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

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Date:	Sat, 11 Oct 2014 11:51:23 -0300
From:	Otavio Salvador <otavio@...ystems.com.br>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Peter Chen <peter.chen@...escale.com>,
	Rik van Riel <riel@...hat.com>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	Linux USB Mailing List <linux-usb@...r.kernel.org>,
	Nathan Lynch <Nathan_Lynch@...tor.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Felipe Balbi <balbi@...com>,
	Josh Triplett <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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 ?

Hello Russell,

On Sat, Oct 11, 2014 at 11:16 AM, Russell King - ARM Linux
<linux@....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.

-- 
Otavio Salvador                             O.S. Systems
http://www.ossystems.com.br        http://code.ossystems.com.br
Mobile: +55 (53) 9981-7854            Mobile: +1 (347) 903-9750
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