[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>
diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
new file mode 100644
index 0000000..1d24f47
--- /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