source: mainline/generic/src/synch/futex.c@ 9aa72b4

/*
 * Copyright (C) 2006 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.
 */

/**
 * Kernel backend for futexes.
 * Deallocation of orphaned kernel-side futex structures is not currently implemented.
 * Timeouting futexes are currently not implemented.
 */

#include <synch/futex.h>
#include <synch/rwlock.h>
#include <synch/synch.h>
#include <mm/frame.h>
#include <mm/page.h>
#include <mm/slab.h>
#include <genarch/mm/page_pt.h>
#include <genarch/mm/page_ht.h>
#include <adt/hash_table.h>
#include <adt/list.h>
#include <arch.h>
#include <align.h>
#include <panic.h>
#include <errno.h>
#include <print.h>

#define FUTEX_HT_SIZE   1024    /* keep it a power of 2 */

static void futex_initialize(futex_t *futex);

static index_t futex_ht_hash(__native *key);
static bool futex_ht_compare(__native *key, count_t keys, link_t *item);
static void futex_ht_remove_callback(link_t *item);

/** Read-write lock protecting global futex hash table. */
static rwlock_t futex_ht_lock;

/** Futex hash table. */
static hash_table_t futex_ht;

/** Futex hash table operations. */
static hash_table_operations_t futex_ht_ops = {
        .hash = futex_ht_hash,
        .compare = futex_ht_compare,
        .remove_callback = futex_ht_remove_callback
};

/** Initialize futex subsystem. */
void futex_init(void)
{
        rwlock_initialize(&futex_ht_lock);
        hash_table_create(&futex_ht, FUTEX_HT_SIZE, 1, &futex_ht_ops);
}

/** Initialize kernel futex structure.
 *
 * @param futex Kernel futex structure.
 */
void futex_initialize(futex_t *futex)
{
        waitq_initialize(&futex->wq);
        link_initialize(&futex->ht_link);
        futex->paddr = 0;
}

/** Sleep in futex wait queue.
 *
 * @param uaddr Userspace address of the futex counter.
 *
 * @return One of ESYNCH_OK_ATOMIC and ESYNCH_OK_BLOCKED. See synch.h.
 *         If there is no physical mapping for uaddr ENOENT is returned.
 */
__native sys_futex_sleep(__address uaddr)
{
        futex_t *futex;
        __address paddr;
        link_t *item;
        pte_t *t;
        
        /*
         * Find physical address of futex counter.
         */
        page_table_lock(AS, true);
        t = page_mapping_find(AS, ALIGN_DOWN(uaddr, PAGE_SIZE));
        if (!t || !PTE_VALID(t) || !PTE_PRESENT(t)) {
                page_table_unlock(AS, true);
                return (__native) ENOENT;
        }
        paddr = PFN2ADDR(PTE_GET_FRAME(t)) + (uaddr - ALIGN_DOWN(uaddr, PAGE_SIZE));
        page_table_unlock(AS, true);
        
        /*
         * Find the respective futex structure
         * or allocate new one if it does not exist already.
         */
        rwlock_read_lock(&futex_ht_lock);
        item = hash_table_find(&futex_ht, &paddr);
        if (item) {
                futex = hash_table_get_instance(item, futex_t, ht_link);
                rwlock_read_unlock(&futex_ht_lock);
        } else {
                /*
                 * Upgrade to writer is not currently supported,
                 * Therefore, it is necessary to release the read lock
                 * and reacquire it as a writer.
                 */
                rwlock_read_unlock(&futex_ht_lock);

                rwlock_write_lock(&futex_ht_lock);
                /*
                 * Detect possible race condition by searching
                 * the hash table once again with write access.
                 */
                item = hash_table_find(&futex_ht, &paddr);
                if (item) {
                        futex = hash_table_get_instance(item, futex_t, ht_link);
                        rwlock_write_unlock(&futex_ht_lock);
                } else {
                        futex = (futex_t *) malloc(sizeof(futex_t), 0);
                        futex_initialize(futex);
                        futex->paddr = paddr;
                        hash_table_insert(&futex_ht, &paddr, &futex->ht_link);
                        rwlock_write_unlock(&futex_ht_lock);
                }
        }
        
        return (__native) waitq_sleep(&futex->wq);
}

/** Wakeup one thread waiting in futex wait queue.
 *
 * @param uaddr Userspace address of the futex counter.
 *
 * @return ENOENT if there is no futex associated with the address
 *         or if there is no physical mapping for uaddr.
 */
__native sys_futex_wakeup(__address uaddr)
{
        futex_t *futex = NULL;
        __address paddr;
        link_t *item;
        pte_t *t;
        
        /*
         * Find physical address of futex counter.
         */
        page_table_lock(AS, true);
        t = page_mapping_find(AS, ALIGN_DOWN(uaddr, PAGE_SIZE));
        if (!t || !PTE_VALID(t) || !PTE_PRESENT(t)) {
                page_table_unlock(AS, true);
                return (__native) ENOENT;
        }
        paddr = PFN2ADDR(PTE_GET_FRAME(t)) + (uaddr - ALIGN_DOWN(uaddr, PAGE_SIZE));
        page_table_unlock(AS, true);
        
        /*
         * Find the respective futex structure.
         */
        rwlock_read_lock(&futex_ht_lock);
        item = hash_table_find(&futex_ht, &paddr);
        if (item) {
                futex = hash_table_get_instance(item, futex_t, ht_link);
        }
        rwlock_read_unlock(&futex_ht_lock);
        
        if (!futex)
                return (__native) ENOENT;       
                
        waitq_wakeup(&futex->wq, WAKEUP_FIRST);
        
        return 0;
}

/** Compute hash index into futex hash table.
 *
 * @param key Address where the key (i.e. physical address of futex counter) is stored.
 *
 * @return Index into futex hash table.
 */
index_t futex_ht_hash(__native *key)
{
        return *key & (FUTEX_HT_SIZE-1);
}

/** Compare futex hash table item with a key.
 *
 * @param key Address where the key (i.e. physical address of futex counter) is stored.
 *
 * @return True if the item matches the key. False otherwise.
 */
bool futex_ht_compare(__native *key, count_t keys, link_t *item)
{
        futex_t *futex;

        ASSERT(keys == 1);

        futex = hash_table_get_instance(item, futex_t, ht_link);
        return *key == futex->paddr;
}

/** Callback for removal items from futex hash table.
 *
 * @param item Item removed from the hash table.
 */
void futex_ht_remove_callback(link_t *item)
{
        futex_t *futex;

        futex = hash_table_get_instance(item, futex_t, ht_link);
        free(futex);
}