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>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 84efb34..1a68241 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -407,19 +407,13 @@
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 @@
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 @@
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)) {
/*
