cgroup: fix cgroupfs_root early destruction path
authorTejun Heo <tj@kernel.org>
Wed, 26 Jun 2013 01:04:54 +0000 (18:04 -0700)
committerTejun Heo <tj@kernel.org>
Wed, 26 Jun 2013 17:39:46 +0000 (10:39 -0700)
commit1672d040709b789671c0502e7aac9d632c2f9175
treeeceebdf6be0c9769e2c10270b4389c402d4a0df9
parentfc76df706123602214da494ba98bccea83e2cfff
cgroup: fix cgroupfs_root early destruction path

cgroupfs_root used to have ->actual_subsys_mask in addition to
->subsys_mask.  a8a648c4ac ("cgroup: remove
cgroup->actual_subsys_mask") removed it noting that the subsys_mask is
essentially temporary and doesn't belong in cgroupfs_root; however,
the patch made it impossible to tell whether a cgroupfs_root actually
has the subsystems bound or just have the bits set leading to the
following BUG when trying to mount with subsystems which are already
mounted elsewhere.

 kernel BUG at kernel/cgroup.c:1038!
 invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
 ...
 CPU: 1 PID: 7973 Comm: mount Tainted: G        W    3.10.0-rc7-next-20130625-sasha-00011-g1c1dc0e #1105
 task: ffff880fc0ae8000 ti: ffff880fc0b9a000 task.ti: ffff880fc0b9a000
 RIP: 0010:[<ffffffff81249b29>]  [<ffffffff81249b29>] rebind_subsystems+0x409/0x5f0
 ...
 Call Trace:
  [<ffffffff8124bd4f>] cgroup_kill_sb+0xff/0x210
  [<ffffffff813d21af>] deactivate_locked_super+0x4f/0x90
  [<ffffffff8124f3b3>] cgroup_mount+0x673/0x6e0
  [<ffffffff81257169>] cpuset_mount+0xd9/0x110
  [<ffffffff813d2580>] mount_fs+0xb0/0x2d0
  [<ffffffff81404afd>] vfs_kern_mount+0xbd/0x180
  [<ffffffff814070b5>] do_new_mount+0x145/0x2c0
  [<ffffffff814085d6>] do_mount+0x356/0x3c0
  [<ffffffff8140873d>] SyS_mount+0xfd/0x140
  [<ffffffff854eb600>] tracesys+0xdd/0xe2

We still want rebind_subsystems() to take added/removed masks, so
let's fix it by marking whether a cgroupfs_root has finished binding
or not.  Also, document what's going on around ->subsys_mask
initialization so that similar mistakes aren't repeated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: Li Zefan <lizefan@huawei.com>
include/linux/cgroup.h
kernel/cgroup.c