netdev - Re: [patch 4/4] tcp: statistics not read

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Message-Id: <20070312.143306.18307677.davem@davemloft.net>
Date:	Mon, 12 Mar 2007 14:33:06 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...ux-foundation.org
Cc:	netdev@...r.kernel.org
Subject: Re: [patch 4/4] tcp: statistics not read_mostly

From: Stephen Hemminger <shemminger@...ux-foundation.org>
Date: Mon, 12 Mar 2007 14:26:47 -0700

> On Mon, 12 Mar 2007 14:15:50 -0700 (PDT)
> David Miller <davem@...emloft.net> wrote:
> 
> > Look at the definition of DEFINE_SNMP_STAT().
> 
> Okay, that's confusing. And maybe the comment suggests future work:

Indeed.

> /* 
>  * FIXME: On x86 and some other CPUs the split into user and softirq parts
>  * is not needed because addl $1,memory is atomic against interrupts (but 
>  * atomic_inc would be overkill because of the lock cycles). Wants new 
>  * nonlocked_atomic_inc() primitives -AK
>  */ 

Yep.  And another interesting case are straight loads and stores of
per-cpu values on platforms that have a PDA'ish thing like x86_64 and
sparc64.  The latter has the per-cpu base in a register so in several
cases the usual preemption protection simply does not matter and we
could optimize them away.
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