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

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Message-Id: <20161027153256.4121871-1-arnd@arndb.de>
Date:   Thu, 27 Oct 2016 17:32:50 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Fan Li <fanofcode.li@...sung.com>,
        Weichao Guo <guoweichao@...wei.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2] f2fs: replace a build-time warning with runtime WARN_ON

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:
-- 
2.9.0