Re: [PATCH 7/7] ixgbevf: eliminate duplicate barriers on weakly-ordered archs

[Sinan Kaya]

On 3/15/2018 12:21 PM, Sinan Kaya wrote:
> On 3/15/2018 10:32 AM, Alexander Duyck wrote:
>> We tend to do something like:
>>   update tx_buffer_info
>>   update tx_desc
>>   wmb()
>>   point first tx_buffer_info next_to_watch value at last tx_desc
>>   update next_to_use
>>   notify device via writel
>>
>> We do it this way because we have to synchronize between the Tx
>> cleanup path and the hardware so we basically lump the two barriers
>> together. instead of invoking both a smp_wmb and a wmb. Now that I
>> look at the pseudocode though I wonder if we shouldn't move the
>> next_to_use update before the wmb, but that might be material for
>> another patch. Anyway, in the Tx cleanup path we should have an
>> smp_rmb() after we read the next_to_watch values so that we avoid
>> reading any of the other fields in the buffer_info if either the field
>> is NULL or the descriptor pointed to has not been written back.
> 
> How do you feel about keeping wmb() very close to writel_relaxed() like this?
> 
>    update tx_buffer_info
>    update tx_desc
>    point first tx_buffer_info next_to_watch value at last tx_desc
>    update next_to_use
>    wmb()
>    notify device via writel_relaxed()
> 
> I'm afraid that if the order of wmb() and writel() is not very
> obvious or hidden in multiple functions, somebody can introduce a very nasty
> bug in the future.
> 
> We also have to think about code maintenance.
> 

Now that I read your email again, I think this is the reason if I understood you
correctly. 

"instead of invoking both a smp_wmb and a wmb"

You'd need something like

    update tx_buffer_info
    update tx_desc
    smp_wmb()
    point first tx_buffer_info next_to_watch value at last tx_desc
    update next_to_use
    wmb()
    notify device via writel_relaxed()

Let me work on your comments.

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.