Re: [PATCH 7/7] xen/evtchn: read producer index only once

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.02.2021 11:41, Jürgen Groß wrote:
> On 08.02.21 10:48, Jan Beulich wrote:
>> On 06.02.2021 11:49, Juergen Gross wrote:
>>> In evtchn_read() use READ_ONCE() for reading the producer index in
>>> order to avoid the compiler generating multiple accesses.
>>>
>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>>> ---
>>>   drivers/xen/evtchn.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
>>> index 421382c73d88..f6b199b597bf 100644
>>> --- a/drivers/xen/evtchn.c
>>> +++ b/drivers/xen/evtchn.c
>>> @@ -211,7 +211,7 @@ static ssize_t evtchn_read(struct file *file, char 
>>> __user *buf,
>>>                     goto unlock_out;
>>>   
>>>             c = u->ring_cons;
>>> -           p = u->ring_prod;
>>> +           p = READ_ONCE(u->ring_prod);
>>>             if (c != p)
>>>                     break;
>>
>> Why only here and not also in
>>
>>              rc = wait_event_interruptible(u->evtchn_wait,
>>                                            u->ring_cons != u->ring_prod);
>>
>> or in evtchn_poll()? I understand it's not needed when
>> ring_prod_lock is held, but that's not the case in the two
>> afaics named places. Plus isn't the same then true for
>> ring_cons and ring_cons_mutex, i.e. aren't the two named
>> places plus evtchn_interrupt() also in need of READ_ONCE()
>> for ring_cons?
> 
> The problem solved here is the further processing using "p" multiple
> times. p must not be silently replaced with u->ring_prod by the
> compiler, so I probably should reword the commit message to say:
> 
> ... in order to not allow the compiler to refetch p.

I still wouldn't understand the change (and the lack of
further changes) then: The first further use of p is
outside the loop, alongside one of c. IOW why would c
then not need treating the same as p?

I also still don't see the difference between latching a
value into a local variable vs a "freestanding" access -
neither are guaranteed to result in exactly one memory
access afaict.

And of course there's also our beloved topic of access
tearing here: READ_ONCE() also excludes that (at least as
per its intentions aiui).

Jan