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

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Date:	Thu, 25 Feb 2010 17:16:54 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	David Miller <davem@...emloft.net>
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

On Thu, 25 Feb 2010 08:06:24 -0800 (PST)
David Miller <davem@...emloft.net> wrote:

> > 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.

But barrier is missing on cpu which call bnx2x_pull(), how spinlock
before bnx2x_start_xmit() helps with that? Below again the whole
picture:

cpu0: (in bnx2x_poll)                   cpu1: (transferring data)

bnx2x_tx_int():                         bnx2x_start_xmit():
local fp->tx_bd_cons = 3980;            send more data to device
return

bnx2x_tx_int():                         local fp->tx_bd_prod = 4000
local fp->tx_bd_cons = 4000;            local fp->tx_bd_cons still 0
queue not stopped                       no avail space in queue
return;                                 stop queue
                                        smp_mb() - not paired, cpu1 does not "see" cpu0 caches changes
                                        local fp->tx_bd_cons still 0
                                        no wake
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