netdev - Re: [PATCH net-next-2.6] net: inet_add

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Message-ID: <20100909094211.GA8404@ff.dom.local>
Date:	Thu, 9 Sep 2010 09:42:11 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, jesse@...ira.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next-2.6] net: inet_add_protocol() can use cmpxchg()

On Thu, Sep 09, 2010 at 11:23:57AM +0200, Eric Dumazet wrote:
> Le jeudi 09 septembre 2010 ?? 09:08 +0000, Jarek Poplawski a écrit :
> > xactly, without "after some time" I couldn't tell this
> > either. Is seems some barriers might have been changed, but I might
> > be wrong. Anyway, it would be nice to have some comment on that in
> > the changelog.
> 
> Jarek, cmpxchg() is a very strong primitive, as documented
> in Documentation/memory-barriers.txt :
> 
> <begin of quote>
> 
> Any atomic operation that modifies some state in memory and returns information
> about the state (old or new) implies an SMP-conditional general memory barrier
> (smp_mb()) on each side of the actual operation (with the exception of
> explicit lock operations, described later).  These include:
> 
>         xchg();
>         cmpxchg();
> ...
> These are used for such things as implementing LOCK-class and UNLOCK-class
> operations and adjusting reference counters towards object destruction, and as
> such the implicit memory barrier effects are necessary.
> 
> <end of quote>
> 
> Its not needed to document each cmpxchg() use, because of these
> strong requirements.

OK! I should remember this until "after some time". ;-)

Thanks for the explanation,
Jarek P.
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