linux-kernel - Re: [PATCH 1/2] scalable rw

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Message-Id: <20070516172435.bd3270bd.akpm@linux-foundation.org>
Date:	Wed, 16 May 2007 17:24:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...sign.ru>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 1/2] scalable rw_mutex

On Wed, 16 May 2007 16:40:59 -0700 (PDT) Christoph Lameter <clameter@....com> wrote:

> On Wed, 16 May 2007, Andrew Morton wrote:
> 
> > (I hope.  Might have race windows in which the percpu_counter_sum() count is
> > inaccurate?)
> 
> The question is how do these race windows affect the locking scheme?

The race to which I refer here is if another CPU is running
percpu_counter_sum() in the window between the clearing of the bit in
cpu_online_map and the CPU_DEAD callout.  Maybe that's too small to care
about in the short-term, dunno.

Officially we should fix that by taking lock_cpu_hotplug() in
percpu_counter_sum(), but I hate that thing.

I was thinking of putting a cpumask into the counter.  If we do that then
there's no race at all: everything happens under fbc->lock.  This would be
a preferable fix, if we need to fix it.

But I'd prefer that freezer-based cpu-hotplug comes along and saves us
again.

umm, actually, we can fix the race by using CPU_DOWN_PREPARE instead of
CPU_DEAD.  Because it's OK if percpu_counter_sum() looks at a gone-away
CPU's slot.
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