linux-kernel - Re: [PATCH] arm64: percpu: Initialize ret in the default case

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Date:   Tue, 25 Sep 2018 13:29:11 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     dennis@...nel.org, tj@...nel.org, Christoph Lameter <cl@...ux.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>, psodagud@...eaurora.org
Subject: Re: [PATCH] arm64: percpu: Initialize ret in the default case

On Tue, Sep 25, 2018 at 1:24 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, Sep 25, 2018 at 12:45 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > Clang warns that if the default case is taken, ret will be
> > uninitialized.
> >
> > ./arch/arm64/include/asm/percpu.h:196:2: warning: variable 'ret' is used
> > uninitialized whenever switch default is taken
> > [-Wsometimes-uninitialized]
> >         default:
> >         ^~~~~~~
> > ./arch/arm64/include/asm/percpu.h:200:9: note: uninitialized use occurs
> > here
> >         return ret;
> >                ^~~
> > ./arch/arm64/include/asm/percpu.h:157:19: note: initialize the variable
> > 'ret' to silence this warning
> >         unsigned long ret, loop;
> >                          ^
> >                           = 0
> >
> > This warning appears several times while building the erofs filesystem.
> > While it's not strictly wrong, the BUILD_BUG will prevent this from
> > becoming a true problem. Initialize ret to 0 in the default case right
> > before the BUILD_BUG to silence all of these warnings.
>
> Clang does semantic analysis BEFORE inlining/optimizations, so I can't

s/I/it/

> determine that default is never reachable.  Nathan, thanks for this
> patch.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> >
> > Reported-by: Prasad Sodagudi <psodagud@...eaurora.org>
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >  arch/arm64/include/asm/percpu.h | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/percpu.h b/arch/arm64/include/asm/percpu.h
> > index 9234013e759e..21a81b59a0cc 100644
> > --- a/arch/arm64/include/asm/percpu.h
> > +++ b/arch/arm64/include/asm/percpu.h
> > @@ -96,6 +96,7 @@ static inline unsigned long __percpu_##op(void *ptr,                  \
> >                 : [val] "Ir" (val));                                    \
> >                 break;                                                  \
> >         default:                                                        \
> > +               ret = 0;                                                \
> >                 BUILD_BUG();                                            \
> >         }                                                               \
> >                                                                         \
> > @@ -125,6 +126,7 @@ static inline unsigned long __percpu_read(void *ptr, int size)
> >                 ret = READ_ONCE(*(u64 *)ptr);
> >                 break;
> >         default:
> > +               ret = 0;
> >                 BUILD_BUG();
> >         }
> >
> > @@ -194,6 +196,7 @@ static inline unsigned long __percpu_xchg(void *ptr, unsigned long val,
> >                 : [val] "r" (val));
> >                 break;
> >         default:
> > +               ret = 0;
> >                 BUILD_BUG();
> >         }
> >
> > --
> > 2.19.0
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers