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

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Date:   Thu, 15 Mar 2018 12:21:17 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Timur Tabi <timur@...eaurora.org>, Netdev <netdev@...r.kernel.org>,
        sulrich@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/7] ixgbevf: eliminate duplicate barriers on
 weakly-ordered archs

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.

-- 
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.