linux-kernel - Re: RCU bug with v3.17-rc3 ?

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Date:	Sat, 11 Oct 2014 14:15:37 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Otavio Salvador <otavio@...ystems.com.br>,
	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Peter Chen <peter.chen@...escale.com>,
	Rik van Riel <riel@...hat.com>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	Linux USB Mailing List <linux-usb@...r.kernel.org>,
	Nathan Lynch <Nathan_Lynch@...tor.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Felipe Balbi <balbi@...com>,
	Josh Triplett <josh@...htriplett.org>,
	Rabin Vincent <rabin@....in>,
	Alan Stern <stern@...land.harvard.edu>,
	Johannes Weiner <hannes@...xchg.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux ARM Kernel Mailing List 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: RCU bug with v3.17-rc3 ?

On 10/11/2014 10:51 AM, Otavio Salvador wrote:
> Hello Russell,
> 
> On Sat, Oct 11, 2014 at 11:16 AM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
>> On Sat, Oct 11, 2014 at 11:54:32AM +0800, Peter Chen wrote:
>>> On Fri, Oct 10, 2014 at 08:44:33PM -0500, Nathan Lynch wrote:
>>>> 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 http://gcc.gnu.org/PR58854 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.
>>>
>>> Yocto has PR58854 problem patch.
>>>
>>> http://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/meta/recipes-devtools/gcc/gcc-4.8/0048-PR58854_fix_arm_apcs_epilogue.patch?h=daisy
>>
>> Right, and we can provide links to these in the comments above the #error
>> so people have the right places to do a bit of research into whether their
>> compiler is safe.
>>
>> It is unfortunate that they are indistinguishable from the broken versions,
>> but that's really a distro problem for causing that issue themselves -
>> especially given how serious this bug is.
> 
> What about checking if GCC_PR58854_FIXED is not defined for error? So
> build systems and people could easily define it if they know their GCC
> has the fix applied.

If the distro/build system/individual is capable of patching gcc, then it
seems reasonable that the same distro/build system/individual is capable
of carrying a patch on top of mainline kernel for building with their
"special" compiler.

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