1 files changed, 110 insertions, 0 deletions

diff --git a/include/asm-generic/mutex-dec.h b/include/asm-generic/mutex-dec.h
new file mode 100644
index 000000000000..74b18cda169f
--- /dev/null
+++ b/include/asm-generic/mutex-dec.h
@@ -0,0 +1,110 @@
+/*
+ * 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
+ *  @fail_fn: function to call if the original value was not 1
+ *
+ * Change the count from 1 to a value lower than 1, and call <fail_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, fail_fn)                           \
+do {                                                                    \
+        if (unlikely(atomic_dec_return(count) < 0))                     \
+                fail_fn(count);                                         \
+        else                                                            \
+                smp_mb();                                               \
+} 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
+ *  @fail_fn: function to call if the original value was not 1
+ *
+ * Change the count from 1 to a value lower than 1, and call <fail_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 (*fail_fn)(atomic_t *))
+{
+        if (unlikely(atomic_dec_return(count) < 0))
+                return fail_fn(count);
+        else {
+                smp_mb();
+                return 0;
+        }
+}
+
+/**
+ *  __mutex_fastpath_unlock - try to promote the count from 0 to 1
+ *  @count: pointer of type atomic_t
+ *  @fail_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 <fail_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, fail_fn)                         \
+do {                                                                    \
+        smp_mb();                                                       \
+        if (unlikely(atomic_inc_return(count) <= 0))                    \
+                fail_fn(count);                                         \
+} while (0)
+
+#define __mutex_slowpath_needs_to_unlock()              1
+
+/**
+ * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
+ *
+ *  @count: pointer of type atomic_t
+ *  @fail_fn: fallback function
+ *
+ * Change the count from 1 to a value lower than 1, and return 0 (failure)
+ * if it wasn't 1 originally, or return 1 (success) otherwise. This function
+ * MUST leave the value lower than 1 even when the "1" assertion wasn't true.
+ * Additionally, if the value was < 0 originally, this function must not leave
+ * it to 0 on failure.
+ *
+ * If the architecture has no effective trylock variant, it should call the
+ * <fail_fn> spinlock-based trylock variant unconditionally.
+ */
+static inline int
+__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
+{
+        /*
+         * We have two variants here. The cmpxchg based one is the best one
+         * because it never induce a false contention state.  It is included
+         * here because architectures using the inc/dec algorithms over the
+         * xchg ones are much more likely to support cmpxchg natively.
+         *
+         * If not we fall back to the spinlock based variant - that is
+         * just as efficient (and simpler) as a 'destructive' probing of
+         * the mutex state would be.
+         */
+#ifdef __HAVE_ARCH_CMPXCHG
+        if (likely(atomic_cmpxchg(count, 1, 0)) == 1) {
+                smp_mb();
+                return 1;
+        }
+        return 0;
+#else
+        return fail_fn(count);
+#endif
+}
+
+#endif