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

Jürgen Groß
2021-02-08T12:31:30Z

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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?

Juergen

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