Re: [PATCH v6 1/3] evtchn: slightly defer lock acquire where possible

[Julien Grall <julien@xen.org>]

Hi Jan,

On 28/05/2021 11:23, Jan Beulich wrote:
> On 28.05.2021 10:30, Roger Pau Monné wrote:
>> On Thu, May 27, 2021 at 07:48:41PM +0100, Julien Grall wrote:
>>> On 27/05/2021 12:28, Jan Beulich wrote:
>>>> port_is_valid() and evtchn_from_port() are fine to use without holding
>>>> any locks. Accordingly acquire the per-domain lock slightly later in
>>>> evtchn_close() and evtchn_bind_vcpu().
>>>
>>> So I agree that port_is_valid() and evtchn_from_port() are fine to use
>>> without holding any locks in evtchn_bind_vcpu(). However, this is misleading
>>> to say there is no problem with evtchn_close().
>>>
>>> evtchn_close() can be called with current != d and therefore, there is a
>>
>> The only instances evtchn_close is called with current != d and the
>> domain could be unpaused is in free_xen_event_channel AFAICT.
> 
> As long as the domain is not paused, ->valid_evtchns can't ever
> decrease: The only point where this gets done is in evtchn_destroy().
> Hence ...
> 
>>> risk that port_is_valid() may be valid and then invalid because
>>> d->valid_evtchns is decremented in evtchn_destroy().
>>
>> Hm, I guess you could indeed have parallel calls to
>> free_xen_event_channel and evtchn_destroy in a way that
>> free_xen_event_channel could race with valid_evtchns getting
>> decreased?
> 
> ... I don't see this as relevant.
> 
>>> Thankfully the memory is still there. So the current code is okayish and I
>>> could reluctantly accept this behavior to be spread. However, I don't think
>>> this should be left uncommented in both the code (maybe on top of
>>> port_is_valid()?) and the commit message.
>>
>> Indeed, I think we need some expansion of the comment in port_is_valid
>> to clarify all this. I'm not sure I understand it properly myself when
>> it's fine to use port_is_valid without holding the per domain event
>> lock.
> 
> Because of the above property plus the fact that even if
> ->valid_evtchns decreases, the underlying struct evtchn instance
> will remain valid (i.e. won't get de-allocated, which happens only
> in evtchn_destroy_final()), it is always fine to use it without
> lock. With this I'm having trouble seeing what would need adding
> to port_is_valid()'s commentary.

Lets take the example of free_xen_event_channel(). The function is 
checking if the port is valid. If it is, then evtchn_close() will be called.

At this point, it would be fair for a developper to assume that 
port_is_valid() will also return true in event_close().

To push to the extreme, if free_xen_event_channel() was the only caller 
of evtchn_close(), one could argue that the check in evtchn_close() 
could be a BUG_ON().

However, this can't be because this would sooner or later turn to an XSA.

Effectively, it means that is_port_valid() *cannot* be used in an 
ASSERT()/BUG_ON() and every caller should check the return even if the 
port was previously validated.

So I think a comment on top of is_port_valid() would be really useful 
before someone rediscover it the hard way.

Cheers,

-- 
Julien Grall