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Date:	Wed, 11 Nov 2009 21:06:48 +0100
From:	Daniel Lezcano <daniel.lezcano@...e.fr>
To:	Li Zefan <lizf@...fujitsu.com>
CC:	bblum@...rew.cmu.edu, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	serue@...ibm.com, menage@...gle.com
Subject: Re: [PATCH 5/6] Makes procs file writable to move all threads by
 tgid at	once

Li Zefan wrote:
>>>> Hi Ben,
>>>>
>>>> The current code (with or without your patch) may lead to an error
>>>> because the fork hook can fail and the exit hook is called in all the
>>>> cases making the fork / exit asymmetric.
>>>>
>>>>     
>>>>         
>>> The _current_ code won't lead to this error, because the fork hook
>>> can't fail.
>>>   
>>>       
>> Right, as no subsystem is using both hooks right now, the bug is never
>> triggered and the current code won't lead to an error.
>> But from my POV, there is a bug hidden in a corner waiting for a
>> subsystem to make use of the fail-able fork / exit :)
>>
>>     
>
> Actually the freezer subsystem is using the fork hook, but it doesn't
> need to be able to fail it.
>
> I don't think we can claim this a bug. If there is a new subsystem
> that needs fail-able fork hook, it has to extent the hook interface
> and adjust the code to meet its needs.
>
> We always adjust our code to meet new needs, don't we?
>   
Sure.

>>>> I will take the usual example with a cgroup with a counter of tasks, in
>>>> the fork hook it increments the counter, in the exit hook it decrements
>>>> the counter. There is one process in the cgroup, hence the counter value
>>>> is 1. Now this process forks and the fork hook fails before the task
>>>> counter is incremented to 2, this is not detected in copy process
>>>> function because the cgroup_fork_callbacks does not return an error, so
>>>> the process will be forked without error and when the process will exits
>>>> the counter will be decremented reaching 0 instead of 1.
>>>>
>>>> IMO, the correct fix should be to make the fork hook to return an error
>>>> and have the cgroup to call the exit method of the subsystem where the
>>>> fork hook was called. For example, there are 10 subsystems using the
>>>> fork / exit hooks, when the a process forks, the fork callbacks is
>>>> called for these subsystems but, let's say, the 3rd fails. So we undo,
>>>> by calling the exit hooks of the first two.
>>>>
>>>> I wrote a patchset to consolidate the hooks called in the cgroump for
>>>> fork and exit, and one of them does a rollback for the fork hook when an
>>>> error occurs. I added an attachment the patch as an example.
>>>>
>>>>     
>>>>         
>>> I'd like to see this patch sent with another patch that needs this
>>> fail-able fork() hook.
>>>
>>> Note this patch is not doing a _fix_, but does an extension. And
>>> for now, this extension is not needed.
>>>   
>>>       
>> I don't know, may be it could be interesting to implement that before
>> more subsystems make use of these hooks.
>> This is not critical, that can be sent later, separately from this
>> patchset of course.
>>
>>     
>
> We tend to remove code that is not used. For example, we may remove
> subsys->bind() interface, because no one is using it, though it has
> been there for years.
>
> So adding things that are not used is normally not good.
>   
Makes sense.

Thanks
  -- Daniel

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