Re: [PATCH] cgroup: fix a race condition in manipulating tsk->cg_list

[Paul Menage]

On Wed, Apr 16, 2008 at 8:37 PM, Li Zefan <lizf@xxxxxxxxxxxxxx> wrote:
> When I ran a test program to fork mass processes and at the same time
>  'cat /cgroup/tasks', I got the following oops:
>
>  ------------[ cut here ]------------
>  kernel BUG at lib/list_debug.c:72!
>  invalid opcode: 0000 [#1] SMP
>  Pid: 4178, comm: a.out Not tainted (2.6.25-rc9 #72)
>  ...
>  Call Trace:
>   [<c044a5f9>] ? cgroup_exit+0x55/0x94
>   [<c0427acf>] ? do_exit+0x217/0x5ba
>   [<c0427ed7>] ? do_group_exit+0.65/0x7c
>   [<c0427efd>] ? sys_exit_group+0xf/0x11
>   [<c0404842>] ? syscall_call+0x7/0xb
>   [<c05e0000>] ? init_cyrix+0x2fa/0x479
>  ...
>  EIP: [<c04df671>] list_del+0x35/0x53 SS:ESP 0068:ebc7df4
>  ---[ end trace caffb7332252612b ]---
>  Fixing recursive fault but reboot is needed!
>
>  After digging into the code and debugging, I finlly found out a race
>  situation:
>                                 do_exit()
>                                   ->cgroup_exit()
>                                     ->if (!list_empty(&tsk->cg_list))
>                                         list_del(&tsk->cg_list);
>
>  cgroup_iter_start()
>   ->cgroup_enable_task_cg_list()
>     ->list_add(&tsk->cg_list, ..);
>
>  In this case the list won't be deleted though the process has exited.
>
>  We got two bug reports in the past, which seem to be the same bug as
>  this one:
>         http://lkml.org/lkml/2008/3/5/332
>         http://lkml.org/lkml/2007/10/17/224

Yes, that looks like it could be the same one - great. But this
corruption can only be triggered the first time you cat a tasks file
after a reboot, right? That would partly explain why it was hard to
reproduce (at least, I had trouble).

My only thought about the downside of this is that an exiting task
that gets stuck somewhere  between setting PF_EXITING and calling
cgroup_exit() won't show up in its cgroup's tasks file, since we'll
enable cgroup links but skip it. I guess that's not a big deal.

Maybe it would be better to not do a cgroup_exit() until we're
unhashed, so that cgroup_enable_task_cg_list() can't find the exiting
task?

Paul
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