netdev - Re: [PATCH net-next v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb in rtl8169_mark_to

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Date:   Sun, 19 Apr 2020 15:33:17 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Heiner Kallweit <hkallweit1@...il.com>,
        Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 3/3] r8169: use WRITE_ONCE instead of dma_wmb
 in rtl8169_mark_to_asic



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@...il.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
>>>
>>>
>