Re: [Xen-devel] [PATCH v3 2/2] xen: merge temporary vcpu pinning scenarios

[Juergen Gross <jgross@xxxxxxxx>]

On 26.07.19 13:50, Andrew Cooper wrote:
> On 26/07/2019 12:42, Juergen Gross wrote:
> > On 26.07.19 13:35, Jan Beulich wrote:
> > > On 26.07.2019 11:46, Andrew Cooper wrote:
> > > > On 24/07/2019 12:26, Juergen Gross wrote:
> > > > > @@ -182,30 +178,24 @@ static void __prepare_to_wait(struct
> > > > > waitqueue_vcpu *wqv)
> > > > >     static void __finish_wait(struct waitqueue_vcpu *wqv)
> > > > >     {
> > > > >         wqv->esp = NULL;
> > > > > -    (void)vcpu_set_hard_affinity(current, &wqv->saved_affinity);
> > > > > +    vcpu_temporary_affinity(current, NR_CPUS, VCPU_AFFINITY_WAIT);
> > > > >     }
> > > > >        void check_wakeup_from_wait(void)
> > > > >     {
> > > > > -    struct waitqueue_vcpu *wqv = current->waitqueue_vcpu;
> > > > > +    struct vcpu *curr = current;
> > > > > +    struct waitqueue_vcpu *wqv = curr->waitqueue_vcpu;
> > > > >            ASSERT(list_empty(&wqv->list));
> > > > >            if ( likely(wqv->esp == NULL) )
> > > > >             return;
> > > > >     -    /* Check if we woke up on the wrong CPU. */
> > > > > -    if ( unlikely(smp_processor_id() != wqv->wakeup_cpu) )
> > > > > +    /* Check if we are still pinned. */
> > > > > +    if ( unlikely(!(curr->affinity_broken & VCPU_AFFINITY_WAIT)) )
> > > > >         {
> > > > > -        /* Re-set VCPU affinity and re-enter the scheduler. */
> > > > > -        struct vcpu *curr = current;
> > > > > -        cpumask_copy(&wqv->saved_affinity, curr->cpu_hard_affinity);
> > > > > -        if ( vcpu_set_hard_affinity(curr,
> > > > > cpumask_of(wqv->wakeup_cpu)) )
> > > > > -        {
> > > > > -            gdprintk(XENLOG_ERR, "Unable to set vcpu affinity\n");
> > > > > -            domain_crash(current->domain);
> > > > > -        }
> > > > > -        wait(); /* takes us back into the scheduler */
> > > > > +        gdprintk(XENLOG_ERR, "vcpu affinity lost\n");
> > > > > +        domain_crash(curr->domain);
> > > > >         }
> > > > I'm sorry to retract my R-by after the fact, but I've only just noticed
> > > > (while rebasing some of my pending work over this) that it is buggy.
> > > >
> > > > The reason wait() was called is because it is not safe to leave that
> > > > if() clause.
> > > >
> > > > With this change in place, we'll arrange for the VM to be crashed, then
> > > > longjump back into the stack from from the waiting vCPU, on the wrong
> > > > CPU.  Any caller with smp_processor_id() or thread-local variables
> > > > cache
> > > > by pointer on the stack will then cause memory corruption.
> > > >
> > > > Its not immediately obvious how to fix this, but bear in mind that as
> > > > soon as the vm-event interface is done, I plan to delete this whole
> > > > waitqueue infrastructure anyway.
> > > In which case - should we revert the commit until this is resolved?
> > In my opinion it is not that urgent. I don't think any of our OSStests
> > will ever be able to trigger this issue, as AFAIK no test is using the
> > wait_event() interface nor do they test suspend/resume. And both need
> > to be true (at the same time!) plus a cpu needs to fail coming up when
> > resuming again.
> Yeah - I don't think reverting it is necessary, but I will flag
> "resolving this somehow" as a 4.12 blocker.
>
> The HVI scale tests trigger this path.  Guess how I discovered that
> Introspection + Livepatching = boom.
>
> I am leaning on the side of panic().  I agree that if the APIs are used
> correctly, it can't occur.
Hmm, shouldn't

   domain_crash();
   raise_softirq(SCHEDULE_SOFTIRQ);
   return;

in check_wakeup_from_wait() just work?


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel