linux-kernel - RE: [2.6.25-rc5-mm1][regression] ia64: hackbench doesn't finish>12 hour

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Date:	Thu, 20 Mar 2008 17:16:42 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	"Yu, Fenghua" <fenghua.yu@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-ia64@...r.kernel.org,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: RE: [2.6.25-rc5-mm1][regression] ia64: hackbench doesn't finish>12
	hour

On Thu, 2008-03-20 at 09:04 -0700, Luck, Tony wrote:
> > Which makes me wonder, why did you ever use a semaphore here? Looking at
> > the code its a straight forward mutex. And when you would have used a
> > mutex lockdep would have warned about this.
> 
> The functionality that we are trying to add is to allow up to N
> simultaneous processors to execute the critical region.  On current
> processors/platforms N=1 so a spinlock or mutex would be fine, but
> there will be platforms for which N is a small integer greater than
> one.  Semaphore initialized to N looked to be the ideal primitive
> for this (until Motohiro-san ran the test case that showed the path
> where we call this code with a spinlock held).

Right, no alternative there.

> Next question is whether it is reasonable to get to this code
> while holding a spinlock.  Isn't this a problem for architectures
> that need to use cross-processor interrupts to do a global TLB
> shootdown?

Yeah, semaphores can't be used from hardirq contexts for much the same
reasons. But its all ia64 code, right? So I'm not directly seeing how
other archs are affected here.

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