On Tuesday 17 November 2015 15:38:55 Andrew Morton wrote:
On Fri, 13 Nov 2015 10:26:41 -0800 Yang Shi yang.shi@linaro.org 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 consistency.
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 warning.
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 Wrong.
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).
How about initializing only m_dirty but not the others?
Arnd