netdev - Re: [PATCH 1/1] bnx2x: Tx barriers and locks

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Message-ID: <20100302165946.GC6491@dhcp-lab-161.englab.brq.redhat.com>
Date:	Tue, 2 Mar 2010 17:59:46 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Michael Chan <mchan@...adcom.com>
Cc:	Vladislav Zolotarov <vladz@...adcom.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	Eilon Greenstein <eilong@...adcom.com>,
	Matthew Carlson <mcarlson@...adcom.com>
Subject: Re: [PATCH 1/1] bnx2x: Tx barriers and locks

On Tue, Mar 02, 2010 at 08:18:44AM -0800, Michael Chan wrote:
> Stanislaw Gruszka wrote:
> 
> > On Tue, Mar 02, 2010 at 04:50:59AM -0800, Vladislav Zolotarov wrote:
> > > Stanislaw barrier() is not a memory barrier - it's a
> > compiler barrier. I don't think removing it from
> > bnx2x_tx_avail() will improve anything. If u think I'm wrong,
> > could u, pls., provide a specific example.
> >
> > Only improvement is removing confusing code, And comment like
> > "Tell compiler that prod and cons can change" is even more
> > confusing. If you think I'm wrong, just tell as why that
> > barrier is needed :)
> 
> The barrier (compiler barrier at least) is required in
> bnx2x_tx_avail().  The status block index can be updated by DMA and
> the compiler doesn't know it (because it is considered wrong to

If you are telling status block index you mean which variable ?
 
> declare the status block as volatile).  Near the end of
> bnx2x_start_xmit() where we call bnx2x_tx_avail() twice.  It is
> possible that the compiler will optimize it and not look at the
> status block in memory the second time.

Ok, I'm trying to understand.
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