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Subject: Re: [PATCH] synchronize_irq needs a barrier
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reply-To: benh@kernel.crashing.org
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: torvalds@linux-foundation.org, akpm@linux-foundation.org,
       linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org
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Date: Fri, 19 Oct 2007 07:35:05 +1000
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On Thu, 2007-10-18 at 22:35 +0800, Herbert Xu wrote:
> Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> > 
> > Note that some kind of read barrier or compiler barrier should be needed
> > regardless, or we are just not sync'ing with anything at all (we may
> > have loaded the value ages ago and thus operate on a totally stale
> > value). I prefer a full barrier to also ensure all previous stores are
> > pushed out.
> 
> We already have a compiler barrier there in the form of cpu_relax.

Isn't it too late ? The barrier should be before the test_bit, to
prevent it from moving up.

Ben.