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

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Date:   Fri, 23 Mar 2018 12:43:26 -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 12:31:12 -0400

> Sorry, you got me confused now.
> 
> If you look at the code closer, you'll see this.
> 
> 	wmb();
> 
> 	txdata->tx_db.data.prod += nbd;
> 	barrier();
> 
> 	DOORBELL(bp, txdata->cid, txdata->tx_db.raw);
> 
> and you also asked me to rename DOORBELL to DOORBELL_RELAXED() to make
> it obvious that we have a relaxed operator inside the macro.

This still doesn't match the stated pattern.

	wmb();
	/* no other memory or I/O or IOMEM operation */
	writel();

There is a write to a producer index there and then no non-compiler
barrier or any kind before the writel().

So, in fact, it might really need that implicit writel() barrier here!