Commit 91486f61 authored Oct 15, 2015 by Tejun Heo

cgroup: make cgroup_destroy_locked() test cgroup_is_populated()



cgroup_destroy_locked() currently tests whether any css_sets are
associated to reject removal if the cgroup contains tasks.  This works
because a css_set's refcnt converges with the number of tasks linked
to it and thus there's no css_set linked to a cgroup if it doesn't
have any live tasks.

To help tracking resource usage of zombie tasks, putting the ref of
css_set will be separated from disassociating the task from the
css_set which means that a cgroup may have css_sets linked to it even
when it doesn't have any live tasks.

This patch updates cgroup_destroy_locked() so that it tests
cgroup_is_populated(), which counts the number of populated css_sets,
instead of whether cgrp->cset_links is empty to determine whether the
cgroup is populated or not.  This ensures that rmdirs won't be
incorrectly rejected for cgroups which only contain zombie tasks.

Signed-off-by: Tejun Heo <tj@kernel.org>

parent 2ceb231b

kernel/cgroup.c

+5 −6

Original line number	Diff line number	Diff line
		@@ -4998,16 +4998,15 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
		__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
		{
		struct cgroup_subsys_state *css;
		bool empty;
		int ssid;

		lockdep_assert_held(&cgroup_mutex);

		/* css_set_rwsem synchronizes access to ->cset_links */
		down_read(&css_set_rwsem);
		empty = list_empty(&cgrp->cset_links);
		up_read(&css_set_rwsem);
		if (!empty)
		/*
		* Only migration can raise populated from zero and we're already
		* holding cgroup_mutex.
		*/
		if (cgroup_is_populated(cgrp))
		return -EBUSY;

		/*

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