linux-kernel - Re: Linux 2.6.27.27

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Date:	Mon, 20 Jul 2009 23:45:04 +0200 (CEST)
From:	Krzysztof Oledzki <olel@....pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Greg KH <gregkh@...e.de>, Krzysztof Oledzki <olel@....pl>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, stable@...nel.org,
	lwn@....net
Subject: Re: Linux 2.6.27.27



On Mon, 20 Jul 2009, Linus Torvalds wrote:

>
>
> On Mon, 20 Jul 2009, Greg KH wrote:
>>
>>>> Linus Torvalds (1):
>>>>      Don't use '-fwrapv' compiler option: it's buggy in gcc-4.1.x
>>>
>>> I'm very sorry to say that but 2.6.27.27 compiled with gcc-4.2.4
>>> hangs during boot and it is caused by this fix.
>>>
>>> I extended http://bugzilla.kernel.org/show_bug.cgi?id=13012 with
>>> additional info & .config.
>>
>> Wierd.  Linus, any thoughts?  Do we need to check the version of the
>> compiler that we are using before turning that option off?
>
> Dang. That was what we hoped to avoid by not using -fwrapv - we didn't
> know which compilers were buggy, although it _looked_ like just 4.1.x (I
> have some dim memory of somebody suspecting a 4.2.x version too, but that
> may have been just a "we know it should only be needed in 4.3.x, so maybe
> we shouldn't use it in 4.2.x either").
>
> But it looks like the "buggy window" for -fno-strict-overflow is
> potentially even _larger_ than it ever was with -fwrapv. And I suspect
> that in both cases the real problem is that almost nobody ever uses either
> optimization flag, so coverage is way way smaller.
>
> For now, I suspect we need to do something like this: go back to -fwrapv
> (because it has gotten more testing), but limit it to 4.2.x and newer.
>
> However, it would be really good to figure out _why_ it breaks. It's
> probably some specific configuration issue in addition to the particular
> kernel vesion (and maybe it even needs some particular hardware to show
> the bug - ie a specific driver that triggers it or whatever).

No problem. Please let me know what should I do to help tracking this 
issue.

Best regards,

 				Krzysztof Olędzki