linux-kernel - Re: linux-next: build warning after merge of the tip tree

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Date:   Thu, 2 Apr 2020 09:00:51 +1100
From:   Stephen Rothwell <sfr@...b.auug.org.au>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: linux-next: build warning after merge of the tip tree

Hi Thomas,

On Wed, 01 Apr 2020 12:25:25 +0200 Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Stephen Rothwell <sfr@...b.auug.org.au> writes:
> >
> > On Mon, 30 Mar 2020 13:47:46 +1100 Stephen Rothwell <sfr@...b.auug.org.au> wrote:  
> >>
> >> After merging the tip tree, today's linux-next build (arm
> >> multi_v7_defconfig) produced this warning:
> >> 
> >> kernel/futex.c: In function 'do_futex':
> >> kernel/futex.c:1676:17: warning: 'oldval' may be used uninitialized in this function [-Wmaybe-uninitialized]
> >>  1676 |   return oldval == cmparg;
> >>       |          ~~~~~~~^~~~~~~~~
> >> kernel/futex.c:1652:6: note: 'oldval' was declared here
> >>  1652 |  int oldval, ret;
> >>       |      ^~~~~~
> >> 
> >> Introduced by commit
> >> 
> >>   a08971e9488d ("futex: arch_futex_atomic_op_inuser() calling
> >>   conventions change")  
> 
> Huch?
>  
> >> but I don't arm-linux-gnueabi-gcc (Debian 9.2.1-21) 9.2.1 20191130see how it makes this difference :-(  
> 
> Me neither. Which compiler version?

arm-linux-gnueabi-gcc (Debian 9.2.1-21) 9.2.1 20191130

> I'm using arm-linux-gnueabi-gcc (Debian 8.3.0-2) 8.3.0 which does not
> show that oddity.

I assume it is because of the change to arch_futex_atomic_op_inuser()
for arm and the compiler is not clever enough to work out that the early
return from arch_futex_atomic_op_inuser() means that oldval is not
referenced in its caller.

-- 
Cheers,
Stephen Rothwell

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