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From: Qu Wenruo <quwenruo.btrfs@gmx.com>
To: dsterba@suse.cz, Qu Wenruo <wqu@suse.com>, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH v2 07/13] btrfs-progs: Fix Wmaybe-uninitialized warning
Date: Wed, 5 Dec 2018 22:03:30 +0800
Message-ID: <926908c6-14c0-5c10-7ce2-de4b810e702b@gmx.com> (raw)
In-Reply-To: <20181205134005.GB3905@twin.jikos.cz>

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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.

Thanks,
Qu


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  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-05  6:40 [PATCH v2 00/13] btrfs-progs: Make W=1 great (no "again") Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 01/13] btrfs-progs: Makefile.extrawarn: Import cc-disable-warning Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 02/13] btrfs-progs: fix gcc8 default build warning caused by '-Wformat-truncation' Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 03/13] btrfs-progs: Makefile.extrawarn: Don't warn on sign compare Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 04/13] btrfs-progs: Fix Wempty-body warning Qu Wenruo
2018-12-13 18:59   ` David Sterba
2018-12-13 23:54     ` Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 05/13] btrfs-progs: Fix Wimplicit-fallthrough warning Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 06/13] btrfs-progs: Fix Wsuggest-attribute=format warning Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 07/13] btrfs-progs: Fix Wmaybe-uninitialized warning Qu Wenruo
2018-12-05 13:40   ` David Sterba
2018-12-05 14:03     ` Qu Wenruo [this message]
2019-01-15 17:42       ` David Sterba
2018-12-05  6:40 ` [PATCH v2 08/13] btrfs-progs: Fix Wtype-limits warning Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 09/13] btrfs-progs: Fix missing-prototypes warning caused by non-static functions Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 10/13] btrfs-progs: Move btrfs_check_nodesize to fsfeatures.c to fix missing-prototypes warning Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 11/13] btrfs-progs: Introduce rescue.h to resolve missing-prototypes for chunk and super rescue Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 12/13] btrfs-progs: Add utils.h include to solve missing-prototypes warning Qu Wenruo
2018-12-05  6:40 ` [PATCH v2 13/13] btrfs-progs: free-space-tree: Remove unsued function Qu Wenruo

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