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Date:	Tue, 14 Oct 2014 06:27:52 -0400
From:	Peter Hurley <>
To:	Greg KH <>,
	Russell King - ARM Linux <>
CC:	David Laight <David.Laight@...LAB.COM>,
	'Nathan Lynch' <>,
	Felipe Balbi <>, Rik van Riel <>,
	"Paul E. McKenney" <>,
	Tony Lindgren <>,
	Linux USB Mailing List <>,
	Linux Kernel Mailing List <>,
	"" <>,
	Rabin Vincent <>,
	Alan Stern <>,
	Johannes Weiner <>,
	Sasha Levin <>,
	Andrew Morton <>,
	Linux OMAP Mailing List <>,
	Linus Torvalds <>,
	Linux ARM Kernel Mailing List 
Subject: Re: RCU bug with v3.17-rc3 ?

On 10/13/2014 10:06 PM, Greg KH wrote:
> On Mon, Oct 13, 2014 at 12:43:07PM +0100, Russell King - ARM Linux wrote:
>> On Mon, Oct 13, 2014 at 09:11:34AM +0000, David Laight wrote:
>>> From: Nathan Lynch
>>>> On 10/10/2014 11:25 AM, Russell King - ARM Linux wrote:
>>>>> Right, so GCC 4.8.{1,2} are totally unsuitable for kernel building (and
>>>>> it seems that this has been known about for some time.)
>>>> Looking at it seems that all 4.8.x for x < 3
>>>> are affected, as well as 4.9.0.
>>>>> We can blacklist these GCC versions quite easily.  We already have GCC
>>>>> 3.3 blacklisted, and it's trivial to add others.  I would want to include
>>>>> some proper details about the bug, just like the other existing entries
>>>>> we already have in asm-offsets.c, where we name the functions that the
>>>>> compiler is known to break where appropriate.
>>>> Before blacklisting anything, it's worth considering that simple version
>>>> checks would break existing pre-4.8.3 compilers that have been patched
>>>> for PR58854.  It looks like Yocto and Buildroot issued releases with
>>>> patched 4.8.2 compilers well before the (fixed) 4.8.3 release.  I think
>>>> the most we can reasonably do without breaking some correctly-behaving
>>>> toolchains is to emit a warning.
>>> Is it possible to compile a small code fragment and check the generated
>>> code for the bug?
>>> Possibly predicated on the broken version number to avoid false positives.
>> I don't see how - it looks like it requires an interrupt to occur at an
>> opportune moment to provoke the function to fail.  The alternative would
>> be to parse the assembly generated by the compiler to determine how it
>> is dealing with the stack.
>> I think the only viable solution here is that:
>> 1. We blacklist the bad compiler versions outright in the kernel.
> Yes, please do this, it's what we have done for other buggy compiler
> versions, no need to do something different here.
>> Remember, it's the distro's choice to fix these buggy compilers, so the
>> onus is on _them_ to deal with the mess they've created by doing so.
> I totally agree.
> Is someone going to send this patch, or do I have to write it myself?

I did on Friday (arm: Blacklist gcc 4.8.[012] ...) but Russell said he
was doing it himself.

Peter Hurley

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