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

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Date:	Wed, 12 Aug 2015 08:49:45 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	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 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.

                 Linus
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