linux-kernel - Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after

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Message-ID: <20090708071831.GB3148@ami.dom.local>
Date:	Wed, 8 Jul 2009 09:18:31 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	htejun@...il.com, davidel@...ilserver.org
Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

On Wed, Jul 08, 2009 at 12:34:32AM -0400, Mathieu Desnoyers wrote:
...
> Because adding smp_mb__after_lock() is _only_ useful on x86. Most other
> architectures _will_ suffer from a performance degradation, unless you
> implement the __read_lock_noacquire.

It's completely backwards: processor barriers are just expected to
add a performance degradation. That's like:

x86 developer:
OK, we need to add a barrier here: even x86 might need this.

Alpha developer:
Right, than we need this even more.

x86 developer:
But wait, we can avoid it using a dummy after some locks, because they
have such a barrier already.

Alpha developer:
Then it's not OK: it's _only_ useful on x86; our architecture _will_
suffer from a performance degradation...

Cheers,
Jarek P.
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