Re: [PATCH v2 07/13] btrfs-progs: Fix Wmaybe-uninitialized warning

[David Sterba <dsterba@suse.cz>]

On Wed, Dec 05, 2018 at 10:03:30PM +0800, Qu Wenruo wrote:
> 
> 
> On 2018/12/5 下午9:40, David Sterba wrote:
> > On Wed, Dec 05, 2018 at 02:40:12PM +0800, Qu Wenruo wrote:
> >> GCC 8.2.1 will report the following warning with "make W=1":
> >>
> >>   ctree.c: In function 'btrfs_next_sibling_tree_block':
> >>   ctree.c:2990:21: warning: 'slot' may be used uninitialized in this function [-Wmaybe-uninitialized]
> >>     path->slots[level] = slot;
> >>     ~~~~~~~~~~~~~~~~~~~^~~~~~
> >>
> >> The culprit is the following code:
> >>
> >> 	int slot;		<< Not initialized
> >> 	int level = path->lowest_level + 1;
> >>  	BUG_ON(path->lowest_level + 1 >= BTRFS_MAX_LEVEL);
> >>  	while(level < BTRFS_MAX_LEVEL) {
> >> 		slot = path->slots[level] + 1;
> >> 		^^^^^^ but we initialize @slot here.
> >> 		...
> >> 	}
> >> 	path->slots[level] = slot;
> >>
> >> It's possible that compiler doesn't get enough hint for BUG_ON() on
> >> lowest_level + 1 >= BTRFS_MAX_LEVEL case.
> >>
> >> Fix it by using a do {} while() loop other than while() {} loop, to
> >> ensure we will run the loop for at least once.
> > 
> > I was hoping that we can actually add the hint to BUG_ON so the code
> > does not continue if the condition is true.
> > 
> I checked that method, but I'm not that confident about things like:
> 
> bugon_trace()
> {
> 	if (!val)
> 		return;
> 	__bugon_trace();
> }
> 
> __attribute__ ((noreturn))
> static inline void __bugon_trace();
> 
> This is as simple as just one extra function call, but the original
> problem is just one more function call before hitting abort().
> 
> So I just give up screwing up things I'm not comfort enough to tweaking.
> 
> The current do {} while() loop is the most direct solution, if gcc one
> day still gives such warning then I could say some harsh word then.

I was not able to make it work properly, patch applied so we rid of the
warning. Thanks.