Re: [Qemu-devel] kvm PCI assignment & VFIO ramblings

[Avi Kivity]

On 08/28/2011 04:56 PM, Joerg Roedel wrote:
> On Sun, Aug 28, 2011 at 04:14:00PM +0300, Avi Kivity wrote:
> >  On 08/26/2011 12:24 PM, Roedel, Joerg wrote:
>
> >>  The biggest problem with this approach is that it has to happen in the
> >>  context of the given process. Linux can't really modify an mm which
> >>  which belong to another context in a safe way.
> >>
> >
> >  Is use_mm() insufficient?
>
> Yes, it introduces a set of race conditions when a process that already
> has an mm wants to take over another processes mm temporarily (and when
> use_mm is modified to actually provide this functionality). It is only
> save when used from kernel-thread context.
>
> One example:
>
>         Process A               Process B                       Process C
>         .                       .                               .
>         .               <--  takes A->mm                  .
>         .                       and assignes as B->mm                .
>         .                       .                       -->  Wants to take
>         .                       .                               B->mm, but gets
>                                                                 A->mm now

Good catch.

> This can't be secured by a lock, because it introduces potential
> A->B<-->B->A lock problem when two processes try to take each others mm.
> It could probably be solved by a task->real_mm pointer, havn't thought
> about this yet...

Or a workqueue -  you get a kernel thread context with a bit of boilerplate.

--
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