linux-kernel - Re: [PATCH] synchronize

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Message-ID: <alpine.LFD.0.999.0710181549330.26902@woody.linux-foundation.org>
Date:	Thu, 18 Oct 2007 15:52:13 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org
Subject: Re: [PATCH] synchronize_irq needs a barrier

On Fri, 19 Oct 2007, Benjamin Herrenschmidt wrote:
> 
> The barrier would guarantee that ioc->active (and in fact the write to
> the chip too above) are globally visible

No, it doesn't really guarantee that.

The thing is, there is no such thing as "globally visible".

There is a "ordering of visibility wrt CPU's", but it's not global, it's 
quite potentially per-CPU. So a barrier on one CPU doesn't guarantee 
anything at all without a barrier on the *other* CPU.

That said, the interrupt handling itself contains various barriers on the 
CPU's that receive interrupts, thanks to the spinlocking. But I do agree 
with Herbert that adding a "smb_mb()" is certainly in no way "obviously 
correct", because it doesn't talk about what the other side does wrt 
barriers and that word in memory.

		Linus
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