Re: [PATCH v2 1/2] xen/events: access last_priority and last_vcpu_id together

[Julien Grall <julien@xen.org>]

On 15/10/2020 13:07, Jan Beulich wrote:
> On 14.10.2020 13:40, Julien Grall wrote:
>> Hi Jan,
>>
>> On 13/10/2020 15:26, Jan Beulich wrote:
>>> On 13.10.2020 16:20, Jürgen Groß wrote:
>>>> On 13.10.20 15:58, Jan Beulich wrote:
>>>>> On 12.10.2020 11:27, Juergen Gross wrote:
>>>>>> The queue for a fifo event is depending on the vcpu_id and the
>>>>>> priority of the event. When sending an event it might happen the
>>>>>> event needs to change queues and the old queue needs to be kept for
>>>>>> keeping the links between queue elements intact. For this purpose
>>>>>> the event channel contains last_priority and last_vcpu_id values
>>>>>> elements for being able to identify the old queue.
>>>>>>
>>>>>> In order to avoid races always access last_priority and last_vcpu_id
>>>>>> with a single atomic operation avoiding any inconsistencies.
>>>>>>
>>>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>>>
>>>>> I seem to vaguely recall that at the time this seemingly racy
>>>>> access was done on purpose by David. Did you go look at the
>>>>> old commits to understand whether there really is a race which
>>>>> can't be tolerated within the spec?
>>>>
>>>> At least the comments in the code tell us that the race regarding
>>>> the writing of priority (not last_priority) is acceptable.
>>>
>>> Ah, then it was comments. I knew I read something to this effect
>>> somewhere, recently.
>>>
>>>> Especially Julien was rather worried by the current situation. In
>>>> case you can convince him the current handling is fine, we can
>>>> easily drop this patch.
>>>
>>> Julien, in the light of the above - can you clarify the specific
>>> concerns you (still) have?
>>
>> Let me start with that the assumption if evtchn->lock is not held when
>> evtchn_fifo_set_pending() is called. If it is held, then my comment is moot.
> 
> But this isn't interesting - we know there are paths where it is
> held, and ones (interdomain sending) where it's the remote port's
> lock instead which is held. What's important here is that a
> _consistent_ lock be held (but it doesn't need to be evtchn's).

Yes, a _consistent_ lock *should* be sufficient. But it is better to use 
the same lock everywhere so it is easier to reason (see more below).

> 
>>   From my understanding, the goal of lock_old_queue() is to return the
>> old queue used.
>>
>> last_priority and last_vcpu_id may be updated separately and I could not
>> convince myself that it would not be possible to return a queue that is
>> neither the current one nor the old one.
>>
>> The following could happen if evtchn->priority and
>> evtchn->notify_vcpu_id keeps changing between calls.
>>
>> pCPU0				| pCPU1
>> 				|
>> evtchn_fifo_set_pending(v0,...)	|
>> 				| evtchn_fifo_set_pending(v1, ...)
>>    [...]				|
>>    /* Queue has changed */	|
>>    evtchn->last_vcpu_id = v0 	|
>> 				| -> evtchn_old_queue()
>> 				| v = d->vcpu[evtchn->last_vcpu_id];
>>     				| old_q = ...
>> 				| spin_lock(old_q->...)
>> 				| v = ...
>> 				| q = ...
>> 				| /* q and old_q would be the same */
>> 				|
>>    evtchn->las_priority = priority|
>>
>> If my diagram is correct, then pCPU1 would return a queue that is
>> neither the current nor old one.
> 
> I think I agree.
> 
>> In which case, I think it would at least be possible to corrupt the
>> queue. From evtchn_fifo_set_pending():
>>
>>           /*
>>            * If this event was a tail, the old queue is now empty and
>>            * its tail must be invalidated to prevent adding an event to
>>            * the old queue from corrupting the new queue.
>>            */
>>           if ( old_q->tail == port )
>>               old_q->tail = 0;
>>
>> Did I miss anything?
> 
> I don't think you did. The important point though is that a consistent
> lock is being held whenever we come here, so two racing set_pending()
> aren't possible for one and the same evtchn. As a result I don't think
> the patch here is actually needed.

I haven't yet read in full details the rest of the patches to say 
whether this is necessary or not. However, at a first glance, I think 
this is not a sane to rely on different lock to protect us. And don't 
get me started on the lack of documentation...

Furthermore, the implementation of old_lock_queue() suggests that the 
code was planned to be lockless. Why would you need the loop otherwise?

Therefore, regardless the rest of the discussion, I think this patch 
would be useful to have for our peace of mind.

> 
> If I take this further, then I think I can reason why it wasn't
> necessary to add further locking to send_guest_{global,vcpu}_virq():
> The virq_lock is the "consistent lock" protecting ECS_VIRQ ports. The
> spin_barrier() while closing the port guards that side against the
> port changing to a different ECS_* behind the sending functions' backs.
> And binding such ports sets ->virq_to_evtchn[] last, with a suitable
> barrier (the unlock).

This makes sense.

> 
> Which leaves send_guest_pirq() before we can drop the IRQ-safe locking
> again. I guess we would need to work towards using the underlying
> irq_desc's lock as consistent lock here, but this certainly isn't the
> case just yet, and I'm not really certain this can be achieved.
I can't comment on the PIRQ code but I think this is a risky approach 
(see more above).

Cheers,

-- 
Julien Grall