cgroup: reorder the operations in cgroup_destroy_locked()

This patch reorders the operations in cgroup_destroy_locked() such
that the userland visible parts happen before css offlining and
removal from the ->sibling list.  This will be used to make css use
percpu refcnt.

While at it, split out CGRP_DEAD related comment from the refcnt
deactivation one and correct / clarify how different guarantees are
met.

While this patch changes the specific order of operations, it
shouldn't cause any noticeable behavior difference.

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

1 files changed, 35 insertions, 26 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 49bfd7b0bbda..5a1ddecc3cfa 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4379,13 +4379,8 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 
         /*
          * Block new css_tryget() by deactivating refcnt and mark @cgrp
-         * removed.  This makes future css_tryget() and child creation
-         * attempts fail thus maintaining the removal conditions verified
-         * above.
-         *
-         * Note that CGRP_DEAD assertion is depended upon by
-         * cgroup_next_sibling() to resume iteration after dropping RCU
-         * read lock.  See cgroup_next_sibling() for details.
+         * removed.  This makes future css_tryget() attempts fail which we
+         * guarantee to ->css_offline() callbacks.
          */
         for_each_subsys(cgrp->root, ss) {
                 struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
@@ -4393,8 +4388,41 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
                 WARN_ON(atomic_read(&css->refcnt) < 0);
                 atomic_add(CSS_DEACT_BIAS, &css->refcnt);
         }
+
+        /*
+         * 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_sibling() to
+         * resume iteration after dropping RCU read lock.  See
+         * cgroup_next_sibling() for details.
+         */
         set_bit(CGRP_DEAD, &cgrp->flags);
 
+        /* CGRP_DEAD is set, remove from ->release_list for the last time */
+        raw_spin_lock(&release_list_lock);
+        if (!list_empty(&cgrp->release_list))
+                list_del_init(&cgrp->release_list);
+        raw_spin_unlock(&release_list_lock);
+
+        /*
+         * Remove @cgrp directory.  The removal puts the base ref but we
+         * aren't quite done with @cgrp yet, so hold onto it.
+         */
+        dget(d);
+        cgroup_d_remove_dir(d);
+
+        /*
+         * Unregister events and notify userspace.
+         * Notify userspace about cgroup removing only after rmdir of cgroup
+         * directory to avoid race between userspace and kernelspace.
+         */
+        spin_lock(&cgrp->event_list_lock);
+        list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
+                list_del_init(&event->list);
+                schedule_work(&event->remove);
+        }
+        spin_unlock(&cgrp->event_list_lock);
+
         /* tell subsystems to initate destruction */
         for_each_subsys(cgrp->root, ss)
                 offline_css(ss, cgrp);
@@ -4409,34 +4437,15 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
         for_each_subsys(cgrp->root, ss)
                 css_put(cgrp->subsys[ss->subsys_id]);
 
-        raw_spin_lock(&release_list_lock);
-        if (!list_empty(&cgrp->release_list))
-                list_del_init(&cgrp->release_list);
-        raw_spin_unlock(&release_list_lock);
-
         /* delete this cgroup from parent->children */
         list_del_rcu(&cgrp->sibling);
         list_del_init(&cgrp->allcg_node);
 
-        dget(d);
-        cgroup_d_remove_dir(d);
         dput(d);
 
         set_bit(CGRP_RELEASABLE, &parent->flags);
         check_for_release(parent);
 
-        /*
-         * Unregister events and notify userspace.
-         * Notify userspace about cgroup removing only after rmdir of cgroup
-         * directory to avoid race between userspace and kernelspace.
-         */
-        spin_lock(&cgrp->event_list_lock);
-        list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
-                list_del_init(&event->list);
-                schedule_work(&event->remove);
-        }
-        spin_unlock(&cgrp->event_list_lock);
-
         return 0;
 }