root/lib/rwsem-spinlock.c

DEFINITIONS

1. __init_rwsem
2. __rwsem_do_wake
3. __rwsem_wake_one_writer
4. __down_read
5. __down_read_trylock
6. __down_write_nested
7. __down_write
8. __down_write_trylock
9. __up_read
10. __up_write
11. __downgrade_write

/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
 * generic spinlock implementation
 *
 * Copyright (c) 2001   David Howells (dhowells@redhat.com).
 * - Derived partially from idea by Andrea Arcangeli <andrea@suse.de>
 * - Derived also from comments by Linus
 */
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/module.h>

struct rwsem_waiter {
        struct list_head list;
        struct task_struct *task;
        unsigned int flags;
#define RWSEM_WAITING_FOR_READ  0x00000001
#define RWSEM_WAITING_FOR_WRITE 0x00000002
};

/*
 * initialise the semaphore
 */
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->activity = 0;
        spin_lock_init(&sem->wait_lock);
        INIT_LIST_HEAD(&sem->wait_list);
}

/*
 * handle the lock release when processes blocked on it that can now run
 * - if we come here, then:
 *   - the 'active count' _reached_ zero
 *   - the 'waiting count' is non-zero
 * - 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 wakewrite is non-zero
 */
static inline struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
{
        struct rwsem_waiter *waiter;
        struct task_struct *tsk;
        int woken;

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

        if (!wakewrite) {
                if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
                        goto out;
                goto dont_wake_writers;
        }

        /* if we are allowed to wake writers try to grant a single write lock
         * if there's a writer at the front of the queue
         * - we leave the 'waiting count' incremented to signify potential
         *   contention
         */
        if (waiter->flags & RWSEM_WAITING_FOR_WRITE) {
                sem->activity = -1;
                list_del(&waiter->list);
                tsk = waiter->task;
                /* Don't touch waiter after ->task has been NULLed */
                smp_mb();
                waiter->task = NULL;
                wake_up_process(tsk);
                put_task_struct(tsk);
                goto out;
        }

        /* grant an infinite number of read locks to the front of the queue */
 dont_wake_writers:
        woken = 0;
        while (waiter->flags & RWSEM_WAITING_FOR_READ) {
                struct list_head *next = waiter->list.next;

                list_del(&waiter->list);
                tsk = waiter->task;
                smp_mb();
                waiter->task = NULL;
                wake_up_process(tsk);
                put_task_struct(tsk);
                woken++;
                if (list_empty(&sem->wait_list))
                        break;
                waiter = list_entry(next, struct rwsem_waiter, list);
        }

        sem->activity += woken;

 out:
        return sem;
}

/*
 * wake a single writer
 */
static inline struct rw_semaphore *
__rwsem_wake_one_writer(struct rw_semaphore *sem)
{
        struct rwsem_waiter *waiter;
        struct task_struct *tsk;

        sem->activity = -1;

        waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
        list_del(&waiter->list);

        tsk = waiter->task;
        smp_mb();
        waiter->task = NULL;
        wake_up_process(tsk);
        put_task_struct(tsk);
        return sem;
}

/*
 * get a read lock on the semaphore
 */
void __sched __down_read(struct rw_semaphore *sem)
{
        struct rwsem_waiter waiter;
        struct task_struct *tsk;

        spin_lock_irq(&sem->wait_lock);

        if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
                /* granted */
                sem->activity++;
                spin_unlock_irq(&sem->wait_lock);
                goto out;
        }

        tsk = current;
        set_task_state(tsk, TASK_UNINTERRUPTIBLE);

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

        list_add_tail(&waiter.list, &sem->wait_list);

        /* we don't need to touch the semaphore struct anymore */
        spin_unlock_irq(&sem->wait_lock);

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

        tsk->state = TASK_RUNNING;
 out:
        ;
}

/*
 * trylock for reading -- returns 1 if successful, 0 if contention
 */
int __down_read_trylock(struct rw_semaphore *sem)
{
        unsigned long flags;
        int ret = 0;


        spin_lock_irqsave(&sem->wait_lock, flags);

        if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
                /* granted */
                sem->activity++;
                ret = 1;
        }

        spin_unlock_irqrestore(&sem->wait_lock, flags);

        return ret;
}

/*
 * get a write lock on the semaphore
 * - we increment the waiting count anyway to indicate an exclusive lock
 */
void __sched __down_write_nested(struct rw_semaphore *sem, int subclass)
{
        struct rwsem_waiter waiter;
        struct task_struct *tsk;

        spin_lock_irq(&sem->wait_lock);

        if (sem->activity == 0 && list_empty(&sem->wait_list)) {
                /* granted */
                sem->activity = -1;
                spin_unlock_irq(&sem->wait_lock);
                goto out;
        }

        tsk = current;
        set_task_state(tsk, TASK_UNINTERRUPTIBLE);

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

        list_add_tail(&waiter.list, &sem->wait_list);

        /* we don't need to touch the semaphore struct anymore */
        spin_unlock_irq(&sem->wait_lock);

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

        tsk->state = TASK_RUNNING;
 out:
        ;
}

void __sched __down_write(struct rw_semaphore *sem)
{
        __down_write_nested(sem, 0);
}

/*
 * trylock for writing -- returns 1 if successful, 0 if contention
 */
int __down_write_trylock(struct rw_semaphore *sem)
{
        unsigned long flags;
        int ret = 0;

        spin_lock_irqsave(&sem->wait_lock, flags);

        if (sem->activity == 0 && list_empty(&sem->wait_list)) {
                /* granted */
                sem->activity = -1;
                ret = 1;
        }

        spin_unlock_irqrestore(&sem->wait_lock, flags);

        return ret;
}

/*
 * release a read lock on the semaphore
 */
void __up_read(struct rw_semaphore *sem)
{
        unsigned long flags;

        spin_lock_irqsave(&sem->wait_lock, flags);

        if (--sem->activity == 0 && !list_empty(&sem->wait_list))
                sem = __rwsem_wake_one_writer(sem);

        spin_unlock_irqrestore(&sem->wait_lock, flags);
}

/*
 * release a write lock on the semaphore
 */
void __up_write(struct rw_semaphore *sem)
{
        unsigned long flags;

        spin_lock_irqsave(&sem->wait_lock, flags);

        sem->activity = 0;
        if (!list_empty(&sem->wait_list))
                sem = __rwsem_do_wake(sem, 1);

        spin_unlock_irqrestore(&sem->wait_lock, flags);
}

/*
 * downgrade a write lock into a read lock
 * - just wake up any readers at the front of the queue
 */
void __downgrade_write(struct rw_semaphore *sem)
{
        unsigned long flags;

        spin_lock_irqsave(&sem->wait_lock, flags);

        sem->activity = 1;
        if (!list_empty(&sem->wait_list))
                sem = __rwsem_do_wake(sem, 0);

        spin_unlock_irqrestore(&sem->wait_lock, flags);
}

EXPORT_SYMBOL(__init_rwsem);
EXPORT_SYMBOL(__down_read);
EXPORT_SYMBOL(__down_read_trylock);
EXPORT_SYMBOL(__down_write_nested);
EXPORT_SYMBOL(__down_write);
EXPORT_SYMBOL(__down_write_trylock);
EXPORT_SYMBOL(__up_read);
EXPORT_SYMBOL(__up_write);
EXPORT_SYMBOL(__downgrade_write);