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, 0 insertions, 208 deletions

diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
deleted file mode 100644
index 9887a905a762..000000000000
--- a/kernel/locking/mcs_spinlock.c
+++ /dev/null
@@ -1,208 +0,0 @@
-#include <linux/percpu.h>
-#include <linux/sched.h>
-#include "mcs_spinlock.h"
-
-#ifdef CONFIG_SMP
-
-/*
- * 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;
-}
-
-#endif
-