mutex subsystem, add asm-generic/mutex-[dec|xchg|null].h implementations

Index: linux/include/asm-generic/mutex-dec.h
===================================================================
@@ -0,0 +1,66 @@
+/*
+ * asm-generic/mutex-dec.h
+ *
+ * Generic implementation of the mutex fastpath, based on atomic
+ * decrement/increment.
+ */
+#ifndef _ASM_GENERIC_MUTEX_DEC_H
+#define _ASM_GENERIC_MUTEX_DEC_H
+
+/**
+ *  __mutex_fastpath_lock - try to take the lock by moving the count
+ *                          from 1 to a 0 value
+ *  @count: pointer of type atomic_t
+ *  @fn: function to call if the original value was not 1
+ *
+ * Change the count from 1 to a value lower than 1, and call <fn> if it
+ * wasn't 1 originally. This function MUST leave the value lower than 1
+ * even when the "1" assertion wasn't true.
+ */
+#define __mutex_fastpath_lock(count, fn)				\
+do {									\
+	if (unlikely(atomic_dec_return(count) < 0))			\
+		fn(count);						\
+} while (0)
+
+/**
+ *  __mutex_fastpath_lock_retval - try to take the lock by moving the count
+ *                                 from 1 to a 0 value
+ *  @count: pointer of type atomic_t
+ *  @fn: function to call if the original value was not 1
+ *
+ * Change the count from 1 to a value lower than 1, and call <fn> if it
+ * wasn't 1 originally. This function returns 0 if the fastpath succeeds,
+ * or anything the slow path function returns.
+ */
+static inline int
+__mutex_fastpath_lock_retval(atomic_t *count, int (*fn)(atomic_t *))
+{
+	if (unlikely(atomic_dec_return(count) < 0))
+		return fn(count);
+	else
+		return 0;
+}
+
+/**
+ *  __mutex_fastpath_unlock - try to promote the count from 0 to 1
+ *  @count: pointer of type atomic_t
+ *  @fn: function to call if the original value was not 0
+ *
+ * Try to promote the count from 0 to 1. If it wasn't 0, call <fn>.
+ * In the failure case, this function is allowed to either set the value to
+ * 1, or to set it to a value lower than 1.
+ *
+ * If the implementation sets it to a value of lower than 1, then the
+ * __mutex_slowpath_needs_to_unlock() macro needs to return 1, it needs
+ * to return 0 otherwise.
+ */
+#define __mutex_fastpath_unlock(count, fn)				\
+do {									\
+	if (unlikely(atomic_inc_return(count) <= 0))			\
+		fn(count);						\
+} while (0)
+
+#define __mutex_slowpath_needs_to_unlock()		1
+
+#endif

Index: linux/include/asm-generic/mutex-null.h
===================================================================
@@ -0,0 +1,23 @@
+/*
+ * asm-generic/mutex-null.h
+ *
+ * Generic implementation of the mutex fastpath, based on NOP :-)
+ *
+ * This is used by the mutex-debugging infrastructure, but it can also
+ * be used by architectures that (for whatever reason) want to use the
+ * spinlock based slowpath.
+ */
+#ifndef _ASM_GENERIC_MUTEX_NULL_H
+#define _ASM_GENERIC_MUTEX_NULL_H
+
+/* extra parameter only needed for mutex debugging: */
+#ifndef __IP__
+# define __IP__
+#endif
+
+#define __mutex_fastpath_lock(count, fn_name)		fn_name(count __IP__)
+#define __mutex_fastpath_lock_retval(count, fn_name)	fn_name(count __IP__)
+#define __mutex_fastpath_unlock(count, fn_name)		fn_name(count __IP__)
+#define __mutex_slowpath_needs_to_unlock()		1
+
+#endif

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