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    SubjectRe: [PATCH] cgroup: fix a race condition in manipulating tsk->cg_list
    On Wed, Apr 16, 2008 at 8:37 PM, Li Zefan <lizf@cn.fujitsu.com> 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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