linux-kernel - Re: [PATCH] [4/4] SLAB: Fix node add timer race in cache

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Date:	Mon, 1 Mar 2010 02:27:31 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Andi Kleen <andi@...stfloor.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, haicheng.li@...el.com
Subject: Re: [PATCH] [4/4] SLAB: Fix node add timer race in cache_reap

On Mon, 1 Mar 2010, KAMEZAWA Hiroyuki wrote:

> > > Well Kamesan indicated that this worked if a cpu became online.
> > 
> > I mean in the general case. There were tons of problems all over.
> > 
> Then, it's cpu hotplug matter, not memory hotplug.
> cpu hotplug callback should prepaare 
> 
> 
> 	l3 = searchp->nodelists[node];
> 	BUG_ON(!l3);
> 
> before onlined. Rather than taking care of races.
> 

I can only speak for x86 and not the abundance of memory hotplug support 
that exists for powerpc, but cpu hotplug doesn't do _anything_ when a 
memory region that has a corresponding ACPI_SRAT_MEM_HOT_PLUGGABLE entry 
in the SRAT is hotadded and requires a new nodeid.  That can be triggered 
via the acpi layer with plug and play or explicitly from the command line 
via CONFIG_ARCH_MEMORY_PROBE.

Relying on cpu hotplug to set up nodelists in such a circumstance simply 
won't work.  You need memory hotplug support such as in my patch.
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