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 09:04:08 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Yu, Fenghua" <fenghua.yu@...el.com>
Cc:	"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

> 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).

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?

-Tony
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