[PATCH net-next 0/4] r8169: improve memory barriers

[PATCH net-next 0/4] r8169: improve memory barriers

[Heiner Kallweit]

This series includes improvements for (too heavy) memory barriers.
Tested on x86-64 with RTL8168g chip version.

Heiner Kallweit (4):
  r8169: use smp_store_mb in rtl_tx
  r8169: change wmb to dma-wmb in rtl8169_start_xmit
  r8169: replace dma_rmb with READ_ONCE in rtl_rx
  r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic

 drivers/net/ethernet/realtek/r8169_main.c | 26 ++++++++---------------
 1 file changed, 9 insertions(+), 17 deletions(-)

-- 
2.26.1

[PATCH net-next 1/4] r8169: use smp_store_mb in rtl_tx

[Heiner Kallweit]

Use helper smp_store_mb() instead of open-coding the functionality.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
---
 drivers/net/ethernet/realtek/r8169_main.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
index 1bc415d00..dd6113fd7 100644
--- a/drivers/net/ethernet/realtek/r8169_main.c
+++ b/drivers/net/ethernet/realtek/r8169_main.c
@@ -4438,7 +4438,6 @@ static void rtl_tx(struct net_device *dev, struct rtl8169_private *tp,
 		tp->tx_stats.bytes += bytes_compl;
 		u64_stats_update_end(&tp->tx_stats.syncp);
 
-		tp->dirty_tx = dirty_tx;
 		/* Sync with rtl8169_start_xmit:
 		 * - publish dirty_tx ring index (write barrier)
 		 * - refresh cur_tx ring index and queue status (read barrier)
@@ -4446,7 +4445,7 @@ static void rtl_tx(struct net_device *dev, struct rtl8169_private *tp,
 		 * a racing xmit thread can only have a right view of the
 		 * ring status.
 		 */
-		smp_mb();
+		smp_store_mb(tp->dirty_tx, dirty_tx);
 		if (netif_queue_stopped(dev) &&
 		    rtl_tx_slots_avail(tp, MAX_SKB_FRAGS)) {
 			netif_wake_queue(dev);
-- 
2.26.1

[PATCH net-next 2/4] r8169: change wmb to dma_wmb in rtl8169_start_xmit

[Heiner Kallweit]

It's the typical dma_wmb() use case to finish descriptor writes before
ringing the device doorbell. Therefore switch to this lighter barrier.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
---
 drivers/net/ethernet/realtek/r8169_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
index dd6113fd7..038cd6fde 100644
--- a/drivers/net/ethernet/realtek/r8169_main.c
+++ b/drivers/net/ethernet/realtek/r8169_main.c
@@ -4259,7 +4259,7 @@ static netdev_tx_t rtl8169_start_xmit(struct sk_buff *skb,
 	txd_first->opts1 |= cpu_to_le32(DescOwn | FirstFrag);
 
 	/* Force all memory writes to complete before notifying device */
-	wmb();
+	dma_wmb();
 
 	tp->cur_tx += frags + 1;
 
-- 
2.26.1

[PATCH net-next 3/4] r8169: replace dma_rmb with READ_ONCE in rtl_rx

[Heiner Kallweit]

We want to ensure that desc->opts1 is read before desc->opts2.
This doesn't require a full compiler barrier. READ_ONCE provides
the ordering guarantee we need.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
---
 drivers/net/ethernet/realtek/r8169_main.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
index 038cd6fde..df788063e 100644
--- a/drivers/net/ethernet/realtek/r8169_main.c
+++ b/drivers/net/ethernet/realtek/r8169_main.c
@@ -1548,7 +1548,7 @@ static inline u32 rtl8169_tx_vlan_tag(struct sk_buff *skb)
 
 static void rtl8169_rx_vlan_tag(struct RxDesc *desc, struct sk_buff *skb)
 {
-	u32 opts2 = le32_to_cpu(desc->opts2);
+	u32 opts2 = le32_to_cpu(READ_ONCE(desc->opts2));
 
 	if (opts2 & RxVlanTag)
 		__vlan_hwaccel_put_tag(skb, htons(ETH_P_8021Q), swab16(opts2 & 0xffff));
@@ -4490,16 +4490,11 @@ static int rtl_rx(struct net_device *dev, struct rtl8169_private *tp, u32 budget
 		struct RxDesc *desc = tp->RxDescArray + entry;
 		u32 status;
 
-		status = le32_to_cpu(desc->opts1);
+		/* Use READ_ONCE to order descriptor field reads */
+		status = le32_to_cpu(READ_ONCE(desc->opts1));
 		if (status & DescOwn)
 			break;
 
-		/* This barrier is needed to keep us from reading
-		 * any other fields out of the Rx descriptor until
-		 * we know the status of DescOwn
-		 */
-		dma_rmb();
-
 		if (unlikely(status & RxRES)) {
 			netif_info(tp, rx_err, dev, "Rx ERROR. status = %08x\n",
 				   status);
-- 
2.26.1

[PATCH net-next 4/4] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic

[Heiner Kallweit]

We want to ensure that desc->opts1 is written as last descriptor field.
This doesn't require a full compiler barrier, WRITE_ONCE provides the
ordering guarantee we need.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
---
 drivers/net/ethernet/realtek/r8169_main.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
index df788063e..8e9944692 100644
--- a/drivers/net/ethernet/realtek/r8169_main.c
+++ b/drivers/net/ethernet/realtek/r8169_main.c
@@ -3892,11 +3892,9 @@ static inline void rtl8169_mark_to_asic(struct RxDesc *desc)
 {
 	u32 eor = le32_to_cpu(desc->opts1) & RingEnd;
 
-	desc->opts2 = 0;
-	/* Force memory writes to complete before releasing descriptor */
-	dma_wmb();
-
-	desc->opts1 = cpu_to_le32(DescOwn | eor | R8169_RX_BUF_SIZE);
+	/* Ensure ordering of writes */
+	WRITE_ONCE(desc->opts2, 0);
+	WRITE_ONCE(desc->opts1, cpu_to_le32(DescOwn | eor | R8169_RX_BUF_SIZE));
 }
 
 static struct page *rtl8169_alloc_rx_data(struct rtl8169_private *tp,
@@ -3919,7 +3917,7 @@ static struct page *rtl8169_alloc_rx_data(struct rtl8169_private *tp,
 		return NULL;
 	}
 
-	desc->addr = cpu_to_le64(mapping);
+	WRITE_ONCE(desc->addr, cpu_to_le64(mapping));
 	rtl8169_mark_to_asic(desc);
 
 	return data;
-- 
2.26.1

Re: [PATCH net-next 0/4] r8169: improve memory barriers

[Heiner Kallweit]

On 19.04.2020 19:38, Heiner Kallweit wrote:
> This series includes improvements for (too heavy) memory barriers.
> Tested on x86-64 with RTL8168g chip version.
> 
> Heiner Kallweit (4):
>   r8169: use smp_store_mb in rtl_tx
>   r8169: change wmb to dma_wmb in rtl8169_start_xmit

I think I'll have to drip this patch from the series. These Realtek
chips are also used on non-x86 platforms, and these platforms may
not be coherent. I'll send a v2.

>   r8169: replace dma_rmb with READ_ONCE in rtl_rx
>   r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic
> 
>  drivers/net/ethernet/realtek/r8169_main.c | 26 ++++++++---------------
>  1 file changed, 9 insertions(+), 17 deletions(-)
>