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

[Juergen Gross <jgross@xxxxxxxx>]

On 26.07.19 11:46, Andrew Cooper wrote:
> On 24/07/2019 12:26, Juergen Gross wrote:
> > diff --git a/xen/common/wait.c b/xen/common/wait.c
> > index 4f830a14e8..3fc5f68611 100644
> > --- a/xen/common/wait.c
> > +++ b/xen/common/wait.c
> > @@ -34,8 +34,6 @@ struct waitqueue_vcpu {
> >        */
> >       void *esp;
> >       char *stack;
> > -    cpumask_t saved_affinity;
> > -    unsigned int wakeup_cpu;
> >   #endif
> >   };
> >   
> > @@ -131,12 +129,10 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv)
> >       ASSERT(wqv->esp == 0);
> >   
> >       /* Save current VCPU affinity; force wakeup on *this* CPU only. */
> > -    wqv->wakeup_cpu = smp_processor_id();
> > -    cpumask_copy(&wqv->saved_affinity, curr->cpu_hard_affinity);
> > -    if ( vcpu_set_hard_affinity(curr, cpumask_of(wqv->wakeup_cpu)) )
> > +    if ( vcpu_temporary_affinity(curr, smp_processor_id(), VCPU_AFFINITY_WAIT) 
> > )
> >       {
> >           gdprintk(XENLOG_ERR, "Unable to set vcpu affinity\n");
> > -        domain_crash(current->domain);
> > +        domain_crash(curr->domain);
> >   
> >           for ( ; ; )
> >               do_softirq();
> > @@ -170,7 +166,7 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv)
> >       if ( unlikely(wqv->esp == 0) )
> >       {
> >           gdprintk(XENLOG_ERR, "Stack too large in %s\n", __func__);
> > -        domain_crash(current->domain);
> > +        domain_crash(curr->domain);
> >   
> >           for ( ; ; )
> >               do_softirq();
> > @@ -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.
Shouldn't just calling wait() after domain_crash() be fine then?

That's what would have happened in the original error case, too.


Juergen

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