From 0e04388f0189fa1f6812a8e1cb6172136eada87e Mon Sep 17 00:00:00 2001 From: Li Zefan Date: Thu, 17 Apr 2008 11:37:15 +0800 Subject: [PATCH] cgroup: fix a race condition in manipulating tsk->cg_list 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: [] ? cgroup_exit+0x55/0x94 [] ? do_exit+0x217/0x5ba [] ? do_group_exit+0.65/0x7c [] ? sys_exit_group+0xf/0x11 [] ? syscall_call+0x7/0xb [] ? init_cyrix+0x2fa/0x479 ... EIP: [] 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 Actually sometimes I got oops on list_del, sometimes oops on list_add. And I can change my test program a bit to trigger other oops. The patch has been tested both on x86_32 and x86_64. Signed-off-by: Li Zefan Acked-by: Paul Menage Cc: Andrew Morton Cc: stable@kernel.org Signed-off-by: Linus Torvalds --- kernel/cgroup.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/kernel/cgroup.c b/kernel/cgroup.c index 2727f9238359..6d8de051382b 100644 --- a/kernel/cgroup.c +++ b/kernel/cgroup.c @@ -1722,7 +1722,12 @@ void cgroup_enable_task_cg_lists(void) use_task_css_set_links = 1; do_each_thread(g, p) { task_lock(p); - if (list_empty(&p->cg_list)) + /* + * We should check if the process is exiting, otherwise + * it will race with cgroup_exit() in that the list + * entry won't be deleted though the process has exited. + */ + if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list)) list_add(&p->cg_list, &p->cgroups->tasks); task_unlock(p); } while_each_thread(g, p); -- 2.20.1