rwsem.c

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/* rwsem.c: R/W semaphores: contention handling functions
 *
 * Written by David Howells ([email protected]).
 * Derived from arch/i386/kernel/semaphore.c
 */
#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);

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
};

/* Wake types for __rwsem_do_wake().  Note that RWSEM_WAKE_NO_ACTIVE and
 * RWSEM_WAKE_READ_OWNED imply that the spinlock must have been kept held
 * since the rwsem value was observed.
 */
#define RWSEM_WAKE_ANY        0 /* Wake whatever's at head of wait list */
#define RWSEM_WAKE_NO_ACTIVE  1 /* rwsem was observed with no active thread */
#define RWSEM_WAKE_READ_OWNED 2 /* rwsem was observed to be read owned */

/*
 * 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, int wake_type)
{
    struct rwsem_waiter *waiter;
    struct task_struct *tsk;
    struct list_head *next;
    signed long oldcount, woken, loop, adjustment;

    waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
    if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
        goto readers_only;

    if (wake_type == RWSEM_WAKE_READ_OWNED)
        /* Another active reader was observed, so wakeup is not
         * likely to succeed. Save the atomic op.
         */
        goto out;

    /* There's a writer at the front of the queue - try to grant it the
     * write lock.  However, we only wake this writer if we can transition
     * the active part of the count from 0 -> 1
     */
    adjustment = RWSEM_ACTIVE_WRITE_BIAS;
    if (waiter->list.next == &sem->wait_list)
        adjustment -= RWSEM_WAITING_BIAS;

 try_again_write:
    oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
    if (oldcount & RWSEM_ACTIVE_MASK)
        /* Someone grabbed the sem already */
        goto undo_write;

    /* We must be careful not to touch 'waiter' after we set ->task = NULL.
     * It is an allocated on the waiter's stack and may become invalid at
     * any time after that point (due to a wakeup from another source).
     */
    list_del(&waiter->list);
    tsk = waiter->task;
    smp_mb();
    waiter->task = NULL;
    wake_up_process(tsk);
    put_task_struct(tsk);
    goto out;

 readers_only:
    /* If we come here from up_xxxx(), another thread might have reached
     * rwsem_down_failed_common() before we acquired the spinlock and
     * woken up a waiter, making it now active.  We prefer to check for
     * this first in order to not spend too much time with the spinlock
     * held if we're not going to be able to wake up readers in the end.
     *
     * Note that we do not need to update the rwsem count: any writer
     * trying to acquire rwsem will run rwsem_down_write_failed() due
     * to the waiting threads and block trying to acquire the spinlock.
     *
     * We use a dummy atomic update in order to acquire the cache line
     * exclusively since we expect to succeed and run the final rwsem
     * count adjustment pretty soon.
     */
    if (wake_type == RWSEM_WAKE_ANY &&
        rwsem_atomic_update(0, sem) < RWSEM_WAITING_BIAS)
        /* Someone grabbed the sem for write already */
        goto out;

    /* 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->flags & RWSEM_WAITING_FOR_READ);

    adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
    if (waiter->flags & RWSEM_WAITING_FOR_READ)
        /* hit end of list above */
        adjustment -= RWSEM_WAITING_BIAS;

    rwsem_atomic_add(adjustment, sem);

    next = sem->wait_list.next;
    for (loop = woken; loop > 0; loop--) {
        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);
    }

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

 out:
    return sem;

    /* undo the change to the active count, but check for a transition
     * 1->0 */
 undo_write:
    if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
        goto out;
    goto try_again_write;
}

/*
 * wait for a lock to be granted
 */
static struct rw_semaphore __sched *
rwsem_down_failed_common(struct rw_semaphore *sem,
             unsigned int flags, signed long adjustment)
{
    struct rwsem_waiter waiter;
    struct task_struct *tsk = current;
    signed long count;

    set_task_state(tsk, TASK_UNINTERRUPTIBLE);

    /* set up my own style of waitqueue */
    raw_spin_lock_irq(&sem->wait_lock);
    waiter.task = tsk;
    waiter.flags = flags;
    get_task_struct(tsk);

    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) up.
     *
     * Alternatively, if we're called from a failed down_write(), 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)
        sem = __rwsem_do_wake(sem, RWSEM_WAKE_NO_ACTIVE);
    else if (count > RWSEM_WAITING_BIAS &&
         adjustment == -RWSEM_ACTIVE_WRITE_BIAS)
        sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);

    raw_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;

    return sem;
}

/*
 * wait for the read lock to be granted
 */
struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
{
    return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_READ,
                    -RWSEM_ACTIVE_READ_BIAS);
}

/*
 * wait for the write lock to be granted
 */
struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
{
    return rwsem_down_failed_common(sem, RWSEM_WAITING_FOR_WRITE,
                    -RWSEM_ACTIVE_WRITE_BIAS);
}

/*
 * 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);