Re: [PATCH cgroup/for-3.11] cgroup: fix cgroupfs_root early destruction path

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2013/6/26 9:04, Tejun Heo wrote:
> 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-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> Reported-by: Sasha Levin <sasha.levin-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>

Acked-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>