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Subject: Re: [PATCH] btrfs: avoid link error with CONFIG_NO_AUTO_INLINE
To:     Arnd Bergmann <arnd@arndb.de>, Chris Mason <clm@fb.com>,
        David Sterba <DSterba@suse.com>,
        Josef Bacik <josef@toxicpanda.com>
Cc:     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
References: <20181103153941.1881966-1-arnd@arndb.de>
From:   Qu Wenruo <wqu@suse.de>
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Date:   Sun, 4 Nov 2018 08:38:24 +0800
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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?

> 
> 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.

Thanks,
Qu

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  fs/btrfs/super.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index c3c1e7bee49d..b7af0b8936ad 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1922,7 +1922,7 @@ static int btrfs_remount(struct super_block *sb, int *flags,
>  }
>  
>  /* Used to sort the devices by max_avail(descending sort) */
> -static int btrfs_cmp_device_free_bytes(const void *dev_info1,
> +static inline int btrfs_cmp_device_free_bytes(const void *dev_info1,
>  				       const void *dev_info2)
>  {
>  	if (((struct btrfs_device_info *)dev_info1)->max_avail >
> @@ -1951,8 +1951,8 @@ static inline void btrfs_descending_sort_devices(
>   * The helper to calc the free space on the devices that can be used to store
>   * file data.
>   */
> -static int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
> -				       u64 *free_bytes)
> +static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
> +					      u64 *free_bytes)
>  {
>  	struct btrfs_device_info *devices_info;
>  	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
>