--- /dev/null
+#include <linux/percpu.h>
+#include <linux/sched.h>
+#include <linux/osq_lock.h>
+
+/*
+ * An MCS like lock especially tailored for optimistic spinning for sleeping
+ * lock implementations (mutex, rwsem, etc).
+ *
+ * Using a single mcs node per CPU is safe because sleeping locks should not be
+ * called from interrupt context and we have preemption disabled while
+ * spinning.
+ */
+static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
+
+/*
+ * We use the value 0 to represent "no CPU", thus the encoded value
+ * will be the CPU number incremented by 1.
+ */
+static inline int encode_cpu(int cpu_nr)
+{
+ return cpu_nr + 1;
+}
+
+static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
+{
+ int cpu_nr = encoded_cpu_val - 1;
+
+ return per_cpu_ptr(&osq_node, cpu_nr);
+}
+
+/*
+ * Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
+ * Can return NULL in case we were the last queued and we updated @lock instead.
+ */
+static inline struct optimistic_spin_node *
+osq_wait_next(struct optimistic_spin_queue *lock,
+ struct optimistic_spin_node *node,
+ struct optimistic_spin_node *prev)
+{
+ struct optimistic_spin_node *next = NULL;
+ int curr = encode_cpu(smp_processor_id());
+ int old;
+
+ /*
+ * If there is a prev node in queue, then the 'old' value will be
+ * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
+ * we're currently last in queue, then the queue will then become empty.
+ */
+ old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
+
+ for (;;) {
+ if (atomic_read(&lock->tail) == curr &&
+ atomic_cmpxchg(&lock->tail, curr, old) == curr) {
+ /*
+ * We were the last queued, we moved @lock back. @prev
+ * will now observe @lock and will complete its
+ * unlock()/unqueue().
+ */
+ break;
+ }
+
+ /*
+ * We must xchg() the @node->next value, because if we were to
+ * leave it in, a concurrent unlock()/unqueue() from
+ * @node->next might complete Step-A and think its @prev is
+ * still valid.
+ *
+ * If the concurrent unlock()/unqueue() wins the race, we'll
+ * wait for either @lock to point to us, through its Step-B, or
+ * wait for a new @node->next from its Step-C.
+ */
+ if (node->next) {
+ next = xchg(&node->next, NULL);
+ if (next)
+ break;
+ }
+
+ cpu_relax_lowlatency();
+ }
+
+ return next;
+}
+
+bool osq_lock(struct optimistic_spin_queue *lock)
+{
+ struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
+ struct optimistic_spin_node *prev, *next;
+ int curr = encode_cpu(smp_processor_id());
+ int old;
+
+ node->locked = 0;
+ node->next = NULL;
+ node->cpu = curr;
+
+ old = atomic_xchg(&lock->tail, curr);
+ if (old == OSQ_UNLOCKED_VAL)
+ return true;
+
+ prev = decode_cpu(old);
+ node->prev = prev;
+ WRITE_ONCE(prev->next, node);
+
+ /*
+ * Normally @prev is untouchable after the above store; because at that
+ * moment unlock can proceed and wipe the node element from stack.
+ *
+ * However, since our nodes are static per-cpu storage, we're
+ * guaranteed their existence -- this allows us to apply
+ * cmpxchg in an attempt to undo our queueing.
+ */
+
+ while (!READ_ONCE(node->locked)) {
+ /*
+ * If we need to reschedule bail... so we can block.
+ */
+ if (need_resched())
+ goto unqueue;
+
+ cpu_relax_lowlatency();
+ }
+ return true;
+
+unqueue:
+ /*
+ * Step - A -- stabilize @prev
+ *
+ * Undo our @prev->next assignment; this will make @prev's
+ * unlock()/unqueue() wait for a next pointer since @lock points to us
+ * (or later).
+ */
+
+ for (;;) {
+ if (prev->next == node &&
+ cmpxchg(&prev->next, node, NULL) == node)
+ break;
+
+ /*
+ * We can only fail the cmpxchg() racing against an unlock(),
+ * in which case we should observe @node->locked becomming
+ * true.
+ */
+ if (smp_load_acquire(&node->locked))
+ return true;
+
+ cpu_relax_lowlatency();
+
+ /*
+ * Or we race against a concurrent unqueue()'s step-B, in which
+ * case its step-C will write us a new @node->prev pointer.
+ */
+ prev = READ_ONCE(node->prev);
+ }
+
+ /*
+ * Step - B -- stabilize @next
+ *
+ * Similar to unlock(), wait for @node->next or move @lock from @node
+ * back to @prev.
+ */
+
+ next = osq_wait_next(lock, node, prev);
+ if (!next)
+ return false;
+
+ /*
+ * Step - C -- unlink
+ *
+ * @prev is stable because its still waiting for a new @prev->next
+ * pointer, @next is stable because our @node->next pointer is NULL and
+ * it will wait in Step-A.
+ */
+
+ WRITE_ONCE(next->prev, prev);
+ WRITE_ONCE(prev->next, next);
+
+ return false;
+}
+
+void osq_unlock(struct optimistic_spin_queue *lock)
+{
+ struct optimistic_spin_node *node, *next;
+ int curr = encode_cpu(smp_processor_id());
+
+ /*
+ * Fast path for the uncontended case.
+ */
+ if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr))
+ return;
+
+ /*
+ * Second most likely case.
+ */
+ node = this_cpu_ptr(&osq_node);
+ next = xchg(&node->next, NULL);
+ if (next) {
+ WRITE_ONCE(next->locked, 1);
+ return;
+ }
+
+ next = osq_wait_next(lock, node, NULL);
+ if (next)
+ WRITE_ONCE(next->locked, 1);
+}
Code Review / kvmfornfv.git / blobdiff