Subversion Repositories HelenOS-historic

/*

 * 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   Reader/Writer locks.

 *

 * A reader/writer lock can be held by multiple readers at a time.

 * Or it can be exclusively held by a sole writer at a time.

 *

 * These locks are not recursive.

 * Because technique called direct hand-off is used, neither readers

 * nor writers will suffer starvation.

 *

 * If there is a writer followed by a reader waiting for the rwlock

 * and the writer times out, all leading readers are automatically woken up

 * and allowed in.

 */

/*

 * NOTE ON rwlock_holder_type

 * This field is set on an attempt to acquire the exclusive mutex

 * to the respective value depending whether the caller is a reader

 * or a writer. The field is examined only if the thread had been

 * previously blocked on the exclusive mutex. Thus it is save

 * to store the rwlock type in the thread structure, because

 * each thread can block on only one rwlock at a time.

 */

#include <synch/rwlock.h>

#include <synch/spinlock.h>

#include <synch/mutex.h>

#include <synch/waitq.h>

#include <synch/synch.h>

#include <adt/list.h>

#include <typedefs.h>

#include <arch/asm.h>

#include <arch.h>

#include <proc/thread.h>

#include <panic.h>

#define ALLOW_ALL       0

#define ALLOW_READERS_ONLY  1

static void let_others_in(rwlock_t *rwl, int readers_only);

static void release_spinlock(void *arg);

/** Initialize reader/writer lock

 *

 * Initialize reader/writer lock.

 *

 * @param rwl Reader/Writer lock.

 */

void rwlock_initialize(rwlock_t *rwl) {

    spinlock_initialize(&rwl->lock, "rwlock_t");

    mutex_initialize(&rwl->exclusive);

    rwl->readers_in = 0;

}

/** Acquire reader/writer lock for reading

 *

 * Acquire reader/writer lock for reading.

 * Timeout and willingness to block may be specified.

 *

 * @param rwl Reader/Writer lock.

 * @param usec Timeout in microseconds.

 * @param flags Specify mode of operation.

 *

 * For exact description of possible combinations of

 * usec and flags, see comment for waitq_sleep_timeout().

 *

 * @return See comment for waitq_sleep_timeout().

 */

int _rwlock_write_lock_timeout(rwlock_t *rwl, __u32 usec, int flags)

{

    ipl_t ipl;

    int rc;

    ipl = interrupts_disable();

    spinlock_lock(&THREAD->lock);

    THREAD->rwlock_holder_type = RWLOCK_WRITER;

    spinlock_unlock(&THREAD->lock);

    interrupts_restore(ipl);

    /*

     * Writers take the easy part.

     * They just need to acquire the exclusive mutex.

     */

    rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);

    if (SYNCH_FAILED(rc)) {

        /*

         * Lock operation timed out or was interrupted.

         * The state of rwl is UNKNOWN at this point.

         * No claims about its holder can be made.

         */

        ipl = interrupts_disable();

        spinlock_lock(&rwl->lock);

        /*

         * Now when rwl is locked, we can inspect it again.

         * If it is held by some readers already, we can let

         * readers from the head of the wait queue in.

         */

        if (rwl->readers_in)

            let_others_in(rwl, ALLOW_READERS_ONLY);

        spinlock_unlock(&rwl->lock);

        interrupts_restore(ipl);

    }

    return rc;

}

/** Acquire reader/writer lock for writing

 *

 * Acquire reader/writer lock for writing.

 * Timeout and willingness to block may be specified.

 *

 * @param rwl Reader/Writer lock.

 * @param usec Timeout in microseconds.

 * @param flags Select mode of operation.

 *

 * For exact description of possible combinations of

 * usec and flags, see comment for waitq_sleep_timeout().

 *

 * @return See comment for waitq_sleep_timeout().

 */

int _rwlock_read_lock_timeout(rwlock_t *rwl, __u32 usec, int flags)

{

    int rc;

    ipl_t ipl;

    ipl = interrupts_disable();

    spinlock_lock(&THREAD->lock);

    THREAD->rwlock_holder_type = RWLOCK_READER;

    spinlock_unlock(&THREAD->lock);

    spinlock_lock(&rwl->lock);

    /*

     * Find out whether we can get what we want without blocking.

     */

    rc = mutex_trylock(&rwl->exclusive);

    if (SYNCH_FAILED(rc)) {

        /*

         * 'exclusive' mutex is being held by someone else.

         * If the holder is a reader and there is no one

         * else waiting for it, we can enter the critical

         * section.

         */

        if (rwl->readers_in) {

            spinlock_lock(&rwl->exclusive.sem.wq.lock);

            if (list_empty(&rwl->exclusive.sem.wq.head)) {

                /*

                 * We can enter.

                 */

                spinlock_unlock(&rwl->exclusive.sem.wq.lock);

                goto shortcut;

            }

            spinlock_unlock(&rwl->exclusive.sem.wq.lock);

        }

        /*

         * In order to prevent a race condition when a reader

         * could block another reader at the head of the waitq,

         * we register a function to unlock rwl->lock

         * after this thread is put asleep.

         */

        #ifdef CONFIG_SMP

        thread_register_call_me(release_spinlock, &rwl->lock);

        #else

        thread_register_call_me(release_spinlock, NULL);

        #endif

        rc = _mutex_lock_timeout(&rwl->exclusive, usec, flags);

        switch (rc) {

            case ESYNCH_WOULD_BLOCK:

                /*

                 * release_spinlock() wasn't called

                 */

                thread_register_call_me(NULL, NULL);

                spinlock_unlock(&rwl->lock);

            case ESYNCH_TIMEOUT:

            case ESYNCH_INTERRUPTED:

                /*

                 * The sleep timed out.

                 * We just restore interrupt priority level.

                 */

            case ESYNCH_OK_BLOCKED:    

                /*

                 * We were woken with rwl->readers_in already incremented.

                 * Note that this arrangement avoids race condition between

                 * two concurrent readers. (Race is avoided if 'exclusive' is

                 * locked at the same time as 'readers_in' is incremented.

                 * Same time means both events happen atomically when

                 * rwl->lock is held.)

                 */

                interrupts_restore(ipl);

                break;

            case ESYNCH_OK_ATOMIC:

                panic("_mutex_lock_timeout()==ESYNCH_OK_ATOMIC\n");

                break;

            default:

                panic("invalid ESYNCH\n");

                break;

        }

        return rc;

    }

shortcut:

    /*

     * We can increment readers_in only if we didn't go to sleep.

     * For sleepers, rwlock_let_others_in() will do the job.

     */

    rwl->readers_in++;

    spinlock_unlock(&rwl->lock);

    interrupts_restore(ipl);

    return ESYNCH_OK_ATOMIC;

}

/** Release reader/writer lock held by writer

 *

 * Release reader/writer lock held by writer.

 * Handoff reader/writer lock ownership directly

 * to waiting readers or a writer.

 *

 * @param rwl Reader/Writer lock.

 */

void rwlock_write_unlock(rwlock_t *rwl)

{

    ipl_t ipl;

    ipl = interrupts_disable();

    spinlock_lock(&rwl->lock);

    let_others_in(rwl, ALLOW_ALL);

    spinlock_unlock(&rwl->lock);

    interrupts_restore(ipl);

}

/** Release reader/writer lock held by reader

 *

 * Release reader/writer lock held by reader.

 * Handoff reader/writer lock ownership directly

 * to a waiting writer or don't do anything if more

 * readers poses the lock.

 *

 * @param rwl Reader/Writer lock.

 */

void rwlock_read_unlock(rwlock_t *rwl)

{

    ipl_t ipl;

    ipl = interrupts_disable();

    spinlock_lock(&rwl->lock);

    if (!--rwl->readers_in)

        let_others_in(rwl, ALLOW_ALL);

    spinlock_unlock(&rwl->lock);

    interrupts_restore(ipl);

}

/** Direct handoff of reader/writer lock ownership.

 *

 * Direct handoff of reader/writer lock ownership

 * to waiting readers or a writer.

 *

 * Must be called with rwl->lock locked.

 * Must be called with interrupts_disable()'d.

 *

 * @param rwl Reader/Writer lock.

 * @param readers_only See the description below.

 *

 * If readers_only is false: (unlock scenario)

 * Let the first sleeper on 'exclusive' mutex in, no matter

 * whether it is a reader or a writer. If there are more leading

 * readers in line, let each of them in.

 *

 * Otherwise: (timeout scenario)

 * Let all leading readers in.

 */

void let_others_in(rwlock_t *rwl, int readers_only)

{

    rwlock_type_t type = RWLOCK_NONE;

    thread_t *t = NULL;

    bool one_more = true;

    spinlock_lock(&rwl->exclusive.sem.wq.lock);

    if (!list_empty(&rwl->exclusive.sem.wq.head))

        t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);

    do {

        if (t) {

            spinlock_lock(&t->lock);

            type = t->rwlock_holder_type;

            spinlock_unlock(&t->lock);         

        }

        /*

         * If readers_only is true, we wake all leading readers

         * if and only if rwl is locked by another reader.

         * Assumption: readers_only ==> rwl->readers_in

         */

        if (readers_only && (type != RWLOCK_READER))

            break;

        if (type == RWLOCK_READER) {

            /*

             * Waking up a reader.

             * We are responsible for incrementing rwl->readers_in for it.

             */

             rwl->readers_in++;

        }

        /*

         * Only the last iteration through this loop can increment

         * rwl->exclusive.sem.wq.missed_wakeup's. All preceeding

         * iterations will wake up a thread.

         */

        /* We call the internal version of waitq_wakeup, which

         * relies on the fact that the waitq is already locked.

         */

        _waitq_wakeup_unsafe(&rwl->exclusive.sem.wq, WAKEUP_FIRST);

        t = NULL;

        if (!list_empty(&rwl->exclusive.sem.wq.head)) {

            t = list_get_instance(rwl->exclusive.sem.wq.head.next, thread_t, wq_link);

            if (t) {

                spinlock_lock(&t->lock);

                if (t->rwlock_holder_type != RWLOCK_READER)

                    one_more = false;

                spinlock_unlock(&t->lock); 

            }

        }

    } while ((type == RWLOCK_READER) && t && one_more);

    spinlock_unlock(&rwl->exclusive.sem.wq.lock);

}

/** Release spinlock callback

 *

 * This is a callback function invoked from the scheduler.

 * The callback is registered in _rwlock_read_lock_timeout().

 *

 * @param arg Spinlock.

 */

void release_spinlock(void *arg)

{

    spinlock_unlock((spinlock_t *) arg);

}

 /** @}

 */