cgroup: remove CGRP_RELEASABLE flag

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@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 13 insertions, 27 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index df7733b48d2e..16e3a4f5c9dc 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);
@@ -4159,7 +4146,6 @@ 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
@@ -4806,7 +4792,6 @@ 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 */
@@ -5244,12 +5229,12 @@ 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);
 }
@@ -5496,7 +5481,8 @@ 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[] =  {