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SubjectRe: [PATCH v2] arm64: mm: Fix dead assignment of old_pte
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@googlegroups.com
> Signed-off-by: Nathan Huckleberry <nhuck@google.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@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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