linux-kernel - Re: [PATCH v5 3/5] bnx2x: Eliminate duplicate barriers on weakly-ordered archs

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Date:   Fri, 23 Mar 2018 13:04:18 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     okaya@...eaurora.org
Cc:     netdev@...r.kernel.org, timur@...eaurora.org,
        sulrich@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, ariel.elior@...ium.com,
        everest-linux-l2@...ium.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/5] bnx2x: Eliminate duplicate barriers on
 weakly-ordered archs

From: Sinan Kaya <okaya@...eaurora.org>
Date: Fri, 23 Mar 2018 12:51:47 -0400

> It could if txdata->tx_db was not a union. There is a data dependency
> between txdata->tx_db.data.prod and txdata->tx_db.raw. 
> 
> So, no reordering.

I don't see it that way, the code requires that:

 	txdata->tx_db.data.prod += nbd;

is visible before the doorbell update.

barrier() doesn't provide that.

Neither does writel_relaxed().  However plain writel() does.

Therefore the code is only correct as-is, and your change potentially
adds a reordering problem.