linux-kernel - Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function

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Message-Id: <1234874455.4744.96.camel@laptop>
Date:	Tue, 17 Feb 2009 13:40:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
 single ipi fallback for smp_call_function_many())

On Tue, 2009-02-17 at 10:29 +0100, Peter Zijlstra wrote:
> > It would be nice to have a comment which explains how can we miss the
> > first addition without read_barrier_depends(). And why only on alpha.
> 
> Paul, care to once again enlighten us? The best I can remember is that
> alpha has split caches, and the rmb is needed for them to become
> coherent -- no other arch is crazy in exactly that way.
> 
> But note that read_barrier_depends() is not quite a NOP for !alpha, it
> does that ACCESS_ONCE() thing, which very much makes a difference, even
> on x86.

I've been saying crazy stuff, read_barrier_depends() is a NOP, I got
confused with rcu_dereference(). My bad.

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