linux-kernel - Re: [PATCH [RT] 05/14] rearrange rt_spin

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Date:	Fri, 22 Feb 2008 08:43:38 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	gregory.haskins@...il.com, Gregory Haskins <ghaskins@...ell.com>,
	a.p.zijlstra@...llo.nl, tglx@...utronix.de,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	bill.huey@...il.com, kevin@...man.org, cminyard@...sta.com,
	dsingleton@...sta.com, dwalker@...sta.com, npiggin@...e.de,
	dsaxena@...xity.net, ak@...e.de, gregkh@...e.de,
	sdietrich@...ell.com, pmorreale@...ell.com, mkohari@...ell.com
Subject: Re: [PATCH [RT] 05/14] rearrange rt_spin_lock sleep


On Fri, 22 Feb 2008, Ingo Molnar wrote:

>
> * Gregory Haskins <gregory.haskins.ml@...il.com> wrote:
>
> > My assumption is that the xchg() (inside update_current()) acts as an
> > effective wmb().  If xchg() does not have this property, then this
> > code is broken and patch 6/14 should also add a:
>
> xchg() is a strong implicit memory barrier, it implies smp_mb().
> (historic sidenote: it was the very first SMP primitive we had in
> Linux.)

OK, I've been proven wrong ;-)

I was just thinking of how an arch would implement it. No need for memory
barriers in just an xchg. But if Linux "implies it" then that's another
story.

Thanks,

-- Steve

/me learns something new everyday.

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