Messages in this thread | | | Date | Thu, 17 Apr 2008 13:04:47 +0800 | From | Li Zefan <> | Subject | Re: [PATCH] cgroup: fix a race condition in manipulating tsk->cg_list |
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Paul Menage wrote: > 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). >
Right. I was lucky to trigger this and thus knew how to reproduce.
> 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. >
Agree. I think it won't be a problem.
> 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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