linux-kernel - Re: [PATCH] arm64: mm: Fix dead assignment of old

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Date:   Tue, 2 Jul 2019 16:26:42 -0700
From:   Nathan Huckleberry <nhuck@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] arm64: mm: Fix dead assignment of old_pte

Oops I forgot moving the variable declaration would cause a warning.
Will send a V2.

Thanks,
Nathan Huckleberry

On Tue, Jul 2, 2019 at 4:25 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Tue, Jul 02, 2019 at 04:13:02PM -0700, 'Nathan Huckleberry' via Clang Built Linux wrote:
> > When analyzed with the clang static analyzer the
> > following warning occurs
> >
> > line 251, column 2
> > Value stored to 'old_pte' is never read
> >
> > This warning is repeated every time pgtable.h is
> > included by another file and produces ~3500
> > extra warnings.
> >
> > Moving old_pte into preprocessor guard.
> >
> > Cc: clang-built-linux@...glegroups.com
> > Signed-off-by: Nathan Huckleberry <nhuck@...gle.com>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index fca26759081a..42ca4fc67f27 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -238,8 +238,6 @@ extern void __sync_icache_dcache(pte_t pteval);
> >  static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> >                             pte_t *ptep, pte_t pte)
> >  {
> > -     pte_t old_pte;
> > -
> >       if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte))
> >               __sync_icache_dcache(pte);
> >
> > @@ -248,8 +246,11 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> >        * hardware updates of the pte (ptep_set_access_flags safely changes
> >        * valid ptes without going through an invalid entry).
> >        */
> > +     #if IS_ENABLED(CONFIG_DEBUG_VM)
> > +     pte_t old_pte;
> > +
> >       old_pte = READ_ONCE(*ptep);
> > -     if (IS_ENABLED(CONFIG_DEBUG_VM) && pte_valid(old_pte) && pte_valid(pte) &&
> > +     if (pte_valid(old_pte) && pte_valid(pte) &&
> >          (mm == current->active_mm || atomic_read(&mm->mm_users) > 1)) {
> >               VM_WARN_ONCE(!pte_young(pte),
> >                            "%s: racy access flag clearing: 0x%016llx -> 0x%016llx",
> > @@ -258,6 +259,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
> >                            "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
> >                            __func__, pte_val(old_pte), pte_val(pte));
> >       }
> > +     #endif
> >
> >       set_pte(ptep, pte);
> >  }
> > --
> > 2.22.0.410.gd8fdbe21b5-goog
> >
>
> Hi Nathan,
>
> This does not apply on -next because of https://git.kernel.org/arm64/c/9b604722059039a1a3ff69fb8dfd024264046024.
> I would get into the habit of testing -next to see if the warning is
> present there first because someone may have independently fixed it
> already (I'd be surprised if it wasn't fixed by that commit from a quick
> glance).
>
> Additionally, when I do apply this patch to mainline and build, I see
> the following warning:
>
> In file included from /home/nathan/cbl/linux/arch/arm64/kernel/asm-offsets.c:10:
> In file included from /home/nathan/cbl/linux/include/linux/arm_sdei.h:14:
> In file included from /home/nathan/cbl/linux/include/acpi/ghes.h:5:
> In file included from /home/nathan/cbl/linux/include/acpi/apei.h:9:
> In file included from /home/nathan/cbl/linux/include/linux/acpi.h:34:
> In file included from /home/nathan/cbl/linux/include/acpi/acpi_io.h:5:
> In file included from /home/nathan/cbl/linux/include/linux/io.h:13:
> In file included from /home/nathan/cbl/linux/arch/arm64/include/asm/io.h:18:
> /home/nathan/cbl/linux/arch/arm64/include/asm/pgtable.h:250:8: warning: ISO C90 forbids mixing declarations and code [-Wdeclaration-after-statement]
>         pte_t old_pte;
>               ^
> 1 warning generated.
>
> Cheers,
> Nathan
>
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