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Re: [PATCH v3 1/8] domain: GADDR based shared guest area registration alternative - teardown
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Wed, 27 Sep 2023 12:50:48 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Wed, 27 Sep 2023 10:51:11 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
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