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

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Message-Id: <20190702234135.78780-1-nhuck@google.com>
Date:   Tue,  2 Jul 2019 16:41:35 -0700
From:   Nathan Huckleberry <nhuck@...gle.com>
To:     catalin.marinas@....com, will@...nel.org
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nathan Huckleberry <nhuck@...gle.com>
Subject: [PATCH v2] arm64: mm: Fix dead assignment of old_pte

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(-)

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