[13/37] cgroup: fix a race condition in manipulating tsk->cg_list

Greg KH
2008-04-29T17:25:16Z

[-- Attachment #1: cgroup-fix-a-race-condition-in-manipulating-tsk-cg_list.patch --]
[-- Type: text/plain, Size: 2474 bytes --]

2.6.25-stable review patch. If anyone has any objections, please let us
know.

------------------
From: Li Zefan <lizf@cn.fujitsu.com>

commit: 0e04388f0189fa1f6812a8e1cb6172136eada87e

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

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 <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/cgroup.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

--- 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);

--