netdev - Re: [RFC PATCH] bnx2x: fix tx queue locking and memory barriers

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Message-Id: <20100225.080624.203625652.davem@davemloft.net>
Date:	Thu, 25 Feb 2010 08:06:24 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	sgruszka@...hat.com
Cc:	vladz@...adcom.com, netdev@...r.kernel.org, eilong@...adcom.com,
	dhowells@...hat.com
Subject: Re: [RFC PATCH] bnx2x: fix tx queue locking and memory barriers

From: Stanislaw Gruszka <sgruszka@...hat.com>
Date: Thu, 25 Feb 2010 17:03:08 +0100

> On Thu, 25 Feb 2010 07:49:48 -0800
> "Vladislav Zolotarov" <vladz@...adcom.com> wrote:
> 
>> In bnx2x_start_xmit(): right after the "stop queue" there is an smp_mb(), which will bring the cpu0 cache and a fresh fp->tx_bd_cons value to cpu1 and the following if() will return true and the queue will be released from bnx2x_start_xmit() flow.
> 
> If I understand correctly what is written in Documentation/memory-barriers.txt
> this smp_mb() need to have another "paired" smp_{w}mb() on cpu0 to make value
> be updated on cpu1, which is missing.

The invocation of ->hard_start_xmit() creates an implicit memory
barrier because all such invocations take the netdev spinlock.
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