RISC-V: Atomic and Locking Code

This contains all the code that directly interfaces with the RISC-V
memory model.  While this code corforms to the current RISC-V ISA
specifications (user 2.2 and priv 1.10), the memory model is somewhat
underspecified in those documents.  There is a working group that hopes
to produce a formal memory model by the end of the year, but my
understanding is that the basic definitions we're relying on here won't
change significantly.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

arch/riscv/include/asm/atomic.h

@@ -0,0 +1,375 @@
+/*
+ * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved.
+ * Copyright (C) 2012 Regents of the University of California
+ * Copyright (C) 2017 SiFive
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public Licence
+ * as published by the Free Software Foundation; either version
+ * 2 of the Licence, or (at your option) any later version.
+ */
+
+#ifndef _ASM_RISCV_ATOMIC_H
+#define _ASM_RISCV_ATOMIC_H
+
+#ifdef CONFIG_GENERIC_ATOMIC64
+# include <asm-generic/atomic64.h>
+#else
+# if (__riscv_xlen < 64)
+#  error "64-bit atomics require XLEN to be at least 64"
+# endif
+#endif
+
+#include <asm/cmpxchg.h>
+#include <asm/barrier.h>
+
+#define ATOMIC_INIT(i)	{ (i) }
+static __always_inline int atomic_read(const atomic_t *v)
+{
+	return READ_ONCE(v->counter);
+}
+static __always_inline void atomic_set(atomic_t *v, int i)
+{
+	WRITE_ONCE(v->counter, i);
+}
+
+#ifndef CONFIG_GENERIC_ATOMIC64
+#define ATOMIC64_INIT(i) { (i) }
+static __always_inline long atomic64_read(const atomic64_t *v)
+{
+	return READ_ONCE(v->counter);
+}
+static __always_inline void atomic64_set(atomic64_t *v, long i)
+{
+	WRITE_ONCE(v->counter, i);
+}
+#endif
+
+/*
+ * First, the atomic ops that have no ordering constraints and therefor don't
+ * have the AQ or RL bits set.  These don't return anything, so there's only
+ * one version to worry about.
+ */
+#define ATOMIC_OP(op, asm_op, c_op, I, asm_type, c_type, prefix)				\
+static __always_inline void atomic##prefix##_##op(c_type i, atomic##prefix##_t *v)		\
+{												\
+	__asm__ __volatile__ (									\
+		"amo" #asm_op "." #asm_type " zero, %1, %0"					\
+		: "+A" (v->counter)								\
+		: "r" (I)									\
+		: "memory");									\
+}
+
+#ifdef CONFIG_GENERIC_ATOMIC64
+#define ATOMIC_OPS(op, asm_op, c_op, I)			\
+        ATOMIC_OP (op, asm_op, c_op, I, w,  int,   )
+#else
+#define ATOMIC_OPS(op, asm_op, c_op, I)			\
+        ATOMIC_OP (op, asm_op, c_op, I, w,  int,   )	\
+        ATOMIC_OP (op, asm_op, c_op, I, d, long, 64)
+#endif
+
+ATOMIC_OPS(add, add, +,  i)
+ATOMIC_OPS(sub, add, +, -i)
+ATOMIC_OPS(and, and, &,  i)
+ATOMIC_OPS( or,  or, |,  i)
+ATOMIC_OPS(xor, xor, ^,  i)
+
+#undef ATOMIC_OP
+#undef ATOMIC_OPS
+
+/*
+ * Atomic ops that have ordered, relaxed, acquire, and relese variants.
+ * There's two flavors of these: the arithmatic ops have both fetch and return
+ * versions, while the logical ops only have fetch versions.
+ */
+#define ATOMIC_FETCH_OP(op, asm_op, c_op, I, asm_or, c_or, asm_type, c_type, prefix)			\
+static __always_inline c_type atomic##prefix##_fetch_##op##c_or(c_type i, atomic##prefix##_t *v)	\
+{													\
+	register c_type ret;										\
+	__asm__ __volatile__ (										\
+		"amo" #asm_op "." #asm_type #asm_or " %1, %2, %0"					\
+		: "+A" (v->counter), "=r" (ret)								\
+		: "r" (I)										\
+		: "memory");										\
+	return ret;											\
+}
+
+#define ATOMIC_OP_RETURN(op, asm_op, c_op, I, asm_or, c_or, asm_type, c_type, prefix)			\
+static __always_inline c_type atomic##prefix##_##op##_return##c_or(c_type i, atomic##prefix##_t *v)	\
+{													\
+        return atomic##prefix##_fetch_##op##c_or(i, v) c_op I;						\
+}
+
+#ifdef CONFIG_GENERIC_ATOMIC64
+#define ATOMIC_OPS(op, asm_op, c_op, I, asm_or, c_or)				\
+        ATOMIC_FETCH_OP (op, asm_op, c_op, I, asm_or, c_or, w,  int,   )	\
+        ATOMIC_OP_RETURN(op, asm_op, c_op, I, asm_or, c_or, w,  int,   )
+#else
+#define ATOMIC_OPS(op, asm_op, c_op, I, asm_or, c_or)				\
+        ATOMIC_FETCH_OP (op, asm_op, c_op, I, asm_or, c_or, w,  int,   )	\
+        ATOMIC_OP_RETURN(op, asm_op, c_op, I, asm_or, c_or, w,  int,   )	\
+        ATOMIC_FETCH_OP (op, asm_op, c_op, I, asm_or, c_or, d, long, 64)	\
+        ATOMIC_OP_RETURN(op, asm_op, c_op, I, asm_or, c_or, d, long, 64)
+#endif
+
+ATOMIC_OPS(add, add, +,  i,      , _relaxed)
+ATOMIC_OPS(add, add, +,  i, .aq  , _acquire)
+ATOMIC_OPS(add, add, +,  i, .rl  , _release)
+ATOMIC_OPS(add, add, +,  i, .aqrl,         )
+
+ATOMIC_OPS(sub, add, +, -i,      , _relaxed)
+ATOMIC_OPS(sub, add, +, -i, .aq  , _acquire)
+ATOMIC_OPS(sub, add, +, -i, .rl  , _release)
+ATOMIC_OPS(sub, add, +, -i, .aqrl,         )
+
+#undef ATOMIC_OPS
+
+#ifdef CONFIG_GENERIC_ATOMIC64
+#define ATOMIC_OPS(op, asm_op, c_op, I, asm_or, c_or)				\
+        ATOMIC_FETCH_OP(op, asm_op, c_op, I, asm_or, c_or, w,  int,   )
+#else
+#define ATOMIC_OPS(op, asm_op, c_op, I, asm_or, c_or)				\
+        ATOMIC_FETCH_OP(op, asm_op, c_op, I, asm_or, c_or, w,  int,   )		\
+        ATOMIC_FETCH_OP(op, asm_op, c_op, I, asm_or, c_or, d, long, 64)
+#endif
+
+ATOMIC_OPS(and, and, &,  i,      , _relaxed)
+ATOMIC_OPS(and, and, &,  i, .aq  , _acquire)
+ATOMIC_OPS(and, and, &,  i, .rl  , _release)
+ATOMIC_OPS(and, and, &,  i, .aqrl,         )
+
+ATOMIC_OPS( or,  or, |,  i,      , _relaxed)
+ATOMIC_OPS( or,  or, |,  i, .aq  , _acquire)
+ATOMIC_OPS( or,  or, |,  i, .rl  , _release)
+ATOMIC_OPS( or,  or, |,  i, .aqrl,         )
+
+ATOMIC_OPS(xor, xor, ^,  i,      , _relaxed)
+ATOMIC_OPS(xor, xor, ^,  i, .aq  , _acquire)
+ATOMIC_OPS(xor, xor, ^,  i, .rl  , _release)
+ATOMIC_OPS(xor, xor, ^,  i, .aqrl,         )
+
+#undef ATOMIC_OPS
+
+#undef ATOMIC_FETCH_OP
+#undef ATOMIC_OP_RETURN
+
+/*
+ * The extra atomic operations that are constructed from one of the core
+ * AMO-based operations above (aside from sub, which is easier to fit above).
+ * These are required to perform a barrier, but they're OK this way because
+ * atomic_*_return is also required to perform a barrier.
+ */
+#define ATOMIC_OP(op, func_op, comp_op, I, c_type, prefix)			\
+static __always_inline bool atomic##prefix##_##op(c_type i, atomic##prefix##_t *v) \
+{										\
+	return atomic##prefix##_##func_op##_return(i, v) comp_op I;		\
+}
+
+#ifdef CONFIG_GENERIC_ATOMIC64
+#define ATOMIC_OPS(op, func_op, comp_op, I)			\
+        ATOMIC_OP (op, func_op, comp_op, I,  int,   )
+#else
+#define ATOMIC_OPS(op, func_op, comp_op, I)			\
+        ATOMIC_OP (op, func_op, comp_op, I,  int,   )		\
+        ATOMIC_OP (op, func_op, comp_op, I, long, 64)
+#endif
+
+ATOMIC_OPS(add_and_test, add, ==, 0)
+ATOMIC_OPS(sub_and_test, sub, ==, 0)
+ATOMIC_OPS(add_negative, add,  <, 0)
+
+#undef ATOMIC_OP
+#undef ATOMIC_OPS
+
+#define ATOMIC_OP(op, func_op, c_op, I, c_type, prefix)				\
+static __always_inline void atomic##prefix##_##op(atomic##prefix##_t *v)	\
+{										\
+	atomic##prefix##_##func_op(I, v);					\
+}
+
+#define ATOMIC_FETCH_OP(op, func_op, c_op, I, c_type, prefix)				\
+static __always_inline c_type atomic##prefix##_fetch_##op(atomic##prefix##_t *v)	\
+{											\
+	return atomic##prefix##_fetch_##func_op(I, v);					\
+}
+
+#define ATOMIC_OP_RETURN(op, asm_op, c_op, I, c_type, prefix)				\
+static __always_inline c_type atomic##prefix##_##op##_return(atomic##prefix##_t *v)	\
+{											\
+        return atomic##prefix##_fetch_##op(v) c_op I;					\
+}
+
+#ifdef CONFIG_GENERIC_ATOMIC64
+#define ATOMIC_OPS(op, asm_op, c_op, I)						\
+        ATOMIC_OP       (op, asm_op, c_op, I,  int,   )				\
+        ATOMIC_FETCH_OP (op, asm_op, c_op, I,  int,   )				\
+        ATOMIC_OP_RETURN(op, asm_op, c_op, I,  int,   )
+#else
+#define ATOMIC_OPS(op, asm_op, c_op, I)						\
+        ATOMIC_OP       (op, asm_op, c_op, I,  int,   )				\
+        ATOMIC_FETCH_OP (op, asm_op, c_op, I,  int,   )				\
+        ATOMIC_OP_RETURN(op, asm_op, c_op, I,  int,   )				\
+        ATOMIC_OP       (op, asm_op, c_op, I, long, 64)				\
+        ATOMIC_FETCH_OP (op, asm_op, c_op, I, long, 64)				\
+        ATOMIC_OP_RETURN(op, asm_op, c_op, I, long, 64)
+#endif
+
+ATOMIC_OPS(inc, add, +,  1)
+ATOMIC_OPS(dec, add, +, -1)
+
+#undef ATOMIC_OPS
+#undef ATOMIC_OP
+#undef ATOMIC_FETCH_OP
+#undef ATOMIC_OP_RETURN
+
+#define ATOMIC_OP(op, func_op, comp_op, I, prefix)				\
+static __always_inline bool atomic##prefix##_##op(atomic##prefix##_t *v)	\
+{										\
+	return atomic##prefix##_##func_op##_return(v) comp_op I;		\
+}
+
+ATOMIC_OP(inc_and_test, inc, ==, 0,   )
+ATOMIC_OP(dec_and_test, dec, ==, 0,   )
+#ifndef CONFIG_GENERIC_ATOMIC64
+ATOMIC_OP(inc_and_test, inc, ==, 0, 64)
+ATOMIC_OP(dec_and_test, dec, ==, 0, 64)
+#endif
+
+#undef ATOMIC_OP
+
+/* This is required to provide a barrier on success. */
+static __always_inline int __atomic_add_unless(atomic_t *v, int a, int u)
+{
+       int prev, rc;
+
+	__asm__ __volatile__ (
+		"0:\n\t"
+		"lr.w.aqrl  %[p],  %[c]\n\t"
+		"beq        %[p],  %[u], 1f\n\t"
+		"add       %[rc],  %[p], %[a]\n\t"
+		"sc.w.aqrl %[rc], %[rc], %[c]\n\t"
+		"bnez      %[rc], 0b\n\t"
+		"1:"
+		: [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
+		: [a]"r" (a), [u]"r" (u)
+		: "memory");
+	return prev;
+}
+
+#ifndef CONFIG_GENERIC_ATOMIC64
+static __always_inline long __atomic64_add_unless(atomic64_t *v, long a, long u)
+{
+       long prev, rc;
+
+	__asm__ __volatile__ (
+		"0:\n\t"
+		"lr.d.aqrl  %[p],  %[c]\n\t"
+		"beq        %[p],  %[u], 1f\n\t"
+		"add       %[rc],  %[p], %[a]\n\t"
+		"sc.d.aqrl %[rc], %[rc], %[c]\n\t"
+		"bnez      %[rc], 0b\n\t"
+		"1:"
+		: [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
+		: [a]"r" (a), [u]"r" (u)
+		: "memory");
+	return prev;
+}
+
+static __always_inline int atomic64_add_unless(atomic64_t *v, long a, long u)
+{
+	return __atomic64_add_unless(v, a, u) != u;
+}
+#endif
+
+/*
+ * The extra atomic operations that are constructed from one of the core
+ * LR/SC-based operations above.
+ */
+static __always_inline int atomic_inc_not_zero(atomic_t *v)
+{
+        return __atomic_add_unless(v, 1, 0);
+}
+
+#ifndef CONFIG_GENERIC_ATOMIC64
+static __always_inline long atomic64_inc_not_zero(atomic64_t *v)
+{
+        return atomic64_add_unless(v, 1, 0);
+}
+#endif
+
+/*
+ * atomic_{cmp,}xchg is required to have exactly the same ordering semantics as
+ * {cmp,}xchg and the operations that return, so they need a barrier.  We just
+ * use the other implementations directly.
+ */
+#define ATOMIC_OP(c_t, prefix, c_or, size, asm_or)						\
+static __always_inline c_t atomic##prefix##_cmpxchg##c_or(atomic##prefix##_t *v, c_t o, c_t n) 	\
+{												\
+	return __cmpxchg(&(v->counter), o, n, size, asm_or, asm_or);				\
+}												\
+static __always_inline c_t atomic##prefix##_xchg##c_or(atomic##prefix##_t *v, c_t n) 		\
+{												\
+	return __xchg(n, &(v->counter), size, asm_or);						\
+}
+
+#ifdef CONFIG_GENERIC_ATOMIC64
+#define ATOMIC_OPS(c_or, asm_or)			\
+	ATOMIC_OP( int,   , c_or, 4, asm_or)
+#else
+#define ATOMIC_OPS(c_or, asm_or)			\
+	ATOMIC_OP( int,   , c_or, 4, asm_or)		\
+	ATOMIC_OP(long, 64, c_or, 8, asm_or)
+#endif
+
+ATOMIC_OPS(        , .aqrl)
+ATOMIC_OPS(_acquire,   .aq)
+ATOMIC_OPS(_release,   .rl)
+ATOMIC_OPS(_relaxed,      )
+
+#undef ATOMIC_OPS
+#undef ATOMIC_OP
+
+static __always_inline int atomic_sub_if_positive(atomic_t *v, int offset)
+{
+       int prev, rc;
+
+	__asm__ __volatile__ (
+		"0:\n\t"
+		"lr.w.aqrl  %[p],  %[c]\n\t"
+		"sub       %[rc],  %[p], %[o]\n\t"
+		"bltz      %[rc],    1f\n\t"
+		"sc.w.aqrl %[rc], %[rc], %[c]\n\t"
+		"bnez      %[rc],    0b\n\t"
+		"1:"
+		: [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
+		: [o]"r" (offset)
+		: "memory");
+	return prev - offset;
+}
+
+#define atomic_dec_if_positive(v)	atomic_sub_if_positive(v, 1)
+
+#ifndef CONFIG_GENERIC_ATOMIC64
+static __always_inline long atomic64_sub_if_positive(atomic64_t *v, int offset)
+{
+       long prev, rc;
+
+	__asm__ __volatile__ (
+		"0:\n\t"
+		"lr.d.aqrl  %[p],  %[c]\n\t"
+		"sub       %[rc],  %[p], %[o]\n\t"
+		"bltz      %[rc],    1f\n\t"
+		"sc.d.aqrl %[rc], %[rc], %[c]\n\t"
+		"bnez      %[rc],    0b\n\t"
+		"1:"
+		: [p]"=&r" (prev), [rc]"=&r" (rc), [c]"+A" (v->counter)
+		: [o]"r" (offset)
+		: "memory");
+	return prev - offset;
+}
+
+#define atomic64_dec_if_positive(v)	atomic64_sub_if_positive(v, 1)
+#endif
+
+#endif /* _ASM_RISCV_ATOMIC_H */