Re: [PATCH] New System call unshare (try 2)

[Janak Desai <janak@us.ibm.com>]

Chris Wright wrote:
> * Jamie Lokier (jamie@shareable.org) wrote:
> 
>>Janak Desai wrote:
>>
>>>	Don't allow sighand unsharing if not unsharing vm
>>
>>Why not?  It's permitted to clone with unshared sighand and shared vm,
>>and it's useful too.
> 
> 
> I think that one's just backwards.  Although I do question how useful it
> is to unshare sighand.  Sharing vm is pretty intimate ;-)
> 
> 
>>It's the combination shared sighand + unshared vm which is not
>>allowed by clone - so I think that's what you should refuse.
>>
>>
>>>	Don't allow vm unsharing if task cloned with CLONE_THREAD
>>
>>It would be better to do what clone does, and say "don't allow sighand
>>unsharing if task cloned with CLONE_THREAD".  This is because
>>CLONE_THREAD tasks must have shared signals.
> 
> 
> Yes, I agree.
> 
> 
>>In combination with the rule above for sighand (my rule, not yours),
>>that implies "don't allow vm unsharing.." as a consequence.
>>
>>
>>>	Don't allow vm unsharing if the task is performing async io
>>
>>Why not?
>>
>>Async ios are tied to an mm (see lookup_ioctx in fs/aio.c), which may
>>be shared among tasks.  I see no reason why the async ios can't
>>continue and be waited in on in other tasks that may be using the old mm.
> 
> 
> My concern was the case where there are no other tasks.  But I don't
> think that's an issue other than having the aio effect of setting up
> aio then exiting.
> 
> thanks,
> -chris
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 
> 

Thanks Chris and Jamie. I understand the issues that you raised and
agree with your change recommendations. I will make the necessary
changes, test aio and unsharing of signal handlers (while keeping
shared vm) and post the updated patch by tomorrow.

-Janak