Re: RCU bug with v3.17-rc3 ?

[Peter Hurley <peter@hurleysoftware.com>]

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@arm.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.