linux-kernel - Re: [PATCH] btrfs: fix uninitialized warning in btrfs_log

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Date:   Wed, 17 May 2023 17:07:55 +0800
From:   Stephen Zhang <starzhangzsd@...il.com>
To:     Qu Wenruo <quwenruo.btrfs@....com>
Cc:     clm@...com, josef@...icpanda.com, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhangshida@...inos.cn, k2ci <kernel-bot@...inos.cn>
Subject: Re: [PATCH] btrfs: fix uninitialized warning in btrfs_log_inode

Qu Wenruo <quwenruo.btrfs@....com> 于2023年5月17日周三 15:47写道：
>
>
>
> On 2023/5/16 09:34, zhangshida wrote:
> > From: Shida Zhang <zhangshida@...inos.cn>
> >
> > From: Shida Zhang <zhangshida@...inos.cn>
> >
> > This fixes the following warning reported by gcc 10 under x86_64:
>
> Full gcc version please.

it's "gcc (Debian 10.2.1-6) 10.2.1 20210110".

> Especially you need to check if your gcc10 is the latest release.
>
> If newer gcc (12.2.1) tested without such error, it may very possible to
> be a false alert.
>
> And in fact it is.
>
> @first_dir_index would only be assigned to @last_range_start if
> last_range_end != 0.
>
> Thus the loop must have to be executed once, and @last_range_start won't
> be zero.
>

Yup, I know it's a false positive. What I don't know is the criterion
that decides whether it is a good patch.
That is,
it doesn't look so good because it is a false alert and the latest gcc
can get rid of such warnings, based on what you said( if I understand
correctly).
Or,
It looks okay because the patch can make some older gcc get a cleaner
build and do no harm to the original code logic.
In fact, I've seen Linus complaining about the warning generated by
some gcc version in another thread.

https://lore.kernel.org/linux-xfs/168384265493.22863.2683852857659893778.pr-tracker-bot@kernel.org/T/#t

so it kinda make me feel confused :<

Nonetheless, I appreciate your review.

Thanks,
Shida

> Please do check your environment (especially your gcc version and
> backports), before sending such trivial patches.
> Under most cases, it helps nobody.
>
> Thanks,
> Qu
>
> >
> > ../fs/btrfs/tree-log.c: In function ‘btrfs_log_inode’:
> > ../fs/btrfs/tree-log.c:6211:9: error: ‘last_range_start’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
> >   6211 |   ret = insert_dir_log_key(trans, log, path, key.objectid,
> >        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >   6212 |       first_dir_index, last_dir_index);
> >        |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ../fs/btrfs/tree-log.c:6161:6: note: ‘last_range_start’ was declared here
> >   6161 |  u64 last_range_start;
> >        |      ^~~~~~~~~~~~~~~~
> >
> > Reported-by: k2ci <kernel-bot@...inos.cn>
> > Signed-off-by: Shida Zhang <zhangshida@...inos.cn>
> > ---
> >   fs/btrfs/tree-log.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
> > index 9b212e8c70cc..d2755d5e338b 100644
> > --- a/fs/btrfs/tree-log.c
> > +++ b/fs/btrfs/tree-log.c
> > @@ -6158,7 +6158,7 @@ static int log_delayed_deletions_incremental(struct btrfs_trans_handle *trans,
> >   {
> >       struct btrfs_root *log = inode->root->log_root;
> >       const struct btrfs_delayed_item *curr;
> > -     u64 last_range_start;
> > +     u64 last_range_start = 0;
> >       u64 last_range_end = 0;
> >       struct btrfs_key key;
> >