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Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
 single ipi fallback for smp_call_function_many())
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Jens Axboe <jens.axboe@oracle.com>,
       Suresh Siddha <suresh.b.siddha@intel.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
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Date: Tue, 17 Feb 2009 13:40:55 +0100
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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.