linux-kernel - Re: [PATCH v2] f2fs: replace a build-time warning with runtime WARN

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Message-ID: <c0d9b688-0ac9-6b45-62cc-02e71f2aa331@huawei.com>
Date:   Wed, 2 Nov 2016 15:09:03 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Arnd Bergmann <arnd@...db.de>, Jaegeuk Kim <jaegeuk@...nel.org>
CC:     Fan Li <fanofcode.li@...sung.com>,
        Weichao Guo <guoweichao@...wei.com>,
        <linux-f2fs-devel@...ts.sourceforge.net>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] f2fs: replace a build-time warning with runtime
 WARN_ON

Hi Arnd, Jaegeuk,

It is trivial, but it's better keep commit *title*, commit log and code
consistent. :)

Thanks,

On 2016/10/27 23:32, Arnd Bergmann wrote:
> gcc is unsure about the use of last_ofs_in_node, which might happen
> without a prior initialization:
> 
> fs/f2fs//git/arm-soc/fs/f2fs/data.c: In function ‘f2fs_map_blocks’:
> fs/f2fs/data.c:799:54: warning: ‘last_ofs_in_node’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>    if (prealloc && dn.ofs_in_node != last_ofs_in_node + 1) {
> 
> As pointed out by Chao Yu, the code is actually correct as 'prealloc'
> is only set if the last_ofs_in_node has been set, the two always
> get updated together.
> 
> This initializes last_ofs_in_node to dn.ofs_in_node for each
> new dnode at the start of the 'next_block' loop, which at that
> point is a correct initialization as well. I assume that compilers
> that correctly track the contents of the variables and do not
> warn about the condition also figure out that they can eliminate
> the extra assignment here.
> 
> Fixes: 46008c6d4232 ("f2fs: support in batch multi blocks preallocation")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  fs/f2fs/data.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> v2: use simpler workaround, as discussed with Chao Yu.
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 68edb47f5f71..cf5ec39f907d 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -713,7 +713,7 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map,
>  	}
>  
>  	prealloc = 0;
> -	ofs_in_node = dn.ofs_in_node;
> +	last_ofs_in_node = ofs_in_node = dn.ofs_in_node;
>  	end_offset = ADDRS_PER_PAGE(dn.node_page, inode);
>  
>  next_block:
>