Re: kvm PCI assignment & VFIO ramblings

Avi Kivity
2011-08-28T14:04:32Z

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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