linux-kernel - Re: [Bugfix v2] sched: fix possible invalid memory access caused by CPU hot-addition

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Date:	Mon, 28 Apr 2014 09:09:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Tony Luck <tony.luck@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [Bugfix v2] sched: fix possible invalid memory access caused by
 CPU hot-addition

On Mon, Apr 28, 2014 at 10:48:13AM +0800, Jiang Liu wrote:
> Intel platforms with Nehalem/Westmere/IvyBridge CPUs may support socket
> hotplug/online at runtime. The CPU hot-addition flow is:
> 1) handle CPU hot-addition event
> 	1.a) gather platform specific information
> 	1.b) associate hot-added CPU with NUMA node
> 	1.c) create CPU device
> 2) online hot-added CPU through sysfs:
> 	2.a)	cpu_up()
> 	2.b)		->try_online_node()
> 	2.c)			->hotadd_new_pgdat()
> 	2.d)			->node_set_online()
> 
> Between 1.b and 2.c, hot-added CPUs are associated with NUMA nodes
> but those NUMA nodes may still be in offlined state. So we should
> check node_online(nid) before calling kmalloc_node(nid) and friends,
> otherwise it may cause invalid memory access as below.

So complete and full NAK on this. This is a workaround for a fucked in
the head BIOS. If you're going to do a work around for that they should
live in arch/ space, not in core code.

The code in question is nearly 7 years old (2.6.24), which leads me to
believe it works just fine for (regular) memory less nodes as I've not
had complaints about it before.
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