[PATCH] x86: s/READ_ONCE_NOCHECK/READ_ONCE/ in arch_atomic_read()

[Dmitry Vyukov]

Two problems was reported with READ_ONCE_NOCHECK in arch_atomic_read:
1. Andrey Ryabinin reported significant binary size increase
(+400K of text). READ_ONCE_NOCHECK is intentionally compiled to
non-inlined function call, and I counted 640 copies of it in my vmlinux.
2. Arnd Bergmann reported a new splat of too large frame sizes.

A single inlined KASAN check is very cheap, a non-inlined function
call with KASAN/KCOV instrumentation can easily be more expensive.

Switch to READ_ONCE() in arch_atomic_read().

Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Reported-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: kasan-dev@xxxxxxxxxxxxxxxx
---
 arch/x86/include/asm/atomic.h | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/atomic.h b/arch/x86/include/asm/atomic.h
index 0cde164f058a..46e53bbf7ce3 100644
--- a/arch/x86/include/asm/atomic.h
+++ b/arch/x86/include/asm/atomic.h
@@ -24,10 +24,13 @@
 static __always_inline int arch_atomic_read(const atomic_t *v)
 {
 	/*
-	 * We use READ_ONCE_NOCHECK() because atomic_read() contains KASAN
-	 * instrumentation. Double instrumentation is unnecessary.
+	 * Note: READ_ONCE() here leads to double instrumentation as
+	 * both READ_ONCE() and atomic_read() contain instrumentation.
+	 * This is deliberate choice. READ_ONCE_NOCHECK() is compiled to a
+	 * non-inlined function call that considerably increases binary size
+	 * and stack usage under KASAN.
 	 */
-	return READ_ONCE_NOCHECK((v)->counter);
+	return READ_ONCE((v)->counter);
 }
 
 /**
@@ -39,12 +42,6 @@ static __always_inline int arch_atomic_read(const atomic_t *v)
  */
 static __always_inline void arch_atomic_set(atomic_t *v, int i)
 {
-	/*
-	 * We could use WRITE_ONCE_NOCHECK() if it exists, similar to
-	 * READ_ONCE_NOCHECK() in arch_atomic_read(). But there is no such
-	 * thing at the moment, and introducing it for this case does not
-	 * worth it.
-	 */
 	WRITE_ONCE(v->counter, i);
 }
 
-- 
2.12.1.500.gab5fba24ee-goog