Re: [PATCH net-next] ptr_ring: make __ptr_ring_empty() checking more reliable

[Jason Wang <jasowang@redhat.com>]

在 2021/5/27 下午2:07, Yunsheng Lin 写道:
> On 2021/5/27 12:57, Jason Wang wrote:
>> 在 2021/5/26 下午8:29, Yunsheng Lin 写道:
>>> Currently r->queue[] is cleared after r->consumer_head is moved
>>> forward, which makes the __ptr_ring_empty() checking called in
>>> page_pool_refill_alloc_cache() unreliable if the checking is done
>>> after the r->queue clearing and before the consumer_head moving
>>> forward.
>>>
>>> Move the r->queue[] clearing after consumer_head moving forward
>>> to make __ptr_ring_empty() checking more reliable.
>>
>> If I understand this correctly, this can only happens if you run __ptr_ring_empty() in parallel with ptr_ring_discard_one().
> Yes.
>
>> I think those two needs to be serialized. Or did I miss anything?
> As the below comment in __ptr_ring_discard_one, if the above is true, I
> do not think we need to keep consumer_head valid at all times, right?
>
>
> 	/* Note: we must keep consumer_head valid at all times for __ptr_ring_empty
> 	 * to work correctly.
> 	 */


I'm not sure I understand. But my point is that you need to synchronize 
the __ptr_ring_discard_one() and __ptr_empty() as explained in the 
comment above __ptr_ring_empty():

/*
  * Test ring empty status without taking any locks.
  *
  * NB: This is only safe to call if ring is never resized.
  *
  * However, if some other CPU consumes ring entries at the same time, 
the value
  * returned is not guaranteed to be correct.
  *
  * In this case - to avoid incorrectly detecting the ring
  * as empty - the CPU consuming the ring entries is responsible
  * for either consuming all ring entries until the ring is empty,
  * or synchronizing with some other CPU and causing it to
  * re-test __ptr_ring_empty and/or consume the ring enteries
  * after the synchronization point.
  *
  * Note: callers invoking this in a loop must use a compiler barrier,
  * for example cpu_relax().
  */

Thanks



>> Thanks
>>
>>
>>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
>>> ---
>>>    include/linux/ptr_ring.h | 26 +++++++++++++++++---------
>>>    1 file changed, 17 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
>>> index 808f9d3..f32f052 100644
>>> --- a/include/linux/ptr_ring.h
>>> +++ b/include/linux/ptr_ring.h
>>> @@ -261,8 +261,7 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r)
>>>        /* Note: we must keep consumer_head valid at all times for __ptr_ring_empty
>>>         * to work correctly.
>>>         */
>>> -    int consumer_head = r->consumer_head;
>>> -    int head = consumer_head++;
>>> +    int consumer_head = r->consumer_head + 1;
>>>          /* Once we have processed enough entries invalidate them in
>>>         * the ring all at once so producer can reuse their space in the ring.
>>> @@ -271,19 +270,28 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r)
>>>         */
>>>        if (unlikely(consumer_head - r->consumer_tail >= r->batch ||
>>>                 consumer_head >= r->size)) {
>>> +        int tail = r->consumer_tail;
>>> +        int head = consumer_head;
>>> +
>>> +        if (unlikely(consumer_head >= r->size)) {
>>> +            r->consumer_tail = 0;
>>> +            WRITE_ONCE(r->consumer_head, 0);
>>> +        } else {
>>> +            r->consumer_tail = consumer_head;
>>> +            WRITE_ONCE(r->consumer_head, consumer_head);
>>> +        }
>>> +
>>>            /* Zero out entries in the reverse order: this way we touch the
>>>             * cache line that producer might currently be reading the last;
>>>             * producer won't make progress and touch other cache lines
>>>             * besides the first one until we write out all entries.
>>>             */
>>> -        while (likely(head >= r->consumer_tail))
>>> -            r->queue[head--] = NULL;
>>> -        r->consumer_tail = consumer_head;
>>> -    }
>>> -    if (unlikely(consumer_head >= r->size)) {
>>> -        consumer_head = 0;
>>> -        r->consumer_tail = 0;
>>> +        while (likely(--head >= tail))
>>> +            r->queue[head] = NULL;
>>> +
>>> +        return;
>>>        }
>>> +
>>>        /* matching READ_ONCE in __ptr_ring_empty for lockless tests */
>>>        WRITE_ONCE(r->consumer_head, consumer_head);
>>>    }
>>
>> .
>>