Messages in this thread | | | Date | Wed, 16 Apr 2008 21:18:41 -0700 | From | "Paul Menage" <> | Subject | Re: [PATCH] cgroup: fix a race condition in manipulating tsk->cg_list |
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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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