Re: [GIT PULL] First batch of KVM changes for 4.1

Marcelo Tosatti
2015-04-23T11:52:17Z

On Thu, Apr 23, 2015 at 11:13:23AM +0200, Paolo Bonzini wrote:
>
>
> On 22/04/2015 23:21, Marcelo Tosatti wrote:
> > On Mon, Apr 20, 2015 at 01:27:58PM -0700, Andy Lutomirski wrote:
> >> On Mon, Apr 20, 2015 at 9:59 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >>>
> >>>
> >>> On 17/04/2015 22:18, Marcelo Tosatti wrote:
> >>>> The bug which this is fixing is very rare, have no memory of a report.
> >>>>
> >>>> In fact, its even difficult to create a synthetic reproducer.
> >>>
> >>> But then why was the task migration notifier even in Jeremy's original
> >>> code for Xen? Was it supposed to work even on non-synchronized TSC?
> >>>
> >>> If that's the case, then it could be reverted indeed; but then why did
> >>> you commit this patch to 4.1? Did you think of something that would
> >>> cause the seqcount-like protocol to fail, and that turned out not to be
> >>> the case later? I was only following the mailing list sparsely in March.
> >>
> >> I don't think anyone ever tried that hard to test this stuff. There
> >> was an infinte loop that Firefox was triggering as a KVM guest
> >> somewhat reliably until a couple months ago in the same vdso code. :(
> >
> > https://bugzilla.redhat.com/show_bug.cgi?id=1174664
>
> That was the missing volatile in an asm. Older compilers didn't catch
> it. :(

How do you know that? It looks like memory corruption (look at the
pattern at the end).