1 files changed, 77 insertions, 0 deletions

diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
new file mode 100644
index 000000000000..9578ef81940b
--- /dev/null
+++ b/include/linux/mcs_spinlock.h
@@ -0,0 +1,77 @@
+/*
+ * MCS lock defines
+ *
+ * This file contains the main data structure and API definitions of MCS lock.
+ *
+ * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
+ * with the desirable properties of being fair, and with each cpu trying
+ * to acquire the lock spinning on a local variable.
+ * It avoids expensive cache bouncings that common test-and-set spin-lock
+ * implementations incur.
+ */
+#ifndef __LINUX_MCS_SPINLOCK_H
+#define __LINUX_MCS_SPINLOCK_H
+
+struct mcs_spinlock {
+        struct mcs_spinlock *next;
+        int locked; /* 1 if lock acquired */
+};
+
+/*
+ * Note: the smp_load_acquire/smp_store_release pair is not
+ * sufficient to form a full memory barrier across
+ * cpus for many architectures (except x86) for mcs_unlock and mcs_lock.
+ * For applications that need a full barrier across multiple cpus
+ * with mcs_unlock and mcs_lock pair, smp_mb__after_unlock_lock() should be
+ * used after mcs_lock.
+ */
+static inline
+void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
+{
+        struct mcs_spinlock *prev;
+
+        /* Init node */
+        node->locked = 0;
+        node->next   = NULL;
+
+        prev = xchg(lock, node);
+        if (likely(prev == NULL)) {
+                /* Lock acquired */
+                node->locked = 1;
+                return;
+        }
+        ACCESS_ONCE(prev->next) = node;
+        /*
+         * Wait until the lock holder passes the lock down.
+         * Using smp_load_acquire() provides a memory barrier that
+         * ensures subsequent operations happen after the lock is acquired.
+         */
+        while (!(smp_load_acquire(&node->locked)))
+                arch_mutex_cpu_relax();
+}
+
+static inline
+void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
+{
+        struct mcs_spinlock *next = ACCESS_ONCE(node->next);
+
+        if (likely(!next)) {
+                /*
+                 * Release the lock by setting it to NULL
+                 */
+                if (cmpxchg(lock, node, NULL) == node)
+                        return;
+                /* Wait until the next pointer is set */
+                while (!(next = ACCESS_ONCE(node->next)))
+                        arch_mutex_cpu_relax();
+        }
+        /*
+         * Pass lock to next waiter.
+         * smp_store_release() provides a memory barrier to ensure
+         * all operations in the critical section has been completed
+         * before unlocking.
+         */
+        smp_store_release(&next->locked, 1);
+}
+
+#endif /* __LINUX_MCS_SPINLOCK_H */