linux-kernel - [RFC PATCH 7/8] locking/barriers: Use '__unqual_scalar

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Message-Id: <20200110165636.28035-8-will@kernel.org>
Date:   Fri, 10 Jan 2020 16:56:35 +0000
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     linux-arch@...r.kernel.org, kernel-team@...roid.com,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: [RFC PATCH 7/8] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros

Passing volatile-qualified pointers to the asm-generic implementations of
the load-acquire macros results in a re-load from the stack due to the
temporary result variable inheriting the volatile semantics thanks to the
use of 'typeof()'.

Define these temporary variables using 'unqual_scalar_typeof' to drop
the volatile qualifier in the case that they are scalar types.

Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Will Deacon <will@...nel.org>
---
 include/asm-generic/barrier.h | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index 85b28eb80b11..2eacaf7d62f6 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -128,10 +128,10 @@ do {									\
 #ifndef __smp_load_acquire
 #define __smp_load_acquire(p)						\
 ({									\
-	typeof(*p) ___p1 = READ_ONCE(*p);				\
+	__unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p);		\
 	compiletime_assert_atomic_type(*p);				\
 	__smp_mb();							\
-	___p1;								\
+	(typeof(*p))___p1;						\
 })
 #endif
 
@@ -183,10 +183,10 @@ do {									\
 #ifndef smp_load_acquire
 #define smp_load_acquire(p)						\
 ({									\
-	typeof(*p) ___p1 = READ_ONCE(*p);				\
+	__unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p);		\
 	compiletime_assert_atomic_type(*p);				\
 	barrier();							\
-	___p1;								\
+	(typeof(*p))___p1;						\
 })
 #endif
 
@@ -229,14 +229,14 @@ do {									\
 #ifndef smp_cond_load_relaxed
 #define smp_cond_load_relaxed(ptr, cond_expr) ({		\
 	typeof(ptr) __PTR = (ptr);				\
-	typeof(*ptr) VAL;					\
+	__unqual_scalar_typeof(*ptr) VAL;			\
 	for (;;) {						\
 		VAL = READ_ONCE(*__PTR);			\
 		if (cond_expr)					\
 			break;					\
 		cpu_relax();					\
 	}							\
-	VAL;							\
+	(typeof(*ptr))VAL;					\
 })
 #endif
 
@@ -250,10 +250,10 @@ do {									\
  */
 #ifndef smp_cond_load_acquire
 #define smp_cond_load_acquire(ptr, cond_expr) ({		\
-	typeof(*ptr) _val;					\
+	__unqual_scalar_typeof(*ptr) _val;			\
 	_val = smp_cond_load_relaxed(ptr, cond_expr);		\
 	smp_acquire__after_ctrl_dep();				\
-	_val;							\
+	(typeof(*ptr))_val;					\
 })
 #endif
 
-- 
2.25.0.rc1.283.g88dfdc4193-goog