linux-kernel - Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM corruption 'Structure needs cleaning'

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Message-ID: <55CBC6C6.3090306@kernel.org>
Date:	Wed, 12 Aug 2015 15:20:54 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>
Cc:	katsuki.uwatoko@...hiba.co.jp,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Dave Chinner <david@...morbit.com>, gangchen@...micro.com,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	karanvir.singh@...t.com, luca@...lable.com,
	christopher.squires@...t.com, edwin@...lable.com,
	wayne.burri@...t.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: enabling libgcc for 64-bit divisions, was Re: PROBLEM: XFS on ARM
 corruption 'Structure needs cleaning'

On 08/12/2015 08:49 AM, Linus Torvalds wrote:
> On Tue, Aug 11, 2015 at 11:24 PM, Christoph Hellwig <hch@...radead.org> wrote:
>>
>> Maybe it's time to rely on gcc to handle 64 bit divisions now?
>
> Ugh. gcc still does a pretty horrible job at it. While gcc knows that
> a widening 32x32->64 multiplication can be simplified, it doesn't do
> the same thing for a 64/32->64 division, and always calls __udivdi3
> for it.
>
> Now, __udivdi3 does avoid the general nasty case by then testing the
> upper 32 bits of the divisor against zero, so it's not entirely
> disastrous. It's just ugly.
>
> But perhaps more importantly, I'm not at all sure libgcc is
> kernel-safe. In particular, I'm not at all sure it *remains*
> kernel-safe. Just as an example: can you guarantee that libgcc doesn't
> implement integer division on some architecture by using the FP
> hardware?
>
> There's been a few cases where not having libgcc saved us headaches. I
> forget the exact details, but it was something like several years ago
> that we had gcc start to generate some insane crap exception handling
> for C code generation, and the fact that we didn't include libgcc was
> what made us catch it because of the resulting link error.
>
> libgcc just isn't reliable in kernel space. I'm not opposed to some
> random architecture using it (arch/tile does include "-lgcc" for
> example), but I _do_ object to the notion that we say "let's use
> libgcc in general".
>
> So no. I do not believe that the occasional pain of a few people who
> do 64-bit divides incorrectly is a good enough argument to start using
> libgcc.
>

Does your objection still apply if we supplied our own implementations 
of a handful of libgcc helpers?

--Andy

>                   Linus
>

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