Re: RCU bug with v3.17-rc3 ?

[Greg KH <greg@kroah.com>]

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 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.
> > 
> > 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?

thanks,

greg k-h