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

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Message-ID: <20090707194533.GB13858@Krystal>
Date:	Tue, 7 Jul 2009 15:45:33 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Oleg Nesterov <oleg@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, fbl@...hat.com, nhorman@...hat.com,
	davem@...hat.com, htejun@...il.com, jarkao2@...il.com,
	davidel@...ilserver.org
Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock

* Peter Zijlstra (a.p.zijlstra@...llo.nl) wrote:
> On Tue, 2009-07-07 at 17:44 +0200, Oleg Nesterov wrote:
> > On 07/07, Mathieu Desnoyers wrote:
> > >
> > > Actually, thinking about it more, to appropriately support x86, as well
> > > as powerpc, arm and mips, we would need something like:
> > >
> > > read_lock_smp_mb()
> > >
> > > Which would be a read_lock with an included memory barrier.
> > 
> > Then we need read_lock_irq_smp_mb, read_lock_irqsave__smp_mb, write_lock_xxx,
> > otherwise it is not clear why only read_lock() has _smp_mb() version.
> > 
> > The same for spin_lock_xxx...
> 
> At which time the smp_mb__{before,after}_{un,}lock become attractive
> again.
> 

Then having a new __read_lock() (without acquire semantic) which would
be required to be followed by a smp__mb_after_lock() would make sense. I
think this would fit all of x86, powerpc, arm, mips without having to
create tons of new primitives. Only "simpler" ones that clearly separate
locking from memory barriers.

Mathieu




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