[PATCH 3/3] cgroup: remove CGRP_RELEASABLE flag

From: Zefan Li
Date: Fri Sep 19 2014 - 04:51:19 EST

We call put_css_set() after setting CGRP_RELEASABLE flag in
cgroup_task_migrate(), but in other places we call it without setting
the flag. I don't see the necessity of this flag.

Moreover once the flag is set, it will never be cleared, unless writing
to the notify_on_release control file, so it can be quite confusing
if we look at the output of debug.releasable.

# mount -t cgroup -o debug xxx /cgroup
# mkdir /cgroup/child
# cat /cgroup/child/debug.releasable
0 <-- shows 0 though the cgroup is empty
# echo $$ > /cgroup/child/tasks
# cat /cgroup/child/debug.releasable
0
# echo $$ > /cgroup/tasks && echo $$ > /cgroup/child/tasks
# cat /proc/child/debug.releasable
1 <-- shows 1 though the cgroup is not empty

This patch removes the flag, and now debug.releasable shows if the
cgroup is empty or not.

Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx>
---
include/linux/cgroup.h | 5 -----
kernel/cgroup.c | 40 +++++++++++++---------------------------
2 files changed, 13 insertions(+), 32 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 818a81f..1d51968 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -161,11 +161,6 @@ static inline void css_put(struct cgroup_subsys_state *css)

/* bits in struct cgroup flags field */
enum {
- /*
- * Control Group has previously had a child cgroup or a task,
- * but no longer (only if CGRP_NOTIFY_ON_RELEASE is set)
- */
- CGRP_RELEASABLE,
/* Control Group requires release notifications to userspace */
CGRP_NOTIFY_ON_RELEASE,
/*
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index de70b63..7f23551 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -329,14 +329,6 @@ bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor)
return false;
}

-static int cgroup_is_releasable(const struct cgroup *cgrp)
-{
- const int bits =
- (1 << CGRP_RELEASABLE) |
- (1 << CGRP_NOTIFY_ON_RELEASE);
- return (cgrp->flags & bits) == bits;
-}
-
static int notify_on_release(const struct cgroup *cgrp)
{
return test_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
@@ -491,7 +483,7 @@ static unsigned long css_set_hash(struct cgroup_subsys_state *css[])
return key;
}

-static void put_css_set_locked(struct css_set *cset, bool taskexit)
+static void put_css_set_locked(struct css_set *cset)
{
struct cgrp_cset_link *link, *tmp_link;
struct cgroup_subsys *ss;
@@ -517,11 +509,7 @@ static void put_css_set_locked(struct css_set *cset, bool taskexit)
/* @cgrp can't go away while we're holding css_set_rwsem */
if (list_empty(&cgrp->cset_links)) {
cgroup_update_populated(cgrp, false);
- if (notify_on_release(cgrp)) {
- if (taskexit)
- set_bit(CGRP_RELEASABLE, &cgrp->flags);
- check_for_release(cgrp);
- }
+ check_for_release(cgrp);
}

kfree(link);
@@ -530,7 +518,7 @@ static void put_css_set_locked(struct css_set *cset, bool taskexit)
kfree_rcu(cset, rcu_head);
}

-static void put_css_set(struct css_set *cset, bool taskexit)
+static void put_css_set(struct css_set *cset)
{
/*
* Ensure that the refcount doesn't hit zero while any readers
@@ -541,7 +529,7 @@ static void put_css_set(struct css_set *cset, bool taskexit)
return;

down_write(&css_set_rwsem);
- put_css_set_locked(cset, taskexit);
+ put_css_set_locked(cset);
up_write(&css_set_rwsem);
}

@@ -2037,8 +2025,7 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
* task. As trading it for new_cset is protected by cgroup_mutex,
* we're safe to drop it here; it will be freed under RCU.
*/
- set_bit(CGRP_RELEASABLE, &old_cgrp->flags);
- put_css_set_locked(old_cset, false);
+ put_css_set_locked(old_cset);
}

/**
@@ -2059,7 +2046,7 @@ static void cgroup_migrate_finish(struct list_head *preloaded_csets)
cset->mg_src_cgrp = NULL;
cset->mg_dst_cset = NULL;
list_del_init(&cset->mg_preload_node);
- put_css_set_locked(cset, false);
+ put_css_set_locked(cset);
}
up_write(&css_set_rwsem);
}
@@ -2153,8 +2140,8 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
if (src_cset == dst_cset) {
src_cset->mg_src_cgrp = NULL;
list_del_init(&src_cset->mg_preload_node);
- put_css_set(src_cset, false);
- put_css_set(dst_cset, false);
+ put_css_set(src_cset);
+ put_css_set(dst_cset);
continue;
}

@@ -2163,7 +2150,7 @@ static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
if (list_empty(&dst_cset->mg_preload_node))
list_add(&dst_cset->mg_preload_node, &csets);
else
- put_css_set(dst_cset, false);
+ put_css_set(dst_cset);
}

list_splice_tail(&csets, preloaded_csets);
@@ -4158,7 +4145,6 @@ static u64 cgroup_read_notify_on_release(struct cgroup_subsys_state *css,
static int cgroup_write_notify_on_release(struct cgroup_subsys_state *css,
struct cftype *cft, u64 val)
{
- clear_bit(CGRP_RELEASABLE, &css->cgroup->flags);
if (val)
set_bit(CGRP_NOTIFY_ON_RELEASE, &css->cgroup->flags);
else
@@ -4805,7 +4791,6 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
*/
kernfs_remove(cgrp->kn);

- set_bit(CGRP_RELEASABLE, &cgroup_parent(cgrp)->flags);
check_for_release(cgroup_parent(cgrp));

/* put the base reference */
@@ -5243,12 +5228,12 @@ void cgroup_exit(struct task_struct *tsk)
}

if (put_cset)
- put_css_set(cset, true);
+ put_css_set(cset);
}

static void check_for_release(struct cgroup *cgrp)
{
- if (cgroup_is_releasable(cgrp) && !cgroup_has_tasks(cgrp) &&
+ if (notify_on_release(cgrp) && !cgroup_has_tasks(cgrp) &&
!css_has_online_children(&cgrp->self) && !cgroup_is_dead(cgrp))
schedule_work(&cgrp->release_agent_work);
}
@@ -5495,7 +5480,8 @@ static int cgroup_css_links_read(struct seq_file *seq, void *v)

static u64 releasable_read(struct cgroup_subsys_state *css, struct cftype *cft)
{
- return test_bit(CGRP_RELEASABLE, &css->cgroup->flags);
+ return (!cgroup_has_tasks(css->cgroup) &&
+ !css_has_online_children(&css->cgroup->self));
}

static struct cftype debug_files[] = {
--
1.8.0.2

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