rwsem-spinlock.c

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/* rwsem-spinlock.c: R/W semaphores: contention handling functions for
 * generic spinlock implementation
 *
 * Copyright (c) 2001   David Howells ([email protected]).
 * - Derived partially from idea by Andrea Arcangeli <[email protected]>
 * - Derived also from comments by Linus
 */
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/export.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
};

int rwsem_is_locked(struct rw_semaphore *sem)
{
    int ret = 1;
    unsigned long flags;

    if (raw_spin_trylock_irqsave(&sem->wait_lock, flags)) {
        ret = (sem->activity != 0);
        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
    }
    return ret;
}
EXPORT_SYMBOL(rwsem_is_locked);

/*
 * 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;
    raw_spin_lock_init(&sem->wait_lock);
    INIT_LIST_HEAD(&sem->wait_list);
}
EXPORT_SYMBOL(__init_rwsem);

/*
 * 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;
    unsigned long flags;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    if (sem->activity >= 0 && list_empty(&sem->wait_list)) {
        /* granted */
        sem->activity++;
        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        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 */
    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

    /* 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;


    raw_spin_lock_irqsave(&sem->wait_lock, flags);

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

    raw_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;
    unsigned long flags;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

    if (sem->activity == 0 && list_empty(&sem->wait_list)) {
        /* granted */
        sem->activity = -1;
        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        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 */
    raw_spin_unlock_irqrestore(&sem->wait_lock, flags);

    /* 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;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

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

    raw_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;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

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

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

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

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

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

    raw_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;

    raw_spin_lock_irqsave(&sem->wait_lock, flags);

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

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