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

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Date:	Tue, 2 Mar 2010 09:26:27 -0800
From:	"Michael Chan" <mchan@...adcom.com>
To:	"Stanislaw Gruszka" <sgruszka@...hat.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, 2010-03-02 at 08:59 -0800, Stanislaw Gruszka wrote:
> 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 ?

The fp-> fields which can be updated by NAPI poll based on new status
block DMA.

>  
> > 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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