[PATCH net-next v2 0/3] r8169: improve memory barriers

[PATCH net-next v2 0/3] r8169: improve memory barriers

[Heiner Kallweit]

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

v2:
- drop patch 2 from the series

Heiner Kallweit (3):
  r8169: use smp_store_mb in rtl_tx
  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 | 24 ++++++++---------------
 1 file changed, 8 insertions(+), 16 deletions(-)

-- 
2.26.1

[PATCH net-next v2 1/3] 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 v2 2/3] 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 dd6113fd7..2fc65aca3 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 v2 3/3] 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 2fc65aca3..3e4ed2528 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 v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic

[Petko Manolov]

On 20-04-19 20:16:21, Heiner Kallweit wrote:
> 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 2fc65aca3..3e4ed2528 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();

If dma_wmb() was really ever needed here you should leave it even after you 
order these writes with WRITE_ONCE().  If not, then good riddance.

Just saying, i am not familiar with the hardware nor with the driver. :)


		Petko


> -
> -	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 v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic

[Heiner Kallweit]

On 19.04.2020 21:00, Petko Manolov wrote:
> On 20-04-19 20:16:21, Heiner Kallweit wrote:
>> 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 2fc65aca3..3e4ed2528 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();
> 
> If dma_wmb() was really ever needed here you should leave it even after you 
> order these writes with WRITE_ONCE().  If not, then good riddance.
> 
My understanding is that we have to avoid transferring ownership of
descriptor to device (by setting DescOwn bit) before opts2 field
has been written. Using WRITE_ONCE() should be sufficient to prevent
the compiler from merging or reordering the writes.
At least that's how I read the process_level() example in
https://www.kernel.org/doc/Documentation/memory-barriers.txt

> Just saying, i am not familiar with the hardware nor with the driver. :)
> 
> 
> 		Petko
> 
> 
>> -
>> -	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 v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic

[Petko Manolov]

On 20-04-19 23:03:57, Heiner Kallweit wrote:
> On 19.04.2020 21:00, Petko Manolov wrote:
> > On 20-04-19 20:16:21, Heiner Kallweit wrote:
> >> 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 2fc65aca3..3e4ed2528 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();
> > 
> > If dma_wmb() was really ever needed here you should leave it even after you 
> > order these writes with WRITE_ONCE().  If not, then good riddance.
> > 
> My understanding is that we have to avoid transferring ownership of
> descriptor to device (by setting DescOwn bit) before opts2 field
> has been written. Using WRITE_ONCE() should be sufficient to prevent
> the compiler from merging or reordering the writes.
> At least that's how I read the process_level() example in
> https://www.kernel.org/doc/Documentation/memory-barriers.txt

WRITE_ONCE() will prevent the compiler from reordering, but not the CPU.  On x86 
this code will run just fine, but not on ARM or PPC.  Based on your description 
above i think dma_wmb() is still needed.


		Petko


> > Just saying, i am not familiar with the hardware nor with the driver. :)
> > 
> > 
> > 		Petko
> > 
> > 
> >> -
> >> -	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 v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic

[Heiner Kallweit]

On 19.04.2020 23:38, Petko Manolov wrote:
> On 20-04-19 23:03:57, Heiner Kallweit wrote:
>> On 19.04.2020 21:00, Petko Manolov wrote:
>>> On 20-04-19 20:16:21, Heiner Kallweit wrote:
>>>> 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 2fc65aca3..3e4ed2528 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();
>>>
>>> If dma_wmb() was really ever needed here you should leave it even after you 
>>> order these writes with WRITE_ONCE().  If not, then good riddance.
>>>
>> My understanding is that we have to avoid transferring ownership of
>> descriptor to device (by setting DescOwn bit) before opts2 field
>> has been written. Using WRITE_ONCE() should be sufficient to prevent
>> the compiler from merging or reordering the writes.
>> At least that's how I read the process_level() example in
>> https://www.kernel.org/doc/Documentation/memory-barriers.txt
> 
> WRITE_ONCE() will prevent the compiler from reordering, but not the CPU.  On x86 
> this code will run just fine, but not on ARM or PPC.  Based on your description 
> above i think dma_wmb() is still needed.
> 

After reading a little about ARMv8-A and weakly-ordered memory I tend to agree
with you. It's a pity that I don't know of any use of Realtek network chips
on such a weakly-ordered platform (what of course doesn't mean it doesn't exist).
This would really help me in testing and avoid such "but it works on x86" ..

> 
> 		Petko
> 
> 
>>> Just saying, i am not familiar with the hardware nor with the driver. :)
>>>
>>>
>>> 		Petko
>>>
>>>
>>>> -
>>>> -	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 v2 0/3] r8169: improve memory barriers

[Heiner Kallweit]

On 19.04.2020 20:13, Heiner Kallweit wrote:
> This series includes improvements for (too heavy) memory barriers.
> Tested on x86-64 with RTL8168g chip version.
> 
A recent conversation convinced me that few of these change are
not safe on platforms with weakly-ordered memory.
Therefore please disregard the series.

> v2:
> - drop patch 2 from the series
> 
> Heiner Kallweit (3):
>   r8169: use smp_store_mb in rtl_tx
>   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 | 24 ++++++++---------------
>  1 file changed, 8 insertions(+), 16 deletions(-)
> 

Re: [PATCH net-next v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to_asic

[Eric Dumazet]

On 4/19/20 2:03 PM, Heiner Kallweit wrote:
> On 19.04.2020 21:00, Petko Manolov wrote:
>> On 20-04-19 20:16:21, Heiner Kallweit wrote:
>>> 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 2fc65aca3..3e4ed2528 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();
>>
>> If dma_wmb() was really ever needed here you should leave it even after you 
>> order these writes with WRITE_ONCE().  If not, then good riddance.
>>
> My understanding is that we have to avoid transferring ownership of
> descriptor to device (by setting DescOwn bit) before opts2 field
> has been written. Using WRITE_ONCE() should be sufficient to prevent
> the compiler from merging or reordering the writes.
> At least that's how I read the process_level() example in
> https://www.kernel.org/doc/Documentation/memory-barriers.txt
> 
>> Just saying, i am not familiar with the hardware nor with the driver. :)
>>
>>
>> 		Petko
>>
>>
>>> -
>>> -	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));


No, this is not correct.

WRITE_ONCE(X, ...)
WRITE_ONCE(Y, ...)

has no guarantee X is written before Y

Sure, the compiler is making sure to perform one write for X, one for Y,
but you want stronger semantic than that. You want to keep dma_wmb()

However, this part of your patch seems fine :

-	desc->opts1 = cpu_to_le32(DescOwn | eor | R8169_RX_BUF_SIZE);
+       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
>>>
>>>
>