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

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Date:   Fri, 22 Mar 2019 15:35:00 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Coly Li <colyli@...e.de>,
        Kent Overstreet <kent.overstreet@...il.com>
Cc:     clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, 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: [PATCH] bcache: avoid clang -Wunintialized warning

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