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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/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

#undef rwsemtrace

void rwsemtrace(struct rw_semaphore *sem, const char *str)

{

        printk("sem=%p\n",sem);

        printk("(sem)=%08lx\n",sem->count);

        if (sem->debug)

                printk("[%d] %s({%08lx})\n",current->pid,str,sem->count);

}

#endif

/*

 * handle the lock being released whilst there are processes blocked on it that can now run

 * - if we come here, then:

 *   - the 'active part' of the count (&0x0000ffff) reached zero but has been re-incremented

 *   - the 'waiting part' of the count (&0xffff0000) is negative (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 flags zeroised

 */

static inline struct rw_semaphore *__rwsem_do_wake(struct rw_semaphore *sem)

{

        struct rwsem_waiter *waiter;

        struct list_head *next;

        signed long oldcount;

        int woken, loop;

        rwsemtrace(sem,"Entering __rwsem_do_wake");

        /* 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;

        list_del(&waiter->list);

        waiter->flags = 0;

        wake_up_process(waiter->task);

        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 (less one

         *   for the activity decrement we've already done) 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;

        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;

                waiter->flags = 0;

                wake_up_process(waiter->task);

        }

        sem->wait_list.next = next;

        next->prev = &sem->wait_list;

 out:

        rwsemtrace(sem,"Leaving __rwsem_do_wake");

        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 inline struct rw_semaphore *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(&sem->wait_lock);

        waiter->task = tsk;

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

        /* note that we're now waiting on the lock, but no longer actively read-locking */

        count = rwsem_atomic_update(adjustment,sem);

        /* if there are no longer active locks, wake the front queued process(es) up

         * - it might even be this process, since the waker takes a more active part

         */

        if (!(count & RWSEM_ACTIVE_MASK))

                sem = __rwsem_do_wake(sem);

        spin_unlock(&sem->wait_lock);

        /* wait to be given the lock */

        for (;;) {

                if (!waiter->flags)

                        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 *rwsem_down_read_failed(struct rw_semaphore *sem)

{

        struct rwsem_waiter waiter;

        rwsemtrace(sem,"Entering rwsem_down_read_failed");

        waiter.flags = RWSEM_WAITING_FOR_READ;

        rwsem_down_failed_common(sem,&waiter,RWSEM_WAITING_BIAS-RWSEM_ACTIVE_BIAS);

        rwsemtrace(sem,"Leaving rwsem_down_read_failed");

        return sem;

}

/*

 * wait for the write lock to be granted

 */

struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem)

{

        struct rwsem_waiter waiter;

        rwsemtrace(sem,"Entering rwsem_down_write_failed");

        waiter.flags = RWSEM_WAITING_FOR_WRITE;

        rwsem_down_failed_common(sem,&waiter,-RWSEM_ACTIVE_BIAS);

        rwsemtrace(sem,"Leaving rwsem_down_write_failed");

        return sem;

}

/*

 * handle waking up a waiter on the semaphore

 * - up_read has decremented the active part of the count if we come here

 */

struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)

{

        rwsemtrace(sem,"Entering rwsem_wake");

        spin_lock(&sem->wait_lock);

        /* do nothing if list empty */

        if (!list_empty(&sem->wait_list))

                sem = __rwsem_do_wake(sem);

        spin_unlock(&sem->wait_lock);

        rwsemtrace(sem,"Leaving rwsem_wake");

        return sem;

}

EXPORT_SYMBOL_NOVERS(rwsem_down_read_failed);

EXPORT_SYMBOL_NOVERS(rwsem_down_write_failed);

EXPORT_SYMBOL_NOVERS(rwsem_wake);

#if RWSEM_DEBUG

EXPORT_SYMBOL(rwsemtrace);

#endif