From: Arnd Bergmann <arnd@arndb.de>
To: Qu Wenruo <wqu@suse.de>
Cc: Chris Mason <clm@fb.com>, David Sterba <DSterba@suse.com>,
Josef Bacik <josef@toxicpanda.com>,
Gu Jinxiang <gujx@cn.fujitsu.com>,
Changbin Du <changbin.du@gmail.com>,
Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>,
Anand Jain <anand.jain@oracle.com>,
Nikolay Borisov <nborisov@suse.com>,
linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] btrfs: avoid link error with CONFIG_NO_AUTO_INLINE
Date: Sun, 4 Nov 2018 23:32:03 +0100 [thread overview]
Message-ID: <CAK8P3a27sNTiKV--smXd1wUzkGY70uUhS1rif6tgUobE+YYcbw@mail.gmail.com> (raw)
In-Reply-To: <87d4e8ce-2142-9d6e-498a-c7e5b83964eb@suse.de>
On 11/4/18, Qu Wenruo <wqu@suse.de> wrote:
>
>
> On 2018/11/3 下午11:39, Arnd Bergmann wrote:
>> On 32-bit ARM with gcc-8, I see a link error with the addition of the
>> CONFIG_NO_AUTO_INLINE option:
>>
>> fs/btrfs/super.o: In function `btrfs_statfs':
>> super.c:(.text+0x67b8): undefined reference to `__aeabi_uldivmod'
>> super.c:(.text+0x67fc): undefined reference to `__aeabi_uldivmod'
>> super.c:(.text+0x6858): undefined reference to `__aeabi_uldivmod'
>> super.c:(.text+0x6920): undefined reference to `__aeabi_uldivmod'
>> super.c:(.text+0x693c): undefined reference to `__aeabi_uldivmod'
>> fs/btrfs/super.o:super.c:(.text+0x6958): more undefined references to
>> `__aeabi_uldivmod' follow
>>
>> So far this is the only file that shows the behavior, so I'd propose
>> to just work around it by marking the functions as 'static inline'
>> that normally get inlined here.
>
> As a workaround it looks OK, but it's definitely not the root cause.
>
>>
>> The reference to __aeabi_uldivmod comes from a div_u64() which has an
>> optimization for a constant division that uses a straight '/' operator
>> when the result should be known to the compiler. My interpretation is
>> that as we turn off inlining, gcc still expects the result to be constant
>> but fails to use that constant value.
>
> It looks more like a bug in div_u64() optimization.
>
> Despite this file in btrfs, did you hit the same problem for any other
> file?
Not this time. I've done a creduce on the file and got to this code
struct kstatfs {
u64 f_bfree;
};
btrfs_calc_avail_data_space(p1) {}
btrfs_statfs(struct kstatfs *p1) {
u64 d = 0;
unsigned e = 1;
for (; a;)
e = btrfs_bg_type_to_factor();
p1->f_bfree = div_u64(0, e) >> c;
__asm__("");
div_u64(d, e);
b = btrfs_calc_avail_data_space(&d);
}
Looking at the assembler code produced by this, it seems to be
the same thing that we dealt with in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785
>> Cc: Changbin Du <changbin.du@gmail.com>
>> Fixes: 943b8435c3bd ("kernel hacking: add a config option to disable
>> compiler auto-inlining")
>
> I can't find it in the mainline kernel, is the commit hash correct?
> If not merged, we should still has a chance to further polish that patch.
It's in linux-next.
Arnd
next prev parent reply other threads:[~2018-11-04 22:32 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-03 15:39 [PATCH] btrfs: avoid link error with CONFIG_NO_AUTO_INLINE Arnd Bergmann
2018-11-03 20:41 ` Nikolay Borisov
2018-11-03 21:25 ` Arnd Bergmann
2018-11-04 0:38 ` Qu Wenruo
2018-11-04 22:32 ` Arnd Bergmann [this message]
2018-11-05 16:51 ` David Sterba
2018-11-05 21:27 ` Arnd Bergmann
2018-11-06 15:04 ` David Sterba
2018-11-05 9:20 ` Nikolay Borisov
2018-11-05 14:37 ` Changbin Du
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