// SPDX-License-Identifier: GPL-2.0
#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 int node_cpu(struct optimistic_spin_node *node)
{
        return node->cpu - 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();
        }

        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;

        /*
         * 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;

        /*
         * osq_lock()                   unqueue
         *
         * node->prev = prev            osq_wait_next()
         * WMB                          MB
         * prev->next = node            next->prev = prev // unqueue-C
         *
         * Here 'node->prev' and 'next->prev' are the same variable and we need
         * to ensure these stores happen in-order to avoid corrupting the list.
         */
        smp_wmb();

        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.
                 * Use vcpu_is_preempted() to avoid waiting for a preempted
                 * lock holder:
                 */
                if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
                        goto unqueue;

                cpu_relax();
        }
        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();

                /*
                 * 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);
}