linux-kernel - Re: [PATCH] synchronize

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Message-Id: <1192749449.7367.51.camel@pasglop>
Date:	Fri, 19 Oct 2007 09:17:29 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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 Thu, 2007-10-18 at 15:52 -0700, Linus Torvalds wrote:
> 
> 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.

I agree and you can see that in fact, we don't have enough barrier on
the other side since spin_unlock doesn't prevent subsequent loads from
crossing a previous store...

I wonder if that's worth trying to address, adding a barrier in
handle_IRQ_event for example, or we can continue ignoring the barrier
and let some drivers do their own fixes in fancy ways.

Ben.


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