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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
commit57f0f512b273f60d52568b8c6b77e17f5636edc0 (patch)
tree5e910f0e82173f4ef4f51111366a3f1299037a7b /include/asm-generic/mutex-xchg.h
Initial import
Diffstat (limited to 'include/asm-generic/mutex-xchg.h')
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diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
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+/*
+ * include/asm-generic/mutex-xchg.h
+ *
+ * Generic implementation of the mutex fastpath, based on xchg().
+ *
+ * NOTE: An xchg based implementation might be 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.
+ */
+static inline void
+__mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
+{
+ if (unlikely(atomic_xchg(count, 0) != 1))
+ /*
+ * We failed to acquire the lock, so mark it contended
+ * to ensure that any waiting tasks are woken up by the
+ * unlock slow path.
+ */
+ if (likely(atomic_xchg(count, -1) != 1))
+ fail_fn(count);
+}
+
+/**
+ * __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
+ *
+ * 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 *count)
+{
+ if (unlikely(atomic_xchg(count, 0) != 1))
+ if (likely(atomic_xchg(count, -1) != 1))
+ return -1;
+ 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.
+ */
+static inline void
+__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
+{
+ if (unlikely(atomic_xchg(count, 1) != 0))
+ fail_fn(count);
+}
+
+#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;
+ }
+
+ return prev;
+}
+
+#endif