linux-kernel - Re: [GIT PULL] SLAB changes for v2.6.39-rc1

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Message-ID: <AANLkTi=KZQd-GrXaq4472V3XnEGYqnCheYcgrdPFE0LJ@mail.gmail.com>
Date:	Thu, 24 Mar 2011 20:06:10 +0200
From:	Pekka Enberg <penberg@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Ingo Molnar <mingo@...e.hu>, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, tj@...nel.org, npiggin@...nel.dk,
	rientjes@...gle.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [GIT PULL] SLAB changes for v2.6.39-rc1

On Thu, Mar 24, 2011 at 8:01 PM, Christoph Lameter <cl@...ux.com> wrote:
> On Thu, 24 Mar 2011, Pekka Enberg wrote:
>
>> On Thu, Mar 24, 2011 at 7:43 PM, Christoph Lameter <cl@...ux.com> wrote:
>> > The bug should only trigger on old AMD64 boxes that do not support
>> > cmpxchg16b.
>>
>> Yup. Ingo is it possible to see /proc/cpuinfo of one of the affected
>> boxes? I'll try your config but I'm pretty sure the problem doesn't
>> trigger here. Like I said, I think the problem is that alternative
>> instructions are not patched early enough for cmpxchg16b emulation to
>> work for kmem_cache_init(). I tried my check_bugs() patch but it hangs
>> during boot. I'll see if I can cook up a patch that does
>> alternative_instructions() before kmem_cache_init() because I think
>> those *should* be available during boot too.
>
> I forced the fallback to the _emu function to occur but could not trigger
> the bug in kvm.

That's not the problem. I'm sure the fallback is just fine. What I'm
saying is that the fallback is *not patched* to kernel text on Ingo's
machines because alternative_instructions() happens late in the boot!
So the problem is that on Ingo's boxes (that presumably have old AMD
CPUs) we execute cmpxchg16b, not the fallback code.

                        Pekka
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