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

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Date:	Thu, 24 Mar 2011 13:15:42 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Pekka Enberg <penberg@...nel.org>
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, 24 Mar 2011, Pekka Enberg wrote:

> > 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.

But then we would get the bug in kmem_cache_alloc() and not in the
*_emu() function. So the _emu is executing but failing on Ingo's system
but not on mine. Question is why.

For some reason the first reference to %gs:(%rsi) wont work right on his
system:

>From arch/x86/lib/cmpxchg16b_emu

#
# Emulate 'cmpxchg16b %gs:(%rsi)' except we return the result in %al not
# via the ZF.  Caller will access %al to get result.
#
# Note that this is only useful for a cpuops operation.  Meaning that we
# do *not* have a fully atomic operation but just an operation that is
# *atomic* on a single cpu (as provided by the this_cpu_xx class of
# macros).
#
this_cpu_cmpxchg16b_emu:
        pushf
        cli

        cmpq %gs:(%rsi), %rax
        jne not_same
        cmpq %gs:8(%rsi), %rdx
        jne not_same

        movq %rbx, %gs:(%rsi)
        movq %rcx, %gs:8(%rsi)

        popf
        mov $1, %al
        ret

 not_same:
        popf
        xor %al,%al
        ret

CFI_ENDPROC




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