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

[Julien Grall <julien@xxxxxxx>]

On 20/10/2020 10:34, Jan Beulich wrote:
> On 20.10.2020 11:25, Julien Grall wrote:
> > Given that evtchn->last_vcpu_id and evtchn->last_priority can only be
> > modified in evtchn_fifo_set_pending(), this suggests that it is expected
> > for the function to multiple called concurrently on the same event channel.
> >
> > > I'm unconvinced it was really considered
> > > whether racing sending on the same channel is also safe this way.
> > How would you explain the 3 try in lock_old_queue then?
> Queue changes (as said by the gprintk()) can't result from sending
> alone, but require re-binding to a different vCPU or altering the
> priority. 
I agree with that. However, this doesn't change the fact that update to 
evtchn->last_priority and evtchn->last_vcpu can only happen when calling 
evtchn_fifo_set_pending().
If evtchn_fifo_set_pending() cannot be called concurrently for the same 
event, then there is *no* way for evtchn->last_{priority, vcpu} to be 
updated concurrently.
> I'm simply unconvinced that the code indeed fully reflects
> the original intentions. 
Do you mind (re-)sharing what was the original intentions?

Cheers,

--
Julien Grall