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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/07/2019 10:53, Juergen Gross wrote:
> 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.

No - I don't think so.  That was to try and get back into a position
where the scheduler rescheduled this vcpu on the correct cpu, so it
could safely longjmp back into context.

With the domain crash here, nothing will happen[1] until we do
successfully longjmp() back into context, because we've got a stack
frame which needs unwinding before it is safe to start cleaning the
domain up.

~Andrew

[1] If something other than nothing happens, then we've got a
refcounting issue...

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