linux-kernel - [RFC PATCH 14/15] x86: Make the mutex implementation use ISO atomic ops

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Date:	Wed, 18 May 2016 16:12:22 +0100
From:	David Howells <dhowells@...hat.com>
To:	linux-arch@...r.kernel.org
Cc:	x86@...nel.org, will.deacon@....com, linux-kernel@...r.kernel.org,
	dhowells@...hat.com, ramana.radhakrishnan@....com,
	paulmck@...ux.vnet.ibm.com, dwmw2@...radead.org
Subject: [RFC PATCH 14/15] x86: Make the mutex implementation use ISO atomic
 ops

Make the x86 mutex implementation use ISO atomic ops rather than inline
assembly.

Signed-off-by: David Howells <dhowells@...hat.com>
---

 arch/x86/include/asm/mutex.h     |    6 +--
 arch/x86/include/asm/mutex_iso.h |   73 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+), 5 deletions(-)
 create mode 100644 arch/x86/include/asm/mutex_iso.h

diff --git a/arch/x86/include/asm/mutex.h b/arch/x86/include/asm/mutex.h
index 7d3a48275394..831bc5a73886 100644
--- a/arch/x86/include/asm/mutex.h
+++ b/arch/x86/include/asm/mutex.h
@@ -1,5 +1 @@
-#ifdef CONFIG_X86_32
-# include <asm/mutex_32.h>
-#else
-# include <asm/mutex_64.h>
-#endif
+#include <asm/mutex_iso.h>
diff --git a/arch/x86/include/asm/mutex_iso.h b/arch/x86/include/asm/mutex_iso.h
new file mode 100644
index 000000000000..ffdbdb411989
--- /dev/null
+++ b/arch/x86/include/asm/mutex_iso.h
@@ -0,0 +1,73 @@
+/* ISO atomics based mutex
+ *
+ * Copyright (C) 2016 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@...hat.com)
+ *
+ * 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_X86_MUTEX_ISO_H
+#define _ASM_X86_MUTEX_ISO_H
+
+/**
+ * __mutex_fastpath_lock - decrement and call function if negative
+ * @v: pointer of type atomic_t
+ * @fail_fn: function to call if the result is negative
+ *
+ * Atomically decrements @v and calls <fail_fn> if the result is negative.
+ */
+static inline void __mutex_fastpath_lock(atomic_t *v,
+					 void (*fail_fn)(atomic_t *))
+{
+	if (atomic_dec_return_acquire(v) < 0)
+		fail_fn(v);
+}
+
+/**
+ *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
+ *                                 from 1 to a 0 value
+ * @v: pointer of type atomic_t
+ *
+ * Change the count from 1 to a value lower than 1. This function returns 0
+ * if the fastpath succeeds, or -1 otherwise.
+ */
+static inline int __mutex_fastpath_lock_retval(atomic_t *v)
+{
+	return unlikely(atomic_dec_return(v) < 0) ? -1 : 0;
+}
+
+/**
+ * __mutex_fastpath_unlock - increment and call function if nonpositive
+ * @v: pointer of type atomic_t
+ * @fail_fn: function to call if the result is nonpositive
+ *
+ * Atomically increments @v and calls <fail_fn> if the result is nonpositive.
+ */
+static inline void __mutex_fastpath_unlock(atomic_t *v,
+					   void (*fail_fn)(atomic_t *))
+{
+	if (atomic_inc_return_release(v) <= 0)
+		fail_fn(v);
+}
+
+#define __mutex_slowpath_needs_to_unlock()	1
+
+/**
+ * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
+ *
+ *  @v: pointer of type atomic_t
+ *  @fail_fn: fallback function
+ *
+ * Change the count from 1 to 0 and return true (success), or return false
+ * (failure) if it wasn't 1 originally. [the fallback function is never used on
+ * x86_64, because all x86_64 CPUs have a CMPXCHG instruction.]
+ */
+static inline bool __mutex_fastpath_trylock(atomic_t *v,
+					    int (*fail_fn)(atomic_t *))
+{
+	return likely(atomic_try_cmpxchg(v, 1, 0));
+}
+
+#endif /* _ASM_X86_MUTEX_ISO_H */