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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.02.2021 11:59, Jürgen Groß wrote:
> On 08.02.21 11:51, Jan Beulich wrote:
>> 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?
> 
> Its value wouldn't change, as ring_cons is being modified only at
> the bottom of this function, and nowhere else (apart from the reset
> case, but this can't run concurrently due to ring_cons_mutex).
> 
>> 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.
> 
> READ_ONCE() is using a pointer to volatile, so any refetching by
> the compiler would be a bug.

Of course, but this wasn't my point. I was contrasting

                c = u->ring_cons;
                p = u->ring_prod;

which you change with

                rc = wait_event_interruptible(u->evtchn_wait,
                                              u->ring_cons != u->ring_prod);

which you leave alone.

Jan