netdev - Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

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Date:   Sun, 12 Mar 2017 08:29:26 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

On Sun, 2017-03-12 at 07:57 -0700, Eric Dumazet wrote:

> Problem is XDP TX :
> 
> I do not see any guarantee mlx4_en_recycle_tx_desc() runs while the NAPI
> RX is owned by current cpu.
> 
> Since TX completion is using a different NAPI, I really do not believe
> we can avoid an atomic operation, like a spinlock, to protect the list
> of pages ( ring->page_cache )

A quick fix for net-next would be :

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
index aa074e57ce06fb2842fa1faabd156c3cd2fe10f5..e0b2ea8cefd6beef093c41bade199e3ec4f0291c 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -137,13 +137,17 @@ static int mlx4_en_prepare_rx_desc(struct mlx4_en_priv *priv,
 	struct mlx4_en_rx_desc *rx_desc = ring->buf + (index * ring->stride);
 	struct mlx4_en_rx_alloc *frags = ring->rx_info +
 					(index << priv->log_rx_info);
+
 	if (ring->page_cache.index > 0) {
+		spin_lock(&ring->page_cache.lock);
+
 		/* XDP uses a single page per frame */
 		if (!frags->page) {
 			ring->page_cache.index--;
 			frags->page = ring->page_cache.buf[ring->page_cache.index].page;
 			frags->dma  = ring->page_cache.buf[ring->page_cache.index].dma;
 		}
+		spin_unlock(&ring->page_cache.lock);
 		frags->page_offset = XDP_PACKET_HEADROOM;
 		rx_desc->data[0].addr = cpu_to_be64(frags->dma +
 						    XDP_PACKET_HEADROOM);
@@ -277,6 +281,7 @@ int mlx4_en_create_rx_ring(struct mlx4_en_priv *priv,
 		}
 	}
 
+	spin_lock_init(&ring->page_cache.lock);
 	ring->prod = 0;
 	ring->cons = 0;
 	ring->size = size;
@@ -419,10 +424,13 @@ bool mlx4_en_rx_recycle(struct mlx4_en_rx_ring *ring,
 
 	if (cache->index >= MLX4_EN_CACHE_SIZE)
 		return false;
-
-	cache->buf[cache->index].page = frame->page;
-	cache->buf[cache->index].dma = frame->dma;
-	cache->index++;
+	spin_lock(&cache->lock);
+	if (cache->index < MLX4_EN_CACHE_SIZE) {
+		cache->buf[cache->index].page = frame->page;
+		cache->buf[cache->index].dma = frame->dma;
+		cache->index++;
+	}
+	spin_unlock(&cache->lock);
 	return true;
 }
 
diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
index 39f401aa30474e61c0b0029463b23a829ec35fa3..090a08020d13d8e11cc163ac9fc6ac6affccc463 100644
--- a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
+++ b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
@@ -258,7 +258,8 @@ struct mlx4_en_rx_alloc {
 #define MLX4_EN_CACHE_SIZE (2 * NAPI_POLL_WEIGHT)
 
 struct mlx4_en_page_cache {
-	u32 index;
+	u32			index;
+	spinlock_t		lock;
 	struct {
 		struct page	*page;
 		dma_addr_t	dma;