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

[Juergen Gross <jgross@xxxxxxxx>]

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.


Juergen


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