cgroup: drop unnecessary RCU dancing from __put_css_set()

__put_css_set() does RCU read access on @cgrp across dropping
@cgrp->count so that it can continue accessing @cgrp even if the count
reached zero and destruction of the cgroup commenced.  Given that both
sides - __css_put() and cgroup_destroy_locked() - are cold paths, this
is unnecessary.  Just making cgroup_destroy_locked() grab css_set_lock
while checking @cgrp->count is enough.

Remove the RCU read locking from __put_css_set() and make
cgroup_destroy_locked() read-lock css_set_lock when checking
@cgrp->count.  This will also allow removing @cgrp->count.

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

1 files changed, 10 insertions, 10 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 84efb344fdf6..1a68241ca835 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -407,19 +407,13 @@ static void __put_css_set(struct css_set *cset, int taskexit)
                 list_del(&link->cset_link);
                 list_del(&link->cgrp_link);
 
-                /*
-                 * We may not be holding cgroup_mutex, and if cgrp->count is
-                 * dropped to 0 the cgroup can be destroyed at any time, hence
-                 * rcu_read_lock is used to keep it alive.
-                 */
-                rcu_read_lock();
+                /* @cgrp can't go away while we're holding css_set_lock */
                 if (atomic_dec_and_test(&cgrp->count) &&
                     notify_on_release(cgrp)) {
                         if (taskexit)
                                 set_bit(CGRP_RELEASABLE, &cgrp->flags);
                         check_for_release(cgrp);
                 }
-                rcu_read_unlock();
 
                 kfree(link);
         }
@@ -4370,11 +4364,19 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
         struct cgroup *parent = cgrp->parent;
         struct cgroup_event *event, *tmp;
         struct cgroup_subsys *ss;
+        bool empty;
 
         lockdep_assert_held(&d->d_inode->i_mutex);
         lockdep_assert_held(&cgroup_mutex);
 
-        if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children))
+        /*
+         * css_set_lock prevents @cgrp from being removed while
+         * __put_css_set() is in progress.
+         */
+        read_lock(&css_set_lock);
+        empty = !atomic_read(&cgrp->count) && list_empty(&cgrp->children);
+        read_unlock(&css_set_lock);
+        if (!empty)
                 return -EBUSY;
 
         /*
@@ -5051,8 +5053,6 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
 
 static void check_for_release(struct cgroup *cgrp)
 {
-        /* All of these checks rely on RCU to keep the cgroup
-         * structure alive */
         if (cgroup_is_releasable(cgrp) &&
             !atomic_read(&cgrp->count) && list_empty(&cgrp->children)) {
                 /*