Re: [PATCH] writeback: initialize m_dirty to avoid compile warning

From: Shi, Yang
Date: Wed Nov 18 2015 - 12:32:36 EST

On 11/18/2015 1:53 AM, Arnd Bergmann wrote:
On Tuesday 17 November 2015 15:38:55 Andrew Morton wrote:
On Fri, 13 Nov 2015 10:26:41 -0800 Yang Shi <yang.shi@xxxxxxxxxx> wrote:

When building kernel with gcc 5.2, the below warning is raised:

mm/page-writeback.c: In function 'balance_dirty_pages.isra.10':
mm/page-writeback.c:1545:17: warning: 'm_dirty' may be used uninitialized in this function [-Wmaybe-uninitialized]
unsigned long m_dirty, m_thresh, m_bg_thresh;

The m_dirty{thresh, bg_thresh} are initialized in the block of "if (mdtc)",
so if mdts is null, they won't be initialized before being used.
Initialize m_dirty to zero, also initialize m_thresh and m_bg_thresh to keep

They are used later by if condition:
!mdtc || m_dirty <= dirty_freerun_ceiling(m_thresh, m_bg_thresh)

If mdtc is null, dirty_freerun_ceiling will not be called at all, so the
initialization will not change any behavior other than just ceasing the compile

Geeze I hate that warning. gcc really could be a bit smarter about it
and this is such a case.

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1542,7 +1542,7 @@ static void balance_dirty_pages(struct address_space *mapping,
for (;;) {
unsigned long now = jiffies;
unsigned long dirty, thresh, bg_thresh;
- unsigned long m_dirty, m_thresh, m_bg_thresh;
+ unsigned long m_dirty = 0, m_thresh = 0, m_bg_thresh = 0;

* Unstable writes are a feature of certain networked

Adding runtime overhead to suppress a compile-time warning is Just

With gcc-4.4.4 the above patch actually reduces page-writeback.o's
.text by 36 bytes, lol. With gcc-4.8.4 the patch saves 19 bytes. No
idea what's going on there...

I've done tons of build tests and never got the warning for the variables
other than m_dirty, and that one also just with very few configurations
(e.g. ARM omap2plus_defconfig).

Yes, I just got the warning for m_dirty too. Just initialize m_thresh and m_bg_thresh to keep consistency (not sure if it is necessary). And, I'm a little bit confused why gcc just reports m_dirty but ignore others.

How about initializing only m_dirty but not the others?

Fine to me.



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