linux-kernel - [PATCH 05/18] asm/rwonce: Remove smp_read_barrier

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Date:   Tue, 30 Jun 2020 18:37:21 +0100
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     Will Deacon <will@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Matt Turner <mattst88@...il.com>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Richard Henderson <rth@...ddle.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-alpha@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kernel-team@...roid.com
Subject: [PATCH 05/18] asm/rwonce: Remove smp_read_barrier_depends() invocation

Alpha overrides __READ_ONCE() directly, so there's no need to use
smp_read_barrier_depends() in the core code. This also means that
__READ_ONCE() can be relied upon to provide dependency ordering.

Acked-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Will Deacon <will@...nel.org>
---
 include/asm-generic/rwonce.h | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
index f9dfa88fc04d..cc810f1f18ca 100644
--- a/include/asm-generic/rwonce.h
+++ b/include/asm-generic/rwonce.h
@@ -30,24 +30,16 @@
 
 /*
  * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
- * atomicity or dependency ordering guarantees. Note that this may result
- * in tears!
+ * atomicity. Note that this may result in tears!
  */
 #ifndef __READ_ONCE
 #define __READ_ONCE(x)	(*(const volatile __unqual_scalar_typeof(x) *)&(x))
 #endif
 
-#define __READ_ONCE_SCALAR(x)						\
-({									\
-	__unqual_scalar_typeof(x) __x = __READ_ONCE(x);			\
-	smp_read_barrier_depends();					\
-	(typeof(x))__x;							\
-})
-
 #define READ_ONCE(x)							\
 ({									\
 	compiletime_assert_rwonce_type(x);				\
-	__READ_ONCE_SCALAR(x);						\
+	__READ_ONCE(x);							\
 })
 
 #define __WRITE_ONCE(x, val)						\
@@ -74,12 +66,9 @@ unsigned long __read_once_word_nocheck(const void *addr)
  */
 #define READ_ONCE_NOCHECK(x)						\
 ({									\
-	unsigned long __x;						\
-	compiletime_assert(sizeof(x) == sizeof(__x),			\
+	compiletime_assert(sizeof(x) == sizeof(unsigned long),		\
 		"Unsupported access size for READ_ONCE_NOCHECK().");	\
-	__x = __read_once_word_nocheck(&(x));				\
-	smp_read_barrier_depends();					\
-	(typeof(x))__x;							\
+	(typeof(x))__read_once_word_nocheck(&(x));			\
 })
 
 static __no_kasan_or_inline
-- 
2.27.0.212.ge8ba1cc988-goog