Re: Possible usage of uninitalized task_ratelimit variable in mm/page-writeback.c

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Nov 07, 2011 at 04:18:24PM +0800, Witold Baryluk wrote:
> Hi,
> 
> I found a minor issue when compiling kernel today
> 
> 
>   CC      mm/page-writeback.o
> mm/page-writeback.c: In function ‘balance_dirty_pages_ratelimited_nr’:
> include/trace/events/writeback.h:281:1: warning: ‘task_ratelimit’ may be used uninitialized in this function [-Wuninitialized]
> mm/page-writeback.c:1018:16: note: ‘task_ratelimit’ was declared here
> 
> Indeed in balance_dirty_pages a task_ratelimit may be not initialized
> (initialization skiped by goto pause;), and then used when calling
> tracing hook.

Witold, thanks for the report! This patch should fix the bug.

Thanks,
Fengguang
---

writeback: fix uninitialized task_ratelimit

In balance_dirty_pages() task_ratelimit may be not initialized
(initialization skiped by goto pause;), and then used when calling
tracing hook.

Fix it by moving the task_ratelimit assignment before goto pause.

Reported-by: Witold Baryluk <baryluk@smp.if.uj.edu.pl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/page-writeback.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- linux.orig/mm/page-writeback.c	2011-11-07 17:07:04.080000043 +0800
+++ linux/mm/page-writeback.c	2011-11-07 17:08:43.232000031 +0800
@@ -1097,13 +1097,13 @@ static void balance_dirty_pages(struct a
 		pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
 					       background_thresh, nr_dirty,
 					       bdi_thresh, bdi_dirty);
-		if (unlikely(pos_ratio == 0)) {
+		task_ratelimit = (u64)dirty_ratelimit *
+					pos_ratio >> RATELIMIT_CALC_SHIFT;
+		if (unlikely(task_ratelimit == 0)) {
 			pause = max_pause;
 			goto pause;
 		}
-		task_ratelimit = (u64)dirty_ratelimit *
-					pos_ratio >> RATELIMIT_CALC_SHIFT;
-		pause = (HZ * pages_dirtied) / (task_ratelimit | 1);
+		pause = (HZ * pages_dirtied) / task_ratelimit;
 		if (unlikely(pause <= 0)) {
 			trace_balance_dirty_pages(bdi,
 						  dirty_thresh,
--
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