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

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Message-ID: <54388B81.5020306@mentor.com>
Date:	Fri, 10 Oct 2014 20:44:33 -0500
From:	Nathan Lynch <Nathan_Lynch@...tor.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Felipe Balbi <balbi@...com>, Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tony Lindgren <tony@...mide.com>,
	Linux USB Mailing List <linux-usb@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<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>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	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 10/10/2014 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.

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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