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Subject[PATCH cgroup/for-3.12] cgroup: make css_for_each_descendant() and friends include the origin css in the iteration
Hello,

I've been wanting to do this for some time and given all the recent
API updates now seems like a pretty good opportunity. Verified
freezer and blkcg. The conversions are mostly straight forward but
I'd much appreciate acks from controller maintainers.

The patch is on top of

cgroup/for-3.12 61584e3f4 ("cgroup: Merge branch 'for-3.11-fixes' into for-3.12")
+ [1] cgroup: use cgroup_subsys_state as the primary subsystem interface handle
+ [2] cgroup: make cgroup_event specific to memcg

and available in the following git branch.

git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-descendant-update

Thanks.

[1] https://lkml.org/lkml/2013/8/1/722
[2] http://thread.gmane.org/gmane.linux.kernel.cgroups/8726

------- 8< -------

From 0e84b0865ab8a87f1c1443e4777c20c7f14e13b6 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Sun, 4 Aug 2013 19:01:23 -0400

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>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Aristeu Rozanski <aris@redhat.com>
---
block/blk-cgroup.c | 8 ++------
block/blk-cgroup.h | 4 +++-
block/blk-throttle.c | 3 ---
include/linux/cgroup.h | 17 +++++++++--------
kernel/cgroup.c | 29 +++++++++++------------------
kernel/cgroup_freezer.c | 29 ++++++++++++++++-------------
kernel/cpuset.c | 42 ++++++++++++++++++++++++++----------------
mm/memcontrol.c | 9 +--------
security/device_cgroup.c | 2 +-
9 files changed, 69 insertions(+), 74 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 54ad002..e90c7c1 100644
--- a/block/blk-cgroup.c
+++ b/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);
diff --git a/block/blk-cgroup.h b/block/blk-cgroup.h
index 8555386..ae6969a 100644
--- a/block/blk-cgroup.h
+++ b/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) \
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 8cefa7f..8331aba 100644
--- a/block/blk-throttle.c
+++ b/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);

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e33eb7b..ed925f5 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -775,7 +775,8 @@ 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.
@@ -797,13 +798,12 @@ 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;
* }
* }
@@ -841,8 +841,9 @@ 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); \
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2bc45d3..4a19c8d 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2839,17 +2839,6 @@ 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)) {
@@ -2878,7 +2867,6 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
}
rcu_read_unlock();
dput(prev);
-out_deact:
deactivate_super(sb);
return ret;
}
@@ -3070,7 +3058,8 @@ 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
@@ -3085,9 +3074,9 @@ 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);
@@ -3157,7 +3146,8 @@ 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
@@ -3178,14 +3168,17 @@ 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);

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 224da9a..f0ff64d 100644
--- a/kernel/cgroup_freezer.c
+++ b/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();
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index bf69717..72a0383 100644
--- a/kernel/cpuset.c
+++ b/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();

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 240ae72..68a37c0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1129,14 +1129,7 @@ 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);

diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index 9bf230a..c123628 100644
--- a/security/device_cgroup.c
+++ b/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();
--
1.8.3.1


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