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

[Shi, Yang]

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
> > > 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).

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.

Thanks,
Yang

> 	Arnd

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