[CRIU] Fix cpuset restore-in-root bug

[Pavel Emelyanov]

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.

And one more notice -- it looks like we have generic service descriptors 
closing problem. Helpers inherit files table from parent but close the
descriptors.

Thanks,
Pavel