root/lib/rwsem.c

/* 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>

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 from up_xxxx(), then:
 *   - the 'active part' of the count (&0x0000ffff) had reached zero (but may have changed)
 *   - 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 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;

        rwsemtrace(sem,"Entering __rwsem_do_wake");

        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;

        list_del(&waiter->list);
        tsk = waiter->task;
        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)
                woken -= RWSEM_ACTIVE_BIAS; /* we'd already done one increment
                                             * earlier */
        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;
                mb();
                waiter->task = NULL;
                wake_up_process(tsk);
                put_task_struct(tsk);
        }

        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;
        get_task_struct(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, 0);

        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;

        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;

        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 fastcall __sched *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/up_write has decremented the active part of the count if we come here
 */
struct rw_semaphore fastcall *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, 0);

        spin_unlock(&sem->wait_lock);

        rwsemtrace(sem,"Leaving rwsem_wake");

        return sem;
}

/*
 * downgrade a write lock into a read lock
 * - caller incremented waiting part of count, and discovered it to be still negative
 * - just wake up any readers at the front of the queue
 */
struct rw_semaphore fastcall *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
        rwsemtrace(sem,"Entering rwsem_downgrade_wake");

        spin_lock(&sem->wait_lock);

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

        spin_unlock(&sem->wait_lock);

        rwsemtrace(sem,"Leaving rwsem_downgrade_wake");
        return sem;
}

EXPORT_SYMBOL_NOVERS(rwsem_down_read_failed);
EXPORT_SYMBOL_NOVERS(rwsem_down_write_failed);
EXPORT_SYMBOL_NOVERS(rwsem_wake);
EXPORT_SYMBOL_NOVERS(rwsem_downgrade_wake);
#if RWSEM_DEBUG
EXPORT_SYMBOL(rwsemtrace);
#endif