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

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Message-ID: <4A4E25BB.8060408@gmail.com>
Date:	Fri, 03 Jul 2009 17:37:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>, mingo@...e.hu,
	jolsa@...hat.com, a.p.zijlstra@...llo.nl, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, fbl@...hat.com, nhorman@...hat.com,
	davem@...hat.com, htejun@...il.com, jarkao2@...il.com,
	oleg@...hat.com, davidel@...ilserver.org, eric.dumazet@...il.com,
	Paul.McKenney@...ibm.com
Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

Herbert Xu a écrit :
> Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
>> Why don't we create a read_lock without acquire semantic instead (e.g.
>> read_lock_nomb(), or something with a better name like __read_lock()) ?
>> On architectures where memory barriers are needed to provide the acquire
>> semantic, it would be faster to do :
>>
>> __read_lock();
>> smp_mb();
>>
>> than :
>>
>> read_lock(); <- e.g. lwsync + isync or something like that
>> smp_mb(); <- full sync.
> 
> Hmm, why do we even care when read_lock should just die?
> 
> Cheers,

+1 :)

Do you mean using a spinlock instead or what ?

Also, how many arches are able to have a true __read_lock()
(or __spin_lock() if that matters), without acquire semantic ?
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