
+	if (parent_css->ss)
+		return cgroup_css(next, parent_css->ss->subsys_id);
+	else
+		return &next->dummy_css;
 }
-EXPORT_SYMBOL_GPL(cgroup_next_child);
+EXPORT_SYMBOL_GPL(css_next_child);
 
 /**
- * cgroup_next_descendant_pre - find the next descendant for pre-order walk
+ * css_next_descendant_pre - find the next descendant for pre-order walk
  * @pos: the current position (%NULL to initiate traversal)
- * @cgroup: cgroup whose descendants to walk
+ * @root: css whose descendants to walk
  *
- * To be used by cgroup_for_each_descendant_pre().  Find the next
- * descendant to visit for pre-order traversal of @cgroup's descendants.
+ * To be used by css_for_each_descendant_pre().  Find the next descendant
+ * to visit for pre-order traversal of @root's descendants.
  *
  * While this function requires RCU read locking, it doesn't require the
  * whole traversal to be contained in a single RCU 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.
+ * and @root are accessible and @pos is a descendant of @root.
  */
-struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
-					  struct cgroup *cgroup)
+struct cgroup_subsys_state *
+css_next_descendant_pre(struct cgroup_subsys_state *pos,
+			struct cgroup_subsys_state *root)
 {
-	struct cgroup *next;
+	struct cgroup_subsys_state *next;
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
 
-	/* if first iteration, pretend we just visited @cgroup */
+	/* if first iteration, pretend we just visited @root */
 	if (!pos)
-		pos = cgroup;
+		pos = root;
 
 	/* visit the first child if exists */
-	next = cgroup_next_child(NULL, pos);
+	next = css_next_child(NULL, pos);
 	if (next)
 		return next;
 
 	/* no child, visit my or the closest ancestor's next sibling */
-	while (pos != cgroup) {
-		next = cgroup_next_child(pos, pos->parent);
+	while (pos != root) {
+		next = css_next_child(pos, css_parent(pos));
 		if (next)
 			return next;
-		pos = pos->parent;
+		pos = css_parent(pos);
 	}
 
 	return NULL;
 }
-EXPORT_SYMBOL_GPL(cgroup_next_descendant_pre);
+EXPORT_SYMBOL_GPL(css_next_descendant_pre);
 
 /**
- * cgroup_rightmost_descendant - return the rightmost descendant of a cgroup
- * @pos: cgroup of interest
+ * css_rightmost_descendant - return the rightmost descendant of a css
+ * @pos: css of interest
  *
- * Return the rightmost descendant of @pos.  If there's no descendant,
- * @pos is returned.  This can be used during pre-order traversal to skip
+ * Return the rightmost descendant of @pos.  If there's no descendant, @pos
+ * is returned.  This can be used during pre-order traversal to skip
  * subtree of @pos.
  *
  * While this function requires RCU read locking, it doesn't require the
@@ -3141,9 +3153,10 @@ EXPORT_SYMBOL_GPL(cgroup_next_descendant_pre);
  * function will return the correct rightmost descendant as long as @pos is
  * accessible.
  */
-struct cgroup *cgroup_rightmost_descendant(struct cgroup *pos)
+struct cgroup_subsys_state *
+css_rightmost_descendant(struct cgroup_subsys_state *pos)
 {
-	struct cgroup *last, *tmp;
+	struct cgroup_subsys_state *last, *tmp;
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
 
@@ -3151,62 +3164,64 @@ struct cgroup *cgroup_rightmost_descendant(struct cgroup *pos)
 		last = pos;
 		/* ->prev isn't RCU safe, walk ->next till the end */
 		pos = NULL;
-		cgroup_for_each_child(tmp, last)
+		css_for_each_child(tmp, last)
 			pos = tmp;
 	} while (pos);
 
 	return last;
 }
-EXPORT_SYMBOL_GPL(cgroup_rightmost_descendant);
+EXPORT_SYMBOL_GPL(css_rightmost_descendant);
 
-static struct cgroup *cgroup_leftmost_descendant(struct cgroup *pos)
+static struct cgroup_subsys_state *
+css_leftmost_descendant(struct cgroup_subsys_state *pos)
 {
-	struct cgroup *last;
+	struct cgroup_subsys_state *last;
 
 	do {
 		last = pos;
-		pos = cgroup_next_child(NULL, pos);
+		pos = css_next_child(NULL, pos);
 	} while (pos);
 
 	return last;
 }
 
 /**
- * cgroup_next_descendant_post - find the next descendant for post-order walk
+ * css_next_descendant_post - find the next descendant for post-order walk
  * @pos: the current position (%NULL to initiate traversal)
- * @cgroup: cgroup whose descendants to walk
+ * @root: css whose descendants to walk
  *
- * To be used by cgroup_for_each_descendant_post().  Find the next
- * descendant to visit for post-order traversal of @cgroup's descendants.
+ * To be used by css_for_each_descendant_post().  Find the next descendant
+ * to visit for post-order traversal of @root's descendants.
  *
  * While this function requires RCU read locking, it doesn't require the
  * whole traversal to be contained in a single RCU 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.
  */
-struct cgroup *cgroup_next_descendant_post(struct cgroup *pos,
-					   struct cgroup *cgroup)
+struct cgroup_subsys_state *
+css_next_descendant_post(struct cgroup_subsys_state *pos,
+			 struct cgroup_subsys_state *root)
 {
-	struct cgroup *next;
+	struct cgroup_subsys_state *next;
 
 	WARN_ON_ONCE(!rcu_read_lock_held());
 
 	/* if first iteration, visit the leftmost descendant */
 	if (!pos) {
-		next = cgroup_leftmost_descendant(cgroup);
-		return next != cgroup ? next : NULL;
+		next = css_leftmost_descendant(root);
+		return next != root ? next : NULL;
 	}
 
 	/* if there's an unvisited sibling, visit its leftmost descendant */
-	next = cgroup_next_child(pos, pos->parent);
+	next = css_next_child(pos, css_parent(pos));
 	if (next)
-		return cgroup_leftmost_descendant(next);
+		return css_leftmost_descendant(next);
 
 	/* no sibling left, visit parent */
-	next = pos->parent;
-	return next != cgroup ? next : NULL;
+	next = css_parent(pos);
+	return next != root ? next : NULL;
 }
-EXPORT_SYMBOL_GPL(cgroup_next_descendant_post);
+EXPORT_SYMBOL_GPL(css_next_descendant_post);
 
 void cgroup_iter_start(struct cgroup *cgrp, struct cgroup_iter *it)
 	__acquires(css_set_lock)
@@ -4549,9 +4564,9 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	/*
 	 * Mark @cgrp dead.  This prevents further task migration and child
 	 * creation by disabling cgroup_lock_live_group().  Note that
-	 * CGRP_DEAD assertion is depended upon by cgroup_next_child() to
+	 * CGRP_DEAD assertion is depended upon by css_next_child() to
 	 * resume iteration after dropping RCU read lock.  See
-	 * cgroup_next_child() for details.
+	 * css_next_child() for details.
 	 */
 	set_bit(CGRP_DEAD, &cgrp->flags);