linux-kernel - Re: [PATCH v2] arm64: mm: Fix dead assignment of old_pte

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Date:   Wed, 3 Jul 2019 10:24:26 -0700
From:   Nathan Huckleberry <nhuck@...gle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     catalin.marinas@....com, will@...nel.org,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] arm64: mm: Fix dead assignment of old_pte

Warning is not triggered in next. Looks like this patch is unnecessary.

Thanks,
Nathan Huckleberry

On Wed, Jul 3, 2019 at 4:23 AM Mark Rutland <mark.rutland@....com> wrote:
>
> On Tue, Jul 02, 2019 at 04:41:35PM -0700, Nathan Huckleberry 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>
> > ---
> > Changes from v1 -> v2
> > * Added scope to avoid [-Wdeclaration-after-statement]
> >  arch/arm64/include/asm/pgtable.h | 27 ++++++++++++++++-----------
> >  1 file changed, 16 insertions(+), 11 deletions(-)
>
> As Will asked, does this also trigger in linux-next?
>
> I rewrote this code to avoid to only perform the READ_ONCE() when we'd
> use the value, and IIUC that may be sufficient to avoid the warning:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?h=for-next/core&id=9b604722059039a1a3ff69fb8dfd024264046024
>
> Thanks,
> Mark.
>
> >
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index fca26759081a..12b7f08db40d 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);
> >
> > @@ -248,16 +246,23 @@ 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).
> >        */
> > -     old_pte = READ_ONCE(*ptep);
> > -     if (IS_ENABLED(CONFIG_DEBUG_VM) && 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",
> > -                          __func__, pte_val(old_pte), pte_val(pte));
> > -             VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
> > -                          "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
> > -                          __func__, pte_val(old_pte), pte_val(pte));
> > +     #if IS_ENABLED(CONFIG_DEBUG_VM)
> > +     {
> > +             pte_t old_pte;
> > +
> > +             old_pte = READ_ONCE(*ptep);
> > +             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",
> > +                                  __func__, pte_val(old_pte), pte_val(pte));
> > +                     VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
> > +                                  "%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
> >
> >
> > _______________________________________________
> > linux-arm-kernel mailing list
> > linux-arm-kernel@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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