Commit 48573a89 authored Feb 08, 2014 by Tejun Heo

cgroup: fix locking in cgroup_cfts_commit()



cgroup_cfts_commit() walks the cgroup hierarchy that the target
subsystem is attached to and tries to apply the file changes.  Due to
the convolution with inode locking, it can't keep cgroup_mutex locked
while iterating.  It currently holds only RCU read lock around the
actual iteration and then pins the found cgroup using dget().

Unfortunately, this is incorrect.  Although the iteration does check
cgroup_is_dead() before invoking dget(), there's nothing which
prevents the dentry from going away inbetween.  Note that this is
different from the usual css iterations where css_tryget() is used to
pin the css - css_tryget() tests whether the css can be pinned and
fails if not.

The problem can be solved by simply holding cgroup_mutex instead of
RCU read lock around the iteration, which actually reduces LOC.

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

parent b58c8998

kernel/cgroup.c

+2 −9

Original line number	Diff line number	Diff line
		@@ -2763,10 +2763,7 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)
		*/
		update_before = cgroup_serial_nr_next;

		mutex_unlock(&cgroup_mutex);

		/* add/rm files for all cgroups created before */
		rcu_read_lock();
		css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
		struct cgroup *cgrp = css->cgroup;

		@@ -2775,23 +2772,19 @@ static int cgroup_cfts_commit(struct cftype *cfts, bool is_add)

		inode = cgrp->dentry->d_inode;
		dget(cgrp->dentry);
		rcu_read_unlock();

		dput(prev);
		prev = cgrp->dentry;

		mutex_unlock(&cgroup_mutex);
		mutex_lock(&inode->i_mutex);
		mutex_lock(&cgroup_mutex);
		if (cgrp->serial_nr < update_before && !cgroup_is_dead(cgrp))
		ret = cgroup_addrm_files(cgrp, cfts, is_add);
		mutex_unlock(&cgroup_mutex);
		mutex_unlock(&inode->i_mutex);

		rcu_read_lock();
		if (ret)
		break;
		}
		rcu_read_unlock();
		mutex_unlock(&cgroup_mutex);
		dput(prev);
		deactivate_super(sb);
		return ret;

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