Commit ddd69148 authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
parent 54766d4a

kernel/cgroup.c

+10 −10
Original line number Diff line number Diff line
@@ -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)) {

		/*

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