linux-kernel - Re: [PATCH] bcache: avoid clang -Wunintialized warning

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Date:   Fri, 26 Apr 2019 06:01:34 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Coly Li <colyli@...e.de>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Kent Overstreet <kent.overstreet@...il.com>,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jens Axboe <axboe@...nel.dk>,
        Shenghui Wang <shhuiw@...mail.com>,
        Hannes Reinecke <hare@...e.com>, linux-bcache@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bcache: avoid clang -Wunintialized warning

On Fri, Apr 26, 2019 at 10:43:01AM +0800, Coly Li wrote:
> On 2019/4/26 2:08 上午, Nathan Chancellor wrote:
> > On Fri, Mar 22, 2019 at 03:35:00PM +0100, Arnd Bergmann wrote:
> >> clang has identified a code path in which it thinks a
> >> variable may be unused:
> >>
> >> drivers/md/bcache/alloc.c:333:4: error: variable 'bucket' is used uninitialized whenever 'if' condition is false
> >>       [-Werror,-Wsometimes-uninitialized]
> >>                         fifo_pop(&ca->free_inc, bucket);
> >>                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >> drivers/md/bcache/util.h:219:27: note: expanded from macro 'fifo_pop'
> >>  #define fifo_pop(fifo, i)       fifo_pop_front(fifo, (i))
> >>                                 ^~~~~~~~~~~~~~~~~~~~~~~~~
> >> drivers/md/bcache/util.h:189:6: note: expanded from macro 'fifo_pop_front'
> >>         if (_r) {                                                       \
> >>             ^~
> >> drivers/md/bcache/alloc.c:343:46: note: uninitialized use occurs here
> >>                         allocator_wait(ca, bch_allocator_push(ca, bucket));
> >>                                                                   ^~~~~~
> >> drivers/md/bcache/alloc.c:287:7: note: expanded from macro 'allocator_wait'
> >>                 if (cond)                                               \
> >>                     ^~~~
> >> drivers/md/bcache/alloc.c:333:4: note: remove the 'if' if its condition is always true
> >>                         fifo_pop(&ca->free_inc, bucket);
> >>                         ^
> >> drivers/md/bcache/util.h:219:27: note: expanded from macro 'fifo_pop'
> >>  #define fifo_pop(fifo, i)       fifo_pop_front(fifo, (i))
> >>                                 ^
> >> drivers/md/bcache/util.h:189:2: note: expanded from macro 'fifo_pop_front'
> >>         if (_r) {                                                       \
> >>         ^
> >> drivers/md/bcache/alloc.c:331:15: note: initialize the variable 'bucket' to silence this warning
> >>                         long bucket;
> >>                                    ^
> >>
> >> This cannot happen in practice because we only enter the loop
> >> if there is at least one element in the list.
> >>
> >> Slightly rearranging the code makes this clearer to both the
> >> reader and the compiler, which avoids the warning.
> >>
> >> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> >> ---
> >>  drivers/md/bcache/alloc.c | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/md/bcache/alloc.c b/drivers/md/bcache/alloc.c
> >> index 5002838ea476..f8986effcb50 100644
> >> --- a/drivers/md/bcache/alloc.c
> >> +++ b/drivers/md/bcache/alloc.c
> >> @@ -327,10 +327,11 @@ static int bch_allocator_thread(void *arg)
> >>  		 * possibly issue discards to them, then we add the bucket to
> >>  		 * the free list:
> >>  		 */
> >> -		while (!fifo_empty(&ca->free_inc)) {
> >> +		while (1) {
> >>  			long bucket;
> >>  
> >> -			fifo_pop(&ca->free_inc, bucket);
> >> +			if (!fifo_pop(&ca->free_inc, bucket))
> >> +				break;
> >>  
> >>  			if (ca->discard) {
> >>  				mutex_unlock(&ca->set->bucket_lock);
> >> -- 
> >> 2.20.0
> >>
> > 
> > Hi all,
> > 
> > Could someone please review/pick this up? This is one of two remaining
> > -Wsometimes-uninitialized warnings among arm, arm64, and x86_64
> > all{yes,mod}config and I'd like to get it turned on as soon as possible
> > to catch more bugs.
> 
> Hi Nathan,
> 
> It is in Jens' block tree for-next branch already, for Linux v5.2 merge
> window.
> 
> Thanks.
> 
> -- 
> 
> Coly Li

Hi Coly,

Thank you for the reply and heads up, it hadn't hit -next when I sent
that message and I didn't check Jens' tree.

I appreciate you picking it up!

Nathan