source: mainline/generic/src/synch/rwlock.c@ 9179d0a

/*
 * 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.
 */

/**
 * @file        rwlock.c
 * @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 trylock Switches between blocking and non-blocking mode.
 *
 * For exact description of possible combinations of
 * @usec and @trylock, see comment for waitq_sleep_timeout().
 *
 * @return See comment for waitq_sleep_timeout().
 */
int _rwlock_write_lock_timeout(rwlock_t *rwl, __u32 usec, int trylock)
{
        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, trylock);
        if (SYNCH_FAILED(rc)) {

                /*
                 * Lock operation timed out.
                 * 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 trylock Switches between blocking and non-blocking mode.
 *
 * For exact description of possible combinations of
 * usec and trylock, see comment for waitq_sleep_timeout().
 *
 * @return See comment for waitq_sleep_timeout().
 */
int _rwlock_read_lock_timeout(rwlock_t *rwl, __u32 usec, int trylock)
{
        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, trylock);
                switch (rc) {
                        case ESYNCH_WOULD_BLOCK:
                                /*
                                 * release_spinlock() wasn't called
                                 */
                                thread_register_call_me(NULL, NULL);
                                spinlock_unlock(&rwl->lock);
                        case ESYNCH_TIMEOUT:
                                /*
                                 * The sleep timeouted.
                                 * 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);
}