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Date:   Thu, 30 Aug 2018 16:43:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey.Brodkin@...opsys.com" <Alexey.Brodkin@...opsys.com>,
        "Vineet.Gupta1@...opsys.com" <Vineet.Gupta1@...opsys.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_"
 causes kernel crash

On Thu, Aug 30, 2018 at 04:29:20PM +0200, Peter Zijlstra wrote:

> Also, once it all works, they should look at switching to _relaxed
> atomics for LL/SC.

A little something like so.. should save a few smp_mb().

---

diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
index 4e0072730241..714b54c308b0 100644
--- a/arch/arc/include/asm/atomic.h
+++ b/arch/arc/include/asm/atomic.h
@@ -44,7 +44,7 @@ static inline void atomic_##op(int i, atomic_t *v)			\
 }									\
 
 #define ATOMIC_OP_RETURN(op, c_op, asm_op)				\
-static inline int atomic_##op##_return(int i, atomic_t *v)		\
+static inline int atomic_##op##_return_relaxed(int i, atomic_t *v)	\
 {									\
 	unsigned int val;						\
 									\
@@ -69,8 +69,11 @@ static inline int atomic_##op##_return(int i, atomic_t *v)		\
 	return val;							\
 }
 
+#define atomic_add_return_relaxed	atomic_add_return_relaxed
+#define atomic_sub_return_relaxed	atomic_sub_return_relaxed
+
 #define ATOMIC_FETCH_OP(op, c_op, asm_op)				\
-static inline int atomic_fetch_##op(int i, atomic_t *v)			\
+static inline int atomic_fetch_##op##_relaxed(int i, atomic_t *v)	\
 {									\
 	unsigned int val, orig;						\
 									\
@@ -96,6 +99,14 @@ static inline int atomic_fetch_##op(int i, atomic_t *v)			\
 	return orig;							\
 }
 
+#define atomic_fetch_add_relaxed	atomic_fetch_add_relaxed
+#define atomic_fetch_sub_relaxed	atomic_fetch_sub_relaxed
+
+#define atomic_fetch_and_relaxed	atomic_fetch_and_relaxed
+#define atomic_fetch_andnot_relaxed	atomic_fetch_andnot_relaxed
+#define atomic_fetch_or_relaxed		atomic_fetch_or_relaxed
+#define atomic_fetch_xor_relaxed	atomic_fetch_xor_relaxed
+
 #else	/* !CONFIG_ARC_HAS_LLSC */
 
 #ifndef CONFIG_SMP
@@ -379,7 +390,7 @@ static inline void atomic64_##op(long long a, atomic64_t *v)		\
 }									\
 
 #define ATOMIC64_OP_RETURN(op, op1, op2)		        	\
-static inline long long atomic64_##op##_return(long long a, atomic64_t *v)	\
+static inline long long atomic64_##op##_return_relaxed(long long a, atomic64_t *v)	\
 {									\
 	unsigned long long val;						\
 									\
@@ -401,8 +412,11 @@ static inline long long atomic64_##op##_return(long long a, atomic64_t *v)	\
 	return val;							\
 }
 
+#define atomic64_add_return_relaxed	atomic64_add_return_relaxed
+#define atomic64_sub_return_relaxed	atomic64_sub_return_relaxed
+
 #define ATOMIC64_FETCH_OP(op, op1, op2)		        		\
-static inline long long atomic64_fetch_##op(long long a, atomic64_t *v)	\
+static inline long long atomic64_fetch_##op##_relaxed(long long a, atomic64_t *v)	\
 {									\
 	unsigned long long val, orig;					\
 									\
@@ -424,6 +438,14 @@ static inline long long atomic64_fetch_##op(long long a, atomic64_t *v)	\
 	return orig;							\
 }
 
+#define atomic64_fetch_add_relaxed	atomic64_fetch_add_relaxed
+#define atomic64_fetch_sub_relaxed	atomic64_fetch_sub_relaxed
+
+#define atomic64_fetch_and_relaxed	atomic64_fetch_and_relaxed
+#define atomic64_fetch_andnot_relaxed	atomic64_fetch_andnot_relaxed
+#define atomic64_fetch_or_relaxed	atomic64_fetch_or_relaxed
+#define atomic64_fetch_xor_relaxed	atomic64_fetch_xor_relaxed
+
 #define ATOMIC64_OPS(op, op1, op2)					\
 	ATOMIC64_OP(op, op1, op2)					\
 	ATOMIC64_OP_RETURN(op, op1, op2)				\
@@ -434,6 +456,12 @@ static inline long long atomic64_fetch_##op(long long a, atomic64_t *v)	\
 
 ATOMIC64_OPS(add, add.f, adc)
 ATOMIC64_OPS(sub, sub.f, sbc)
+
+#undef ATOMIC64_OPS
+#define ATOMIC64_OPS(op, op1, op2)					\
+	ATOMIC64_OP(op, op1, op2)					\
+	ATOMIC64_FETCH_OP(op, op1, op2)
+
 ATOMIC64_OPS(and, and, and)
 ATOMIC64_OPS(andnot, bic, bic)
 ATOMIC64_OPS(or, or, or)

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