cgroup: check cgroup liveliness before unbreaking kernfs
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: <4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org> # 3.15+
Reported-by: Toralf Förster <24e9a7e86225fc01f236a81fc86e902a74542aa1@gmx.de>
Signed-off-by: Zefan Li <a8167b9d27aca9c6624754deb52c4c6420eded04@huawei.com>
Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs