root/lib/rwsem-spinlock.c

/* 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
};

#if RWSEM_DEBUG
void rwsemtrace(struct rw_semaphore *sem, const char *str)
{
        if (sem->debug)
                printk("[%d] %s({%d,%d})\n",
                       current->pid,str,sem->activity,list_empty(&sem->wait_list)?0:1);
}
#endif

/*
 * initialise the semaphore
 */
void fastcall init_rwsem(struct rw_semaphore *sem)
{
        sem->activity = 0;
        spin_lock_init(&sem->wait_lock);
        INIT_LIST_HEAD(&sem->wait_list);
#if RWSEM_DEBUG
        sem->debug = 0;
#endif
}

/*
 * handle the lock being released whilst there are 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;

        rwsemtrace(sem,"Entering __rwsem_do_wake");

        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;
                mb();
                waiter->task = NULL;
                wake_up_process(tsk);
                put_task_struct(tsk);
                goto out;
        }

        /* grant an infinite number of read locks to the readers at 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;
                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:
        rwsemtrace(sem,"Leaving __rwsem_do_wake");
        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;
        mb();
        waiter->task = NULL;
        wake_up_process(tsk);
        put_task_struct(tsk);
        return sem;
}

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

        rwsemtrace(sem,"Entering __down_read");

        spin_lock(&sem->wait_lock);

        if (sem->activity>=0 && list_empty(&sem->wait_list)) {
                /* granted */
                sem->activity++;
                spin_unlock(&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(&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:
        rwsemtrace(sem,"Leaving __down_read");
}

/*
 * trylock for reading -- returns 1 if successful, 0 if contention
 */
int fastcall __down_read_trylock(struct rw_semaphore *sem)
{
        int ret = 0;
        rwsemtrace(sem,"Entering __down_read_trylock");

        spin_lock(&sem->wait_lock);

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

        spin_unlock(&sem->wait_lock);

        rwsemtrace(sem,"Leaving __down_read_trylock");
        return ret;
}

/*
 * get a write lock on the semaphore
 * - note that we increment the waiting count anyway to indicate an exclusive lock
 */
void fastcall __down_write(struct rw_semaphore *sem)
{
        struct rwsem_waiter waiter;
        struct task_struct *tsk;

        rwsemtrace(sem,"Entering __down_write");

        spin_lock(&sem->wait_lock);

        if (sem->activity==0 && list_empty(&sem->wait_list)) {
                /* granted */
                sem->activity = -1;
                spin_unlock(&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(&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:
        rwsemtrace(sem,"Leaving __down_write");
}

/*
 * trylock for writing -- returns 1 if successful, 0 if contention
 */
int fastcall __down_write_trylock(struct rw_semaphore *sem)
{
        int ret = 0;
        rwsemtrace(sem,"Entering __down_write_trylock");

        spin_lock(&sem->wait_lock);

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

        spin_unlock(&sem->wait_lock);

        rwsemtrace(sem,"Leaving __down_write_trylock");
        return ret;
}

/*
 * release a read lock on the semaphore
 */
void fastcall __up_read(struct rw_semaphore *sem)
{
        rwsemtrace(sem,"Entering __up_read");

        spin_lock(&sem->wait_lock);

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

        spin_unlock(&sem->wait_lock);

        rwsemtrace(sem,"Leaving __up_read");
}

/*
 * release a write lock on the semaphore
 */
void fastcall __up_write(struct rw_semaphore *sem)
{
        rwsemtrace(sem,"Entering __up_write");

        spin_lock(&sem->wait_lock);

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

        spin_unlock(&sem->wait_lock);

        rwsemtrace(sem,"Leaving __up_write");
}

/*
 * downgrade a write lock into a read lock
 * - just wake up any readers at the front of the queue
 */
void fastcall __downgrade_write(struct rw_semaphore *sem)
{
        rwsemtrace(sem,"Entering __downgrade_write");

        spin_lock(&sem->wait_lock);

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

        spin_unlock(&sem->wait_lock);

        rwsemtrace(sem,"Leaving __downgrade_write");
}

EXPORT_SYMBOL(init_rwsem);
EXPORT_SYMBOL(__down_read);
EXPORT_SYMBOL(__down_read_trylock);
EXPORT_SYMBOL(__down_write);
EXPORT_SYMBOL(__down_write_trylock);
EXPORT_SYMBOL(__up_read);
EXPORT_SYMBOL(__up_write);
EXPORT_SYMBOL(__downgrade_write);
#if RWSEM_DEBUG
EXPORT_SYMBOL(rwsemtrace);
#endif