Re: [PATCH v3 1/8] domain: GADDR based shared guest area registration alternative - teardown

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Sep 27, 2023 at 12:46:07PM +0200, Jan Beulich wrote:
> On 27.09.2023 12:42, Roger Pau Monné wrote:
> > On Wed, Sep 27, 2023 at 11:55:19AM +0200, Jan Beulich wrote:
> >> On 27.09.2023 10:51, Roger Pau Monné wrote:
> >>> On Wed, May 03, 2023 at 05:54:47PM +0200, Jan Beulich wrote:
> >>>> +{
> >>>> +    struct domain *d = v->domain;
> >>>> +
> >>>> +    if ( v != current )
> >>>> +        ASSERT(atomic_read(&v->pause_count) | atomic_read(&d->pause_count));
> >>>
> >>> Isn't this racy?
> >>
> >> It is, yes.
> >>
> >>>  What guarantees that the vcpu won't be kicked just
> >>> after the check has been performed?
> >>
> >> Nothing. This check isn't any better than assertions towards an ordinary
> >> spinlock being held. I assume you realize that we've got a number of such
> >> assertions elsewhere already.
> > 
> > Right, but different from spinlock assertions, the code here could be
> > made safe just by pausing the vCPU?
> 
> That's what the assertion is checking (see also the comment ahead of the
> function). It's just that the assertions cannot be made more strict, at
> least from all I can tell.

But the assertion might no longer be true by the time the code
afterwards is executed.  Why not wrap the code in a pair of
vcpu_{,un}pause() calls?

Thanks, Roger.