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Commit 455050d2 authored by Tejun Heo's avatar Tejun Heo
Browse files

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: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarLi Zefan <lizefan@huawei.com>
parent 6f3d828f

kernel/cgroup.c

+35 −26
Original line number Diff line number Diff line
@@ -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;

}