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

[Yunsheng Lin]

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.
	 */

> 
> Thanks
> 
> 
>>
>> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
>> ---
>>   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);
>>   }
> 
> 
> .
>