Re: RCU bug with v3.17-rc3 ?

From: Nathan Lynch
Date: Fri Oct 10 2014 - 21:45:06 EST

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.

Hopefully nobody's still using gcc 4.8 from the Linaro 2013.11 toolchain
release -- since it's a 4.8.3 prerelease from before the fix was
committed you'll get GCC_VERSION == 40803 but still generate bad code.

> However, I'm rather annoyed that there are people here who have known
> for some time that GCC 4.8.1 and GCC 4.8.2 _can_ lead to filesystem
> corruption, and have sat on their backsides doing nothing about getting
> it blacklisted for something like a year.

Mea culpa, although I hadn't drawn the connection to FS corruption
reports until now. I have known about the issue for some time, but
figured the prevalence of the fix in downstream projects largely
mitigated the issue.

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