locking/mcs: Better differentiate between MCS variants

We have two flavors of the MCS spinlock: standard and cancelable (OSQ).
While each one is independent of the other, we currently mix and match
them. This patch:

  - Moves the OSQ code out of mcs_spinlock.h (which only deals with the traditional
    version) into include/linux/osq_lock.h. No unnecessary code is added to the
    more global header file, anything locks that make use of OSQ must include
    it anyway.

  - Renames mcs_spinlock.c to osq_lock.c. This file only contains osq code.

  - Introduces a CONFIG_LOCK_SPIN_ON_OWNER in order to only build osq_lock
    if there is support for it.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Waiman Long <Waiman.Long@hp.com>
Link: http://lkml.kernel.org/r/1420573509-24774-5-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 203 insertions, 0 deletions

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
new file mode 100644
index 000000000000..ec83d4db8ec6
--- /dev/null
+++ b/kernel/locking/osq_lock.c
@@ -0,0 +1,203 @@
+#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;
+        ACCESS_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 (!smp_load_acquire(&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 = ACCESS_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.
+         */
+
+        ACCESS_ONCE(next->prev) = prev;
+        ACCESS_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) {
+                ACCESS_ONCE(next->locked) = 1;
+                return;
+        }
+
+        next = osq_wait_next(lock, node, NULL);
+        if (next)
+                ACCESS_ONCE(next->locked) = 1;
+}