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

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Message-ID: <20090703154700.GD10256@Krystal>
Date:	Fri, 3 Jul 2009 11:47:00 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>, 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, Paul.McKenney@...ibm.com
Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

* Eric Dumazet (eric.dumazet@...il.com) wrote:
> 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 ?
> 

I think he meant RCU.

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

At least PowerPC, MIPS, recent ARM, alpha.

Mathieu


-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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