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

Add three (generic) mutex fastpath implementations.

The mutex-xchg.h implementation is atomic_xchg() based, and should
work fine on every architecture.

The mutex-dec.h implementation is atomic_dec_return() based - this
one too should work on every architecture, but might not perform the
most optimally on architectures that have no atomic-dec/inc instructions.

The mutex-null.h implementation forces all calls into the slowpath. This
is used for mutex debugging, but it can also be used on platforms that do
not want (or need) a fastpath at all.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arjan van de Ven <arjan@infradead.org>

1 files changed, 117 insertions, 0 deletions

diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
new file mode 100644
index 000000000000..1d24f47e6c48
--- /dev/null
+++ b/include/asm-generic/mutex-xchg.h
@@ -0,0 +1,117 @@
+/*
+ * asm-generic/mutex-xchg.h
+ *
+ * Generic implementation of the mutex fastpath, based on xchg().
+ *
+ * NOTE: An xchg based implementation is less optimal than an atomic
+ *       decrement/increment based implementation. If your architecture
+ *       has a reasonable atomic dec/inc then you should probably use
+ *       asm-generic/mutex-dec.h instead, or you could open-code an
+ *       optimized version in asm/mutex.h.
+ */
+#ifndef _ASM_GENERIC_MUTEX_XCHG_H
+#define _ASM_GENERIC_MUTEX_XCHG_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_xchg(count, 0) != 1))                       \
+                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_xchg(count, 0) != 1))
+                return fail_fn(count);
+        else {
+                smp_mb();
+                return 0;
+        }
+}
+
+/**
+ *  __mutex_fastpath_unlock - try to promote the mutex 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 mutex from 0 to 1. if it wasn't 0, call <function>
+ * In the failure case, this function is allowed to either set the value to
+ * 1, or to set it to a value lower than one.
+ * If the implementation sets it to a value of lower than one, 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_xchg(count, 1) != 0))                       \
+                fail_fn(count);                                         \
+} while (0)
+
+#define __mutex_slowpath_needs_to_unlock()              0
+
+/**
+ * __mutex_fastpath_trylock - try to acquire the mutex, without waiting
+ *
+ *  @count: pointer of type atomic_t
+ *  @fail_fn: spinlock based trylock implementation
+ *
+ * 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 *))
+{
+        int prev = atomic_xchg(count, 0);
+
+        if (unlikely(prev < 0)) {
+                /*
+                 * The lock was marked contended so we must restore that
+                 * state. If while doing so we get back a prev value of 1
+                 * then we just own it.
+                 *
+                 * [ In the rare case of the mutex going to 1, to 0, to -1
+                 *   and then back to 0 in this few-instructions window,
+                 *   this has the potential to trigger the slowpath for the
+                 *   owner's unlock path needlessly, but that's not a problem
+                 *   in practice. ]
+                 */
+                prev = atomic_xchg(count, prev);
+                if (prev < 0)
+                        prev = 0;
+        }
+        smp_mb();
+
+        return prev;
+}
+
+#endif