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

[Eric Dumazet]

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;