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/* 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

 */

struct rw_semaphore fastcall __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

 */

struct rw_semaphore fastcall __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

 */

struct rw_semaphore fastcall *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

 */

struct rw_semaphore fastcall *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);