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Date:   Mon, 12 Mar 2018 05:17:09 -0700
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To:     linux-tip-commits@vger.kernel.org
Subject: [tip:locking/core] locking/xchg/alpha: Remove superfluous memory
 barriers from the _local() variants
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Commit-ID:  fbfcd0199170984bd3c2812e49ed0fe7b226959a
Gitweb:     https://git.kernel.org/tip/fbfcd0199170984bd3c2812e49ed0fe7b226959a
Author:     Andrea Parri <parri.andrea@gmail.com>
AuthorDate: Tue, 27 Feb 2018 05:00:58 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 12 Mar 2018 10:59:03 +0100

locking/xchg/alpha: Remove superfluous memory barriers from the _local() variants

The following two commits:

  79d442461df74 ("locking/xchg/alpha: Clean up barrier usage by using smp_mb() in place of __ASM__MB")
  472e8c55cf662 ("locking/xchg/alpha: Fix xchg() and cmpxchg() memory ordering bugs")

... ended up adding unnecessary barriers to the _local() variants on Alpha,
which the previous code took care to avoid.

Fix them by adding the smp_mb() into the cmpxchg() macro rather than into the
____cmpxchg() variants.

Reported-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-alpha@vger.kernel.org
Fixes: 472e8c55cf662 ("locking/xchg/alpha: Fix xchg() and cmpxchg() memory ordering bugs")
Fixes: 79d442461df74 ("locking/xchg/alpha: Clean up barrier usage by using smp_mb() in place of __ASM__MB")
Link: http://lkml.kernel.org/r/1519704058-13430-1-git-send-email-parri.andrea@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/alpha/include/asm/cmpxchg.h | 20 ++++++++++++++++----
 arch/alpha/include/asm/xchg.h    | 27 ---------------------------
 2 files changed, 16 insertions(+), 31 deletions(-)

diff --git a/arch/alpha/include/asm/cmpxchg.h b/arch/alpha/include/asm/cmpxchg.h
index 8a2b331e43fe..6c7c39452471 100644
--- a/arch/alpha/include/asm/cmpxchg.h
+++ b/arch/alpha/include/asm/cmpxchg.h
@@ -38,19 +38,31 @@
 #define ____cmpxchg(type, args...)	__cmpxchg ##type(args)
 #include <asm/xchg.h>
 
+/*
+ * The leading and the trailing memory barriers guarantee that these
+ * operations are fully ordered.
+ */
 #define xchg(ptr, x)							\
 ({									\
+	__typeof__(*(ptr)) __ret;					\
 	__typeof__(*(ptr)) _x_ = (x);					\
-	(__typeof__(*(ptr))) __xchg((ptr), (unsigned long)_x_,		\
-				 sizeof(*(ptr)));			\
+	smp_mb();							\
+	__ret = (__typeof__(*(ptr)))					\
+		__xchg((ptr), (unsigned long)_x_, sizeof(*(ptr)));	\
+	smp_mb();							\
+	__ret;								\
 })
 
 #define cmpxchg(ptr, o, n)						\
 ({									\
+	__typeof__(*(ptr)) __ret;					\
 	__typeof__(*(ptr)) _o_ = (o);					\
 	__typeof__(*(ptr)) _n_ = (n);					\
-	(__typeof__(*(ptr))) __cmpxchg((ptr), (unsigned long)_o_,	\
-				    (unsigned long)_n_,	sizeof(*(ptr)));\
+	smp_mb();							\
+	__ret = (__typeof__(*(ptr))) __cmpxchg((ptr),			\
+		(unsigned long)_o_, (unsigned long)_n_, sizeof(*(ptr)));\
+	smp_mb();							\
+	__ret;								\
 })
 
 #define cmpxchg64(ptr, o, n)						\
diff --git a/arch/alpha/include/asm/xchg.h b/arch/alpha/include/asm/xchg.h
index e2b59fac5257..7adb80c6746a 100644
--- a/arch/alpha/include/asm/xchg.h
+++ b/arch/alpha/include/asm/xchg.h
@@ -12,10 +12,6 @@
  * Atomic exchange.
  * Since it can be used to implement critical sections
  * it must clobber "memory" (also for interrupts in UP).
- *
- * The leading and the trailing memory barriers guarantee that these
- * operations are fully ordered.
- *
  */
 
 static inline unsigned long
@@ -23,7 +19,6 @@ ____xchg(_u8, volatile char *m, unsigned long val)
 {
 	unsigned long ret, tmp, addr64;
 
-	smp_mb();
 	__asm__ __volatile__(
 	"	andnot	%4,7,%3\n"
 	"	insbl	%1,%4,%1\n"
@@ -38,7 +33,6 @@ ____xchg(_u8, volatile char *m, unsigned long val)
 	".previous"
 	: "=&r" (ret), "=&r" (val), "=&r" (tmp), "=&r" (addr64)
 	: "r" ((long)m), "1" (val) : "memory");
-	smp_mb();
 
 	return ret;
 }
@@ -48,7 +42,6 @@ ____xchg(_u16, volatile short *m, unsigned long val)
 {
 	unsigned long ret, tmp, addr64;
 
-	smp_mb();
 	__asm__ __volatile__(
 	"	andnot	%4,7,%3\n"
 	"	inswl	%1,%4,%1\n"
@@ -63,7 +56,6 @@ ____xchg(_u16, volatile short *m, unsigned long val)
 	".previous"
 	: "=&r" (ret), "=&r" (val), "=&r" (tmp), "=&r" (addr64)
 	: "r" ((long)m), "1" (val) : "memory");
-	smp_mb();
 
 	return ret;
 }
@@ -73,7 +65,6 @@ ____xchg(_u32, volatile int *m, unsigned long val)
 {
 	unsigned long dummy;
 
-	smp_mb();
 	__asm__ __volatile__(
 	"1:	ldl_l %0,%4\n"
 	"	bis $31,%3,%1\n"
@@ -84,7 +75,6 @@ ____xchg(_u32, volatile int *m, unsigned long val)
 	".previous"
 	: "=&r" (val), "=&r" (dummy), "=m" (*m)
 	: "rI" (val), "m" (*m) : "memory");
-	smp_mb();
 
 	return val;
 }
@@ -94,7 +84,6 @@ ____xchg(_u64, volatile long *m, unsigned long val)
 {
 	unsigned long dummy;
 
-	smp_mb();
 	__asm__ __volatile__(
 	"1:	ldq_l %0,%4\n"
 	"	bis $31,%3,%1\n"
@@ -105,7 +94,6 @@ ____xchg(_u64, volatile long *m, unsigned long val)
 	".previous"
 	: "=&r" (val), "=&r" (dummy), "=m" (*m)
 	: "rI" (val), "m" (*m) : "memory");
-	smp_mb();
 
 	return val;
 }
@@ -135,13 +123,6 @@ ____xchg(, volatile void *ptr, unsigned long x, int size)
  * Atomic compare and exchange.  Compare OLD with MEM, if identical,
  * store NEW in MEM.  Return the initial value in MEM.  Success is
  * indicated by comparing RETURN with OLD.
- *
- * The leading and the trailing memory barriers guarantee that these
- * operations are fully ordered.
- *
- * The trailing memory barrier is placed in SMP unconditionally, in
- * order to guarantee that dependency ordering is preserved when a
- * dependency is headed by an unsuccessful operation.
  */
 
 static inline unsigned long
@@ -149,7 +130,6 @@ ____cmpxchg(_u8, volatile char *m, unsigned char old, unsigned char new)
 {
 	unsigned long prev, tmp, cmp, addr64;
 
-	smp_mb();
 	__asm__ __volatile__(
 	"	andnot	%5,7,%4\n"
 	"	insbl	%1,%5,%1\n"
@@ -167,7 +147,6 @@ ____cmpxchg(_u8, volatile char *m, unsigned char old, unsigned char new)
 	".previous"
 	: "=&r" (prev), "=&r" (new), "=&r" (tmp), "=&r" (cmp), "=&r" (addr64)
 	: "r" ((long)m), "Ir" (old), "1" (new) : "memory");
-	smp_mb();
 
 	return prev;
 }
@@ -177,7 +156,6 @@ ____cmpxchg(_u16, volatile short *m, unsigned short old, unsigned short new)
 {
 	unsigned long prev, tmp, cmp, addr64;
 
-	smp_mb();
 	__asm__ __volatile__(
 	"	andnot	%5,7,%4\n"
 	"	inswl	%1,%5,%1\n"
@@ -195,7 +173,6 @@ ____cmpxchg(_u16, volatile short *m, unsigned short old, unsigned short new)
 	".previous"
 	: "=&r" (prev), "=&r" (new), "=&r" (tmp), "=&r" (cmp), "=&r" (addr64)
 	: "r" ((long)m), "Ir" (old), "1" (new) : "memory");
-	smp_mb();
 
 	return prev;
 }
@@ -205,7 +182,6 @@ ____cmpxchg(_u32, volatile int *m, int old, int new)
 {
 	unsigned long prev, cmp;
 
-	smp_mb();
 	__asm__ __volatile__(
 	"1:	ldl_l %0,%5\n"
 	"	cmpeq %0,%3,%1\n"
@@ -219,7 +195,6 @@ ____cmpxchg(_u32, volatile int *m, int old, int new)
 	".previous"
 	: "=&r"(prev), "=&r"(cmp), "=m"(*m)
 	: "r"((long) old), "r"(new), "m"(*m) : "memory");
-	smp_mb();
 
 	return prev;
 }
@@ -229,7 +204,6 @@ ____cmpxchg(_u64, volatile long *m, unsigned long old, unsigned long new)
 {
 	unsigned long prev, cmp;
 
-	smp_mb();
 	__asm__ __volatile__(
 	"1:	ldq_l %0,%5\n"
 	"	cmpeq %0,%3,%1\n"
@@ -243,7 +217,6 @@ ____cmpxchg(_u64, volatile long *m, unsigned long old, unsigned long new)
 	".previous"
 	: "=&r"(prev), "=&r"(cmp), "=m"(*m)
 	: "r"((long) old), "r"(new), "m"(*m) : "memory");
-	smp_mb();
 
 	return prev;
 }