[CRIU] Fix cpuset restore-in-root bug

[Pavel Emelyanov]

On 10/08/2014 06:16 PM, Tycho Andersen wrote:
> On Wed, Oct 08, 2014 at 06:02:04PM +0400, Pavel Emelyanov wrote:
>> On 10/08/2014 05:57 PM, Pavel Emelyanov wrote:
>>> On 10/08/2014 02:43 AM, Tycho Andersen wrote:
>>>> Hi Pavel,
>>>>
>>>> On Tue, Oct 07, 2014 at 03:58:39PM -0500, Tycho Andersen wrote:
>>>>> On Tue, Oct 07, 2014 at 11:15:34PM +0400, Pavel Emelyanov wrote:
>>>>>> On 10/07/2014 06:16 PM, Tycho Andersen wrote:
>>>>>>> Hi Pavel,
>>>>>>>
>>>>>>> On Tue, Oct 07, 2014 at 12:58:34PM +0400, Pavel Emelyanov wrote:
>>>>>>>> On 10/06/2014 06:03 PM, Tycho Andersen wrote:
>>>>>>>>> Hi all,
>>>>>>>>>
>>>>>>>>> Based on some discussion, we shouldn't try to do any intelligent copying of any
>>>>>>>>> properties anywhere else in the tree. These cpuset properties are special only
>>>>>>>>> in the sense that they need to be restored before the move_in_cgroup call, and
>>>>>>>>> if the parent cgroups have conflicting options (or empty options), it is best
>>>>>>>>> to just let the restore fail.
>>>>>>>>
>>>>>>>> Patches applied, thanks!
>>>>>>>>
>>>>>>>>> Also, I noticed when making this patch that I am getting a lot of:
>>>>>>>>>
>>>>>>>>> (00.092575)    290: Error (util.c:101): Unable to close fd 1016: Bad file descriptor
>>>>>>>>
>>>>>>>> Ouch. Can you show the full log please?
>>>>>>>
>>>>>>> Yes, these are with the current master:
>>>>>>>
>>>>>>> http://paste.ubuntu.com/8514798/
>>>>>>
>>>>>> Hm... I've run cgroup* tests on recent HEAD and no errors in logs.
>>>>>>
>>>>>>> also, 1016 is the cg service fd, so it seems like maybe a recent
>>>>>>> reordering broke something? I can do a bisect if you take a look at
>>>>>>> the log and nothing obvious pops out.
>>>>>>
>>>>>> Yes, please. If you would be able to shed more light on this, that
>>>>>> would be awesome.
>>>>>
>>>>> Hmm. I tried to bisect with a patch that I know I wasn't seeing this
>>>>> problem with, but even that one has this issue now. Something about my
>>>>> environment must have changed, I guess? I will investigate more and
>>>>> keep you posted.
>>>>
>>>> The patch below fixes the issue for me. Does it make sense? I have no
>>>> idea why we/I didn't see this earlier.
>>>>
>>>> Tycho
>>>>
>>>>
>>>>
>>>> >From 0f3aad8a78aee56ff7794db33cf76c5d5aae5039 Mon Sep 17 00:00:00 2001
>>>> From: Tycho Andersen <tycho.andersen at canonical.com>
>>>> Date: Tue, 7 Oct 2014 17:25:26 -0500
>>>> Subject: [PATCH] restore: don't race when closing cg yard
>>>>
>>>> If CLONE_FILES is set, we only have one copy of the cg yard fd, so we shouldn't
>>>> all close it. Further, we should wait to close it until after the FORKING stage
>>>> is done, since it is needed during that stage.
>>>>
>>>> If CLONE_FILES is not set, each process can safely close their copy of the fd.
>>>>
>>>> Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
>>>> ---
>>>>  cr-restore.c | 17 +++++++++--------
>>>>  1 file changed, 9 insertions(+), 8 deletions(-)
>>>>
>>>> diff --git a/cr-restore.c b/cr-restore.c
>>>> index fd35bef..c12d28e 100644
>>>> --- a/cr-restore.c
>>>> +++ b/cr-restore.c
>>>> @@ -1461,14 +1461,6 @@ static int restore_task_with_children(void *_arg)
>>>>  	if (create_children_and_session())
>>>>  		goto err_fini_mnt;
>>>>  
>>>> -	/*
>>>> -	 * This must be done after forking to allow child
>>>> -	 * to get the cgroup fd so it can move into the
>>>> -	 * correct /tasks file if it is in a different cgroup
>>>> -	 * set than its parent
>>>> -	 */
>>>> -	close_service_fd(CGROUP_YARD);
>>>> -
>>>>  	if (restore_task_mnt_ns(current))
>>>>  		goto err_fini_mnt;
>>>>  
>>>> @@ -1480,6 +1472,15 @@ static int restore_task_with_children(void *_arg)
>>>>  	if (restore_finish_stage(CR_STATE_FORKING) < 0)
>>>>  		goto err_fini_mnt;
>>>>  
>>>> +	/*
>>>> +	 * This must be done after forking to allow child
>>>> +	 * to get the cgroup fd so it can move into the
>>>> +	 * correct /tasks file if it is in a different cgroup
>>>> +	 * set than its parent
>>>> +	 */
>>>> +	if (!(ca->clone_flags & CLONE_FILES) || current->parent == NULL)
>>>> +		close_service_fd(CGROUP_YARD);
>>>
>>> I think what really makes sense here is the CLONE_FILES check, because
>>> all children forking happens in create_children_and_session().
>>>
>>> And it looks like I know why this appeared only recently -- the helper
>>> tasks are now created with CLONE_FILES, so we may have a lot of helpers.
>>
>> After a bit more thinking and reading the code. The place where this close
>> is called also matters -- if we close CGYARD before other tasks forked, the
>> latter ones may fail to obtain this service fd.
> 
> Yeah, it isn't necessarily clear from the diff, but I moved it after
> the FORKING stage to avoid this problem.

Yes, and this is also correct. I'm still thinking about the CLONE_FILES check.

>> And one more notice -- it looks like we have generic service descriptors 
>> closing problem. Helpers inherit files table from parent but close the
>> descriptors.
> 
> By this you mean that there are some other bugs like this as well?

Should be -- other service descriptors are closed just with close_service_fd,
but seem not to emit such warnings -- I've run the session00 test that generates 
helpers and haven't found any errors like that.

Thanks,
Pavel