root/lib/rwsem.c

DEFINITIONS

1. __init_rwsem
2. __rwsem_do_wake
3. rwsem_down_failed_common
4. rwsem_down_read_failed
5. rwsem_down_write_failed
6. rwsem_wake
7. rwsem_downgrade_wake

/* rwsem.c: R/W semaphores: contention handling functions
 *
 * Written by David Howells (dhowells@redhat.com).
 * Derived from arch/i386/kernel/semaphore.c
 */
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/module.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;
        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
};

/*
 * 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 inline struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
{
        struct rwsem_waiter *waiter;
        struct task_struct *tsk;
        struct list_head *next;
        signed long oldcount, woken, loop;

        if (downgrading)
                goto dont_wake_writers;

        /* if we came through an up_xxxx() call, we only only wake someone up
         * if we can transition the active part of the count from 0 -> 1
         */
 try_again:
        oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
                                                - RWSEM_ACTIVE_BIAS;
        if (oldcount & RWSEM_ACTIVE_MASK)
                goto undo;

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

        /* try to grant a single write lock if there's a writer at the front
         * of the queue - note we leave the 'active part' of the count
         * incremented by 1 and the waiting part incremented by 0x00010000
         */
        if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
                goto readers_only;

        /* 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;

        /* don't want to wake any writers */
 dont_wake_writers:
        waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
        if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
                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
         */
 readers_only:
        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);

        loop = woken;
        woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
        if (!downgrading)
                /* we'd already done one increment earlier */
                woken -= RWSEM_ACTIVE_BIAS;

        rwsem_atomic_add(woken, sem);

        next = sem->wait_list.next;
        for (; 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 count, but check for a transition 1->0 */
 undo:
        if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) != 0)
                goto out;
        goto try_again;
}

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

        set_task_state(tsk, TASK_UNINTERRUPTIBLE);

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

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

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

        /* if there are no active locks, wake the front queued process(es) up */
        if (!(count & RWSEM_ACTIVE_MASK))
                sem = __rwsem_do_wake(sem, 0);

        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
 */
asmregparm struct rw_semaphore __sched *
rwsem_down_read_failed(struct rw_semaphore *sem)
{
        struct rwsem_waiter waiter;

        waiter.flags = RWSEM_WAITING_FOR_READ;
        rwsem_down_failed_common(sem, &waiter,
                                RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
        return sem;
}

/*
 * wait for the write lock to be granted
 */
asmregparm struct rw_semaphore __sched *
rwsem_down_write_failed(struct rw_semaphore *sem)
{
        struct rwsem_waiter waiter;

        waiter.flags = RWSEM_WAITING_FOR_WRITE;
        rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);

        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
 */
asmregparm struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
{
        unsigned long flags;

        spin_lock_irqsave(&sem->wait_lock, flags);

        /* do nothing if list empty */
        if (!list_empty(&sem->wait_list))
                sem = __rwsem_do_wake(sem, 0);

        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
 */
asmregparm struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
        unsigned long flags;

        spin_lock_irqsave(&sem->wait_lock, flags);

        /* do nothing if list empty */
        if (!list_empty(&sem->wait_list))
                sem = __rwsem_do_wake(sem, 1);

        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);