linux-kernel - Re: [PATCH] sbitmap: avoid maybe-uninitialized warning

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Date:   Mon, 19 Sep 2016 17:14:48 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Omar Sandoval <osandov@...com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sbitmap: avoid maybe-uninitialized warning

On Monday, September 19, 2016 8:43:12 AM CEST Jens Axboe wrote:
> On 09/19/2016 06:33 AM, Arnd Bergmann wrote:
> > The sbitmap code that has just been turned into a library module
> > returns uninitialized data for sbitmap_weight(), as pointed out by
> > gcc when building with -Wmaybe-uninitialized:
> >
> > lib/sbitmap.c: In function 'sbitmap_weight':
> > lib/sbitmap.c:179:9: error: 'weight' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> >
> > Note that the value is never initialized, we just add data on
> > top, so it is wrong regardless of sb->map_nr.
> >
> > This adds the missing initialization.
> 
> Thanks Arnd, Colin sent the same patch and I applied it. Note that:

Ok, thanks!

> > Fixes: 88459642cba4 ("blk-mq: abstract tag allocation out into sbitmap library")
> 
> that isn't truly correct, that patch is just what moved the code. The 
> bug predates that commit.

It's not important as long as the bug is fixed now, but I think it was
correct before that move, we just lost the 'used=0' initialization,
which also triggered the warning:

-static unsigned int bt_unused_tags(struct blk_mq_bitmap_tags *bt)
-{
-       unsigned int i, used;
-
-       for (i = 0, used = 0; i < bt->map_nr; i++) {
-               struct blk_align_bitmap *bm = &bt->map[i];
-
-               used += bitmap_weight(&bm->word, bm->depth);
        }
 
-       return bt->depth - used;
 }

	Arnd