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

[Jason Wang <jasowang@redhat.com>]

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().

I think those two needs to be serialized. Or did I miss anything?

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);
>   }