source: mainline/kernel/generic/src/synch/waitq.c@ b3f8fb7

/*
 * Copyright (c) 2001-2004 Jakub Jermar
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/** @addtogroup sync
 * @{
 */

/**
 * @file
 * @brief       Wait queue.
 *
 * Wait queue is the basic synchronization primitive upon which all
 * other synchronization primitives build.
 *
 * It allows threads to wait for an event in first-come, first-served
 * fashion. Conditional operation as well as timeouts and interruptions
 * are supported.
 */

#include <synch/waitq.h>
#include <synch/synch.h>
#include <synch/spinlock.h>
#include <proc/thread.h>
#include <proc/scheduler.h>
#include <arch/asm.h>
#include <arch/types.h>
#include <time/timeout.h>
#include <arch.h>
#include <context.h>
#include <adt/list.h>

static void waitq_timeouted_sleep(void *data);

/** Initialize wait queue
 *
 * Initialize wait queue.
 *
 * @param wq Pointer to wait queue to be initialized.
 */
void waitq_initialize(waitq_t *wq)
{
        spinlock_initialize(&wq->lock, "waitq_lock");
        list_initialize(&wq->head);
        wq->missed_wakeups = 0;
}

/** Handle timeout during waitq_sleep_timeout() call
 *
 * This routine is called when waitq_sleep_timeout() timeouts.
 * Interrupts are disabled.
 *
 * It is supposed to try to remove 'its' thread from the wait queue;
 * it can eventually fail to achieve this goal when these two events
 * overlap. In that case it behaves just as though there was no
 * timeout at all.
 *
 * @param data Pointer to the thread that called waitq_sleep_timeout().
 */
void waitq_timeouted_sleep(void *data)
{
        thread_t *t = (thread_t *) data;
        waitq_t *wq;
        bool do_wakeup = false;

        spinlock_lock(&threads_lock);
        if (!thread_exists(t))
                goto out;

grab_locks:
        spinlock_lock(&t->lock);
        if ((wq = t->sleep_queue)) {            /* assignment */
                if (!spinlock_trylock(&wq->lock)) {
                        spinlock_unlock(&t->lock);
                        goto grab_locks;        /* avoid deadlock */
                }

                list_remove(&t->wq_link);
                t->saved_context = t->sleep_timeout_context;
                do_wakeup = true;
                t->sleep_queue = NULL;
                spinlock_unlock(&wq->lock);
        }
        
        t->timeout_pending = false;
        spinlock_unlock(&t->lock);
        
        if (do_wakeup)
                thread_ready(t);

out:
        spinlock_unlock(&threads_lock);
}


/** Sleep until either wakeup, timeout or interruption occurs
 *
 * This is a sleep implementation which allows itself to time out or to be
 * interrupted from the sleep, restoring a failover context.
 *
 * Sleepers are organised in a FIFO fashion in a structure called wait queue.
 *
 * This function is really basic in that other functions as waitq_sleep()
 * and all the *_timeout() functions use it.
 *
 * @param wq Pointer to wait queue.
 * @param usec Timeout in microseconds.
 * @param flags Specify mode of the sleep.
 *
 * The sleep can be interrupted only if the
 * SYNCH_FLAGS_INTERRUPTIBLE bit is specified in flags.
 * 
 * If usec is greater than zero, regardless of the value of the
 * SYNCH_FLAGS_NON_BLOCKING bit in flags, the call will not return until either
 * timeout, interruption or wakeup comes. 
 *
 * If usec is zero and the SYNCH_FLAGS_NON_BLOCKING bit is not set in flags,
 * the call will not return until wakeup or interruption comes.
 *
 * If usec is zero and the SYNCH_FLAGS_NON_BLOCKING bit is set in flags, the
 * call will immediately return, reporting either success or failure.
 *
 * @return One of: ESYNCH_WOULD_BLOCK, ESYNCH_TIMEOUT, ESYNCH_INTERRUPTED,
 * ESYNCH_OK_ATOMIC, ESYNCH_OK_BLOCKED.
 *
 * @li ESYNCH_WOULD_BLOCK means that the sleep failed because at the time of the
 * call there was no pending wakeup.
 *
 * @li ESYNCH_TIMEOUT means that the sleep timed out.
 *
 * @li ESYNCH_INTERRUPTED means that somebody interrupted the sleeping thread.
 *
 * @li ESYNCH_OK_ATOMIC means that the sleep succeeded and that there was
 * a pending wakeup at the time of the call. The caller was not put
 * asleep at all.
 * 
 * @li ESYNCH_OK_BLOCKED means that the sleep succeeded; the full sleep was 
 * attempted.
 */
int waitq_sleep_timeout(waitq_t *wq, uint32_t usec, int flags)
{
        ipl_t ipl;
        int rc;
        
        ipl = waitq_sleep_prepare(wq);
        rc = waitq_sleep_timeout_unsafe(wq, usec, flags);
        waitq_sleep_finish(wq, rc, ipl);
        return rc;
}

/** Prepare to sleep in a waitq.
 *
 * This function will return holding the lock of the wait queue
 * and interrupts disabled.
 *
 * @param wq Wait queue.
 *
 * @return Interrupt level as it existed on entry to this function.
 */
ipl_t waitq_sleep_prepare(waitq_t *wq)
{
        ipl_t ipl;
        
restart:
        ipl = interrupts_disable();

        if (THREAD) {   /* needed during system initiailzation */
                /*
                 * Busy waiting for a delayed timeout.
                 * This is an important fix for the race condition between
                 * a delayed timeout and a next call to waitq_sleep_timeout().
                 * Simply, the thread is not allowed to go to sleep if
                 * there are timeouts in progress.
                 */
                spinlock_lock(&THREAD->lock);
                if (THREAD->timeout_pending) {
                        spinlock_unlock(&THREAD->lock);
                        interrupts_restore(ipl);
                        goto restart;
                }
                spinlock_unlock(&THREAD->lock);
        }
                                                                                                        
        spinlock_lock(&wq->lock);
        return ipl;
}

/** Finish waiting in a wait queue.
 *
 * This function restores interrupts to the state that existed prior
 * to the call to waitq_sleep_prepare(). If necessary, the wait queue
 * lock is released.
 *
 * @param wq Wait queue.
 * @param rc Return code of waitq_sleep_timeout_unsafe().
 * @param ipl Interrupt level returned by waitq_sleep_prepare().
 */
void waitq_sleep_finish(waitq_t *wq, int rc, ipl_t ipl)
{
        switch (rc) {
        case ESYNCH_WOULD_BLOCK:
        case ESYNCH_OK_ATOMIC:
                spinlock_unlock(&wq->lock);
                break;
        default:
                break;
        }
        interrupts_restore(ipl);
}

/** Internal implementation of waitq_sleep_timeout().
 *
 * This function implements logic of sleeping in a wait queue.
 * This call must be preceeded by a call to waitq_sleep_prepare()
 * and followed by a call to waitq_slee_finish().
 *
 * @param wq See waitq_sleep_timeout().
 * @param usec See waitq_sleep_timeout().
 * @param flags See waitq_sleep_timeout().
 *
 * @return See waitq_sleep_timeout().
 */
int waitq_sleep_timeout_unsafe(waitq_t *wq, uint32_t usec, int flags)
{
        /* checks whether to go to sleep at all */
        if (wq->missed_wakeups) {
                wq->missed_wakeups--;
                return ESYNCH_OK_ATOMIC;
        }
        else {
                if ((flags & SYNCH_FLAGS_NON_BLOCKING) && (usec == 0)) {
                        /* return immediatelly instead of going to sleep */
                        return ESYNCH_WOULD_BLOCK;
                }
        }
        
        /*
         * Now we are firmly decided to go to sleep.
         */
        spinlock_lock(&THREAD->lock);

        if (flags & SYNCH_FLAGS_INTERRUPTIBLE) {

                /*
                 * If the thread was already interrupted,
                 * don't go to sleep at all.
                 */
                if (THREAD->interrupted) {
                        spinlock_unlock(&THREAD->lock);
                        spinlock_unlock(&wq->lock);
                        return ESYNCH_INTERRUPTED;
                }

                /*
                 * Set context that will be restored if the sleep
                 * of this thread is ever interrupted.
                 */
                THREAD->sleep_interruptible = true;
                if (!context_save(&THREAD->sleep_interruption_context)) {
                        /* Short emulation of scheduler() return code. */
                        spinlock_unlock(&THREAD->lock);
                        return ESYNCH_INTERRUPTED;
                }

        } else {
                THREAD->sleep_interruptible = false;
        }

        if (usec) {
                /* We use the timeout variant. */
                if (!context_save(&THREAD->sleep_timeout_context)) {
                        /* Short emulation of scheduler() return code. */
                        spinlock_unlock(&THREAD->lock);
                        return ESYNCH_TIMEOUT;
                }
                THREAD->timeout_pending = true;
                timeout_register(&THREAD->sleep_timeout, (uint64_t) usec,
                    waitq_timeouted_sleep, THREAD);
        }

        list_append(&THREAD->wq_link, &wq->head);

        /*
         * Suspend execution.
         */
        THREAD->state = Sleeping;
        THREAD->sleep_queue = wq;

        spinlock_unlock(&THREAD->lock);

        /* wq->lock is released in scheduler_separated_stack() */
        scheduler(); 
        
        return ESYNCH_OK_BLOCKED;
}


/** Wake up first thread sleeping in a wait queue
 *
 * Wake up first thread sleeping in a wait queue. This is the SMP- and IRQ-safe
 * wrapper meant for general use.
 *
 * Besides its 'normal' wakeup operation, it attempts to unregister possible
 * timeout.
 *
 * @param wq Pointer to wait queue.
 * @param all If this is non-zero, all sleeping threads will be woken up and
 *      missed count will be zeroed.
 */
void waitq_wakeup(waitq_t *wq, bool all)
{
        ipl_t ipl;

        ipl = interrupts_disable();
        spinlock_lock(&wq->lock);

        _waitq_wakeup_unsafe(wq, all);

        spinlock_unlock(&wq->lock);     
        interrupts_restore(ipl);        
}

/** Internal SMP- and IRQ-unsafe version of waitq_wakeup()
 *
 * This is the internal SMP- and IRQ-unsafe version of waitq_wakeup(). It
 * assumes wq->lock is already locked and interrupts are already disabled.
 *
 * @param wq Pointer to wait queue.
 * @param all If this is non-zero, all sleeping threads will be woken up and
 *      missed count will be zeroed.
 */
void _waitq_wakeup_unsafe(waitq_t *wq, bool all)
{
        thread_t *t;

loop:   
        if (list_empty(&wq->head)) {
                wq->missed_wakeups++;
                if (all)
                        wq->missed_wakeups = 0;
                return;
        }

        t = list_get_instance(wq->head.next, thread_t, wq_link);
        
        /*
         * Lock the thread prior to removing it from the wq.
         * This is not necessary because of mutual exclusion
         * (the link belongs to the wait queue), but because
         * of synchronization with waitq_timeouted_sleep()
         * and thread_interrupt_sleep().
         *
         * In order for these two functions to work, the following
         * invariant must hold:
         *
         * t->sleep_queue != NULL <=> t sleeps in a wait queue
         *
         * For an observer who locks the thread, the invariant
         * holds only when the lock is held prior to removing
         * it from the wait queue.
         */
        spinlock_lock(&t->lock);
        list_remove(&t->wq_link);
        
        if (t->timeout_pending && timeout_unregister(&t->sleep_timeout))
                t->timeout_pending = false;
        t->sleep_queue = NULL;
        spinlock_unlock(&t->lock);

        thread_ready(t);

        if (all)
                goto loop;
}

/** @}
 */