ixgbe: normalize frag_list usage

[David Miller <davem@davemloft.net>]

Signed-off-by: David S. Miller <davem@davemloft.net>
---

Basically I want to get the whole tree to consistently use
the convention that the frag list is built as packets arrive
by using "skb->prev" of the head skb to track the tail member
of the frag list.

This will allow me to do something like:

struct sk_buff {
       union {
		struct {
			struct sk_buff *next;
			struct sk_buff *prev;
		};
		struct {
			struct sk_buff *frag_next;
			struct sk_buff *frag_tail_tracker;
		};
	}
 ...
}

And have all frag_list code use these members consistently.

While doing this patch I noticed that there are some left-over bits in
the IXGBEVF driver that builds these IXGBE style (before my patch)
skb->prev RSC chains.

But nothing other than the RX ring shutdown code processes them, so
likely if they are created they will leak or cause some other kind of
problem.

Please take a look, thanks.

diff --git a/drivers/net/ixgbe/ixgbe.h b/drivers/net/ixgbe/ixgbe.h
index 5cebc37..434c9fa 100644
--- a/drivers/net/ixgbe/ixgbe.h
+++ b/drivers/net/ixgbe/ixgbe.h
@@ -181,6 +181,7 @@ struct ixgbe_ring {
 	struct ixgbe_queue_stats stats;
 	unsigned long reinit_state;
 	int numa_node;
+	struct sk_buff *rsc_skb;	/* RSC packet being built */
 	u64 rsc_count;			/* stat for coalesced packets */
 	u64 rsc_flush;			/* stats for flushed packets */
 	u32 restart_queue;		/* track tx queue restarts */
diff --git a/drivers/net/ixgbe/ixgbe_main.c b/drivers/net/ixgbe/ixgbe_main.c
index c35e13c..43987a4 100644
--- a/drivers/net/ixgbe/ixgbe_main.c
+++ b/drivers/net/ixgbe/ixgbe_main.c
@@ -1130,36 +1130,6 @@ static inline u32 ixgbe_get_rsc_count(union ixgbe_adv_rx_desc *rx_desc)
 		IXGBE_RXDADV_RSCCNT_SHIFT;
 }
 
-/**
- * ixgbe_transform_rsc_queue - change rsc queue into a full packet
- * @skb: pointer to the last skb in the rsc queue
- * @count: pointer to number of packets coalesced in this context
- *
- * This function changes a queue full of hw rsc buffers into a completed
- * packet.  It uses the ->prev pointers to find the first packet and then
- * turns it into the frag list owner.
- **/
-static inline struct sk_buff *ixgbe_transform_rsc_queue(struct sk_buff *skb,
-							u64 *count)
-{
-	unsigned int frag_list_size = 0;
-
-	while (skb->prev) {
-		struct sk_buff *prev = skb->prev;
-		frag_list_size += skb->len;
-		skb->prev = NULL;
-		skb = prev;
-		*count += 1;
-	}
-
-	skb_shinfo(skb)->frag_list = skb->next;
-	skb->next = NULL;
-	skb->len += frag_list_size;
-	skb->data_len += frag_list_size;
-	skb->truesize += frag_list_size;
-	return skb;
-}
-
 struct ixgbe_rsc_cb {
 	dma_addr_t dma;
 	bool delay_unmap;
@@ -1216,10 +1186,13 @@ static bool ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
 		prefetch(skb->data);
 		rx_buffer_info->skb = NULL;
 
+		if (!(staterr & IXGBE_RXD_STAT_EOP)) {
+			rx_ring->rsc_skb = skb;
+			rx_ring->rsc_count++;
+		}
 		if (rx_buffer_info->dma) {
 			if ((adapter->flags2 & IXGBE_FLAG2_RSC_ENABLED) &&
-			    (!(staterr & IXGBE_RXD_STAT_EOP)) &&
-				 (!(skb->prev))) {
+			    rx_ring->rsc_skb == skb) {
 				/*
 				 * When HWRSC is enabled, delay unmapping
 				 * of the first packet. It carries the
@@ -1279,9 +1252,12 @@ static bool ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
 		}
 
 		if (staterr & IXGBE_RXD_STAT_EOP) {
-			if (skb->prev)
-				skb = ixgbe_transform_rsc_queue(skb,
-								&(rx_ring->rsc_count));
+			if (rx_ring->rsc_skb) {
+				skb = rx_ring->rsc_skb;
+				rx_ring->rsc_skb = NULL;
+
+				skb->prev = NULL;
+			}
 			if (adapter->flags2 & IXGBE_FLAG2_RSC_ENABLED) {
 				if (IXGBE_RSC_CB(skb)->delay_unmap) {
 					dma_unmap_single(&pdev->dev,
@@ -1307,8 +1283,22 @@ static bool ixgbe_clean_rx_irq(struct ixgbe_q_vector *q_vector,
 				next_buffer->skb = skb;
 				next_buffer->dma = 0;
 			} else {
-				skb->next = next_buffer->skb;
-				skb->next->prev = skb;
+				struct sk_buff *head = rx_ring->rsc_skb;
+				struct sk_buff *next = next_buffer->skb;
+
+				if (!skb_shinfo(head)->frag_list) {
+					skb_shinfo(head)->frag_list = next;
+				} else {
+					head->prev->next = next;
+				}
+
+				/* ->prev tracks the last skb in the frag_list */
+				head->prev = next;
+				rx_ring->rsc_count++;
+
+				head->len += next->len;
+				head->data_len += next->len;
+				head->truesize += next->len;
 			}
 			rx_ring->non_eop_descs++;
 			goto next_desc;