/* rwsem.c: R/W semaphores: contention handling functions
 *
 * Written by David Howells (dhowells@redhat.com).
 * Derived from arch/i386/kernel/semaphore.c
 *
 * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
 * and Michel Lespinasse <walken@google.com>
 */
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/export.h>

/*
 * Initialize an rwsem:
 */
void __init_rwsem(struct rw_semaphore *sem, const char *name,
                  struct lock_class_key *key)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
        /*
         * Make sure we are not reinitializing a held semaphore:
         */
        debug_check_no_locks_freed((void *)sem, sizeof(*sem));
        lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
        sem->count = RWSEM_UNLOCKED_VALUE;
        raw_spin_lock_init(&sem->wait_lock);
        INIT_LIST_HEAD(&sem->wait_list);
}

EXPORT_SYMBOL(__init_rwsem);

enum rwsem_waiter_type {
        RWSEM_WAITING_FOR_WRITE,
        RWSEM_WAITING_FOR_READ
};

struct rwsem_waiter {
        struct list_head list;
        struct task_struct *task;
        enum rwsem_waiter_type type;
};

enum rwsem_wake_type {
        RWSEM_WAKE_ANY,         /* Wake whatever's at head of wait list */
        RWSEM_WAKE_READERS,     /* Wake readers only */
        RWSEM_WAKE_READ_OWNED   /* Waker thread holds the read lock */
};

/*
 * handle the lock release when processes blocked on it that can now run
 * - if we come here from up_xxxx(), then:
 *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
 *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
 * - there must be someone on the queue
 * - the spinlock must be held by the caller
 * - woken process blocks are discarded from the list after having task zeroed
 * - writers are only woken if downgrading is false
 */
static struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
{
        struct rwsem_waiter *waiter;
        struct task_struct *tsk;
        struct list_head *next;
        long oldcount, woken, loop, adjustment;

        waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
        if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
                if (wake_type == RWSEM_WAKE_ANY)
                        /* Wake writer at the front of the queue, but do not
                         * grant it the lock yet as we want other writers
                         * to be able to steal it.  Readers, on the other hand,
                         * will block as they will notice the queued writer.
                         */
                        wake_up_process(waiter->task);
                goto out;
        }

        /* Writers might steal the lock before we grant it to the next reader.
         * We prefer to do the first reader grant before counting readers
         * so we can bail out early if a writer stole the lock.
         */
        adjustment = 0;
        if (wake_type != RWSEM_WAKE_READ_OWNED) {
                adjustment = RWSEM_ACTIVE_READ_BIAS;
 try_reader_grant:
                oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
                if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
                        /* A writer stole the lock. Undo our reader grant. */
                        if (rwsem_atomic_update(-adjustment, sem) &
                                                RWSEM_ACTIVE_MASK)
                                goto out;
                        /* Last active locker left. Retry waking readers. */
                        goto try_reader_grant;
                }
        }

        /* Grant an infinite number of read locks to the readers at the front
         * of the queue.  Note we increment the 'active part' of the count by
         * the number of readers before waking any processes up.
         */
        woken = 0;
        do {
                woken++;

                if (waiter->list.next == &sem->wait_list)
                        break;

                waiter = list_entry(waiter->list.next,
                                        struct rwsem_waiter, list);

        } while (waiter->type != RWSEM_WAITING_FOR_WRITE);

        adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
        if (waiter->type != RWSEM_WAITING_FOR_WRITE)
                /* hit end of list above */
                adjustment -= RWSEM_WAITING_BIAS;

        if (adjustment)
                rwsem_atomic_add(adjustment, sem);

        next = sem->wait_list.next;
        loop = woken;
        do {
                waiter = list_entry(next, struct rwsem_waiter, list);
                next = waiter->list.next;
                tsk = waiter->task;
                smp_mb();
                waiter->task = NULL;
                wake_up_process(tsk);
                put_task_struct(tsk);
        } while (--loop);

        sem->wait_list.next = next;
        next->prev = &sem->wait_list;

 out:
        return sem;
}

/*
 * wait for the read lock to be granted
 */
struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
{
        long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
        struct rwsem_waiter waiter;
        struct task_struct *tsk = current;

        /* set up my own style of waitqueue */
        waiter.task = tsk;
        waiter.type = RWSEM_WAITING_FOR_READ;
        get_task_struct(tsk);

        raw_spin_lock_irq(&sem->wait_lock);
        if (list_empty(&sem->wait_list))
                adjustment += RWSEM_WAITING_BIAS;
        list_add_tail(&waiter.list, &sem->wait_list);

        /* we're now waiting on the lock, but no longer actively locking */
        count = rwsem_atomic_update(adjustment, sem);

        /* If there are no active locks, wake the front queued process(es).
         *
         * If there are no writers and we are first in the queue,
         * wake our own waiter to join the existing active readers !
         */
        if (count == RWSEM_WAITING_BIAS ||
            (count > RWSEM_WAITING_BIAS &&
             adjustment != -RWSEM_ACTIVE_READ_BIAS))
                sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);

        raw_spin_unlock_irq(&sem->wait_lock);

        /* wait to be given the lock */
        while (true) {
                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                if (!waiter.task)
                        break;
                schedule();
        }

        tsk->state = TASK_RUNNING;

        return sem;
}

/*
 * wait until we successfully acquire the write lock
 */
struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
{
        long count, adjustment = -RWSEM_ACTIVE_WRITE_BIAS;
        struct rwsem_waiter waiter;
        struct task_struct *tsk = current;

        /* set up my own style of waitqueue */
        waiter.task = tsk;
        waiter.type = RWSEM_WAITING_FOR_WRITE;

        raw_spin_lock_irq(&sem->wait_lock);
        if (list_empty(&sem->wait_list))
                adjustment += RWSEM_WAITING_BIAS;
        list_add_tail(&waiter.list, &sem->wait_list);

        /* we're now waiting on the lock, but no longer actively locking */
        count = rwsem_atomic_update(adjustment, sem);

        /* If there were already threads queued before us and there are no
         * active writers, the lock must be read owned; so we try to wake
         * any read locks that were queued ahead of us. */
        if (count > RWSEM_WAITING_BIAS &&
            adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
                sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);

        /* wait until we successfully acquire the lock */
        set_task_state(tsk, TASK_UNINTERRUPTIBLE);
        while (true) {
                if (!(count & RWSEM_ACTIVE_MASK)) {
                        /* Try acquiring the write lock. */
                        count = RWSEM_ACTIVE_WRITE_BIAS;
                        if (!list_is_singular(&sem->wait_list))
                                count += RWSEM_WAITING_BIAS;

                        if (sem->count == RWSEM_WAITING_BIAS &&
                            cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
                                                        RWSEM_WAITING_BIAS)
                                break;
                }

                raw_spin_unlock_irq(&sem->wait_lock);

                /* Block until there are no active lockers. */
                do {
                        schedule();
                        set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                } while ((count = sem->count) & RWSEM_ACTIVE_MASK);

                raw_spin_lock_irq(&sem->wait_lock);
        }

        list_del(&waiter.list);
        raw_spin_unlock_irq(&sem->wait_lock);
        tsk->state = TASK_RUNNING;

        return sem;
}

/*
 * handle waking up a waiter on the semaphore
 * - up_read/up_write has decremented the active part of count if we come here
 */
struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
{
        unsigned long flags;

        raw_spin_lock_irqsave(&sem->wait_lock, flags);

        /* do nothing if list empty */
        if (!list_empty(&sem->wait_list))
                sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);

        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

        return sem;
}

/*
 * downgrade a write lock into a read lock
 * - caller incremented waiting part of count and discovered it still negative
 * - just wake up any readers at the front of the queue
 */
struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
        unsigned long flags;

        raw_spin_lock_irqsave(&sem->wait_lock, flags);

        /* do nothing if list empty */
        if (!list_empty(&sem->wait_list))
                sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);

        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

        return sem;
}

EXPORT_SYMBOL(rwsem_down_read_failed);
EXPORT_SYMBOL(rwsem_down_write_failed);
EXPORT_SYMBOL(rwsem_wake);
EXPORT_SYMBOL(rwsem_downgrade_wake);