Re: RCU bug with v3.17-rc3 ?

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

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.

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