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

[Chris Wright]

* Jamie Lokier (jamie@xxxxxxxxxxxxx) 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
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