[PATCH v3 2/4] locking/atomic/x86: Introduce arch_atomic64_read_nonatomic to x86_32

[Uros Bizjak <ubizjak@gmail.com>]

Introduce arch_atomic64_read_nonatomic for 32-bit targets to load
the value from atomic64_t location in a non-atomic way. This
function is intended to be used in cases where a subsequent atomic
operation will handle the torn value, and can be used to prime the
first iteration of unconditional try_cmpxchg() loops.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
v2: New patch.
v3: Expand function comment.
---
 arch/x86/include/asm/atomic64_32.h | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/include/asm/atomic64_32.h b/arch/x86/include/asm/atomic64_32.h
index 11e817dab44a..bc76a88ae481 100644
--- a/arch/x86/include/asm/atomic64_32.h
+++ b/arch/x86/include/asm/atomic64_32.h
@@ -14,6 +14,32 @@ typedef struct {
 
 #define ATOMIC64_INIT(val)	{ (val) }
 
+/*
+ * Read an atomic64_t non-atomically.
+ *
+ * This is intended to be used in cases where a subsequent atomic operation
+ * will handle the torn value, and can be used to prime the first iteration
+ * of unconditional try_cmpxchg() loops, e.g.:
+ *
+ * 	s64 val = arch_atomic64_read_nonatomic(v);
+ * 	do { } while (!arch_atomic64_try_cmpxchg(v, &val, val OP i);
+ *
+ * This is NOT safe to use where the value is not always checked by a
+ * subsequent atomic operation, such as in conditional try_cmpxchg() loops
+ * that can break before the atomic operation, e.g.:
+ *
+ * 	s64 val = arch_atomic64_read_nonatomic(v);
+ * 	do {
+ * 		if (condition(val))
+ * 			break;
+ * 	} while (!arch_atomic64_try_cmpxchg(v, &val, val OP i);
+ */
+static __always_inline s64 arch_atomic64_read_nonatomic(const atomic64_t *v)
+{
+	/* See comment in arch_atomic_read(). */
+	return __READ_ONCE(v->counter);
+}
+
 #define __ATOMIC64_DECL(sym) void atomic64_##sym(atomic64_t *, ...)
 #ifndef ATOMIC64_EXPORT
 #define ATOMIC64_DECL_ONE __ATOMIC64_DECL
-- 
2.44.0