Commit aa32362f authored Sep 04, 2014 by Li Zefan Committed by Tejun Heo Sep 05, 2014

cgroup: check cgroup liveliness before unbreaking kernfs



When cgroup_kn_lock_live() is called through some kernfs operation and
another thread is calling cgroup_rmdir(), we'll trigger the warning in
cgroup_get().

------------[ cut here ]------------
WARNING: CPU: 1 PID: 1228 at kernel/cgroup.c:1034 cgroup_get+0x89/0xa0()
...
Call Trace:
 [<c16ee73d>] dump_stack+0x41/0x52
 [<c10468ef>] warn_slowpath_common+0x7f/0xa0
 [<c104692d>] warn_slowpath_null+0x1d/0x20
 [<c10bb999>] cgroup_get+0x89/0xa0
 [<c10bbe58>] cgroup_kn_lock_live+0x28/0x70
 [<c10be3c1>] __cgroup_procs_write.isra.26+0x51/0x230
 [<c10be5b2>] cgroup_tasks_write+0x12/0x20
 [<c10bb7b0>] cgroup_file_write+0x40/0x130
 [<c11aee71>] kernfs_fop_write+0xd1/0x160
 [<c1148e58>] vfs_write+0x98/0x1e0
 [<c114934d>] SyS_write+0x4d/0xa0
 [<c16f656b>] sysenter_do_call+0x12/0x12
---[ end trace 6f2e0c38c2108a74 ]---

Fix this by calling css_tryget() instead of cgroup_get().

v2:
- move cgroup_tryget() right below cgroup_get() definition. (Tejun)

Cc: <stable@vger.kernel.org> # 3.15+
Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

parent a4189487

kernel/cgroup.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -1035,6 +1035,11 @@ static void cgroup_get(struct cgroup *cgrp)
		css_get(&cgrp->self);
		}

		static bool cgroup_tryget(struct cgroup *cgrp)
		{
		return css_tryget(&cgrp->self);
		}

		static void cgroup_put(struct cgroup *cgrp)
		{
		css_put(&cgrp->self);
		@@ -1147,7 +1152,8 @@ static struct cgroup cgroup_kn_lock_live(struct kernfs_node kn)
		* protection against removal. Ensure @cgrp stays accessible and
		* break the active_ref protection.
		*/
		cgroup_get(cgrp);
		if (!cgroup_tryget(cgrp))
		return NULL;
		kernfs_break_active_protection(kn);

		mutex_lock(&cgroup_mutex);

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