path: root/kernel/locking/osq_lock.c

#include <linux/percpu.h>
#include <linux/sched.h>
#include <linux/osq_lock.h>
#include <linux/sched/rt.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_acquire(&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;
	struct task_struct *task = current;
	int curr = encode_cpu(smp_processor_id());
	int old;

	node->locked = 0;
	node->next = NULL;
	node->cpu = curr;

	/*
	 * We need both ACQUIRE (pairs with corresponding RELEASE in
	 * unlock() uncontended, or fastpath) and RELEASE (to publish
	 * the node fields we just initialised) semantics when updating
	 * the lock tail.
	 */
	old = atomic_xchg(&lock->tail, curr);
	if (old == OSQ_UNLOCKED_VAL)
		return true;

	prev = decode_cpu(old);
	node->prev = prev;

	/*
	 * We need to avoid reordering of link updation sequence of osq.
	 * A case in which the status of optimistic spin queue is
	 * CPU6->CPU2 in which CPU6 has acquired the lock. At this point
	 * if CPU0 comes in to acquire osq_lock, it will update the tail
	 * count. After tail count update if CPU2 starts to unqueue itself
	 * from optimistic spin queue, it will find updated tail count with
	 * CPU0 and update CPU2 node->next to NULL in osq_wait_next(). If
	 * reordering of following stores happen then prev->next where prev
	 * being CPU2 would be updated to point to CPU0 node:
	 *      node->prev = prev;
	 *      WRITE_ONCE(prev->next, node);
	 *
	 * At this point if next instruction
	 *      WRITE_ONCE(next->prev, prev);
	 * in CPU2 path is committed before the update of CPU0 node->prev =
	 * prev then CPU0 node->prev will point to CPU6 node. At this point
	 * if CPU0 path's node->prev = prev is committed resulting in change
	 * of CPU0 prev back to CPU2 node. CPU2 node->next is NULL, so if
	 * CPU0 gets into unqueue path of osq_lock it will keep spinning
	 * in infinite loop as condition prev->next == node will never be
	 * true.
	 */
	smp_mb();

	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 a task spins on owner on a CPU after acquiring
		 * osq_lock while a RT task spins on another CPU  to
		 * acquire osq_lock, it will starve the owner from
		 * completing if owner is to be scheduled on the same CPU.
		 * It will be a live lock.
		 */
		if (need_resched() || rt_task(task))
			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_release(&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);
}