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

Jan Beulich
2021-02-08T12:33:55Z

On 08.02.2021 13:15, Jürgen Groß wrote:
> On 08.02.21 12:54, Jan Beulich wrote:
>> 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@suse.com>
>>>>>>> ---
>>>>>>> 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.
>
> Can you point out which problem might arise from that?

Not any particular active one. Yet enhancing some accesses
but not others seems to me like a recipe for new problems
down the road.

Jan