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

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Message-ID: <20090703170621.GS8943@linux.vnet.ibm.com>
Date:	Fri, 3 Jul 2009 10:06:22 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	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

On Fri, Jul 03, 2009 at 11:47:00AM -0400, Mathieu Desnoyers wrote:
> * 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.

Are you guys sure you are in agreement about what you all mean by
"acquire semantics"?

Clearly, any correct __read_lock() implementation must enforce ordering
with respect to the most recent __write_unlock(), but this does not
necesarily imply all possible definitions of "acquire semantics".

							Thanx, Paul
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