source: mainline/kernel/generic/src/synch/futex.c@ 4774a32

/*
 * 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.
 */

/** @addtogroup sync
 * @{
 */

/**
 * @file
 * @brief       Kernel backend for futexes.
 */

#include <synch/futex.h>
#include <synch/rwlock.h>
#include <synch/spinlock.h>
#include <synch/synch.h>
#include <mm/frame.h>
#include <mm/page.h>
#include <mm/slab.h>
#include <proc/thread.h>
#include <proc/task.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 futex_t *futex_find(uintptr_t paddr);
static size_t futex_ht_hash(unative_t *key);
static bool futex_ht_compare(unative_t *key, size_t keys, link_t *item);
static void futex_ht_remove_callback(link_t *item);

/**
 * Read-write lock protecting global futex hash table.
 * It is also used to serialize access to all futex_t structures.
 * Must be acquired before the task futex B+tree lock.
 */
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;
        futex->refcount = 1;
}

/** Sleep in futex wait queue.
 *
 * @param uaddr         Userspace address of the futex counter.
 *
 * @return              If there is no physical mapping for uaddr ENOENT is
 *                      returned. Otherwise returns a wait result as defined in
 *                      synch.h.
 */
unative_t sys_futex_sleep(uintptr_t uaddr)
{
        futex_t *futex;
        uintptr_t paddr;
        pte_t *t;
        ipl_t ipl;
        int rc;
        
        ipl = interrupts_disable();

        /*
         * 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);
                interrupts_restore(ipl);
                return (unative_t) ENOENT;
        }
        paddr = PTE_GET_FRAME(t) + (uaddr - ALIGN_DOWN(uaddr, PAGE_SIZE));
        page_table_unlock(AS, true);
        
        interrupts_restore(ipl);        

        futex = futex_find(paddr);

#ifdef CONFIG_UDEBUG
        udebug_stoppable_begin();
#endif
        rc = waitq_sleep_timeout(&futex->wq, 0, SYNCH_FLAGS_INTERRUPTIBLE); 
#ifdef CONFIG_UDEBUG
        udebug_stoppable_end();
#endif
        return (unative_t) rc;
}

/** Wakeup one thread waiting in futex wait queue.
 *
 * @param uaddr         Userspace address of the futex counter.
 *
 * @return              ENOENT if there is no physical mapping for uaddr.
 */
unative_t sys_futex_wakeup(uintptr_t uaddr)
{
        futex_t *futex;
        uintptr_t paddr;
        pte_t *t;
        ipl_t ipl;
        
        ipl = interrupts_disable();
        
        /*
         * 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);
                interrupts_restore(ipl);
                return (unative_t) ENOENT;
        }
        paddr = PTE_GET_FRAME(t) + (uaddr - ALIGN_DOWN(uaddr, PAGE_SIZE));
        page_table_unlock(AS, true);
        
        interrupts_restore(ipl);

        futex = futex_find(paddr);
                
        waitq_wakeup(&futex->wq, WAKEUP_FIRST);
        
        return 0;
}

/** Find kernel address of the futex structure corresponding to paddr.
 *
 * If the structure does not exist already, a new one is created.
 *
 * @param paddr         Physical address of the userspace futex counter.
 *
 * @return              Address of the kernel futex structure.
 */
futex_t *futex_find(uintptr_t paddr)
{
        link_t *item;
        futex_t *futex;
        btree_node_t *leaf;
        
        /*
         * 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);

                /*
                 * See if the current task knows this futex.
                 */
                mutex_lock(&TASK->futexes_lock);
                if (!btree_search(&TASK->futexes, paddr, &leaf)) {
                        /*
                         * The futex is new to the current task.
                         * However, we only have read access.
                         * Gain write access and try again.
                         */
                        mutex_unlock(&TASK->futexes_lock);
                        goto gain_write_access;
                }
                mutex_unlock(&TASK->futexes_lock);

                rwlock_read_unlock(&futex_ht_lock);
        } else {
gain_write_access:
                /*
                 * 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);
                /*
                 * Avoid 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);
                        
                        /*
                         * See if this futex is known to the current task.
                         */
                        mutex_lock(&TASK->futexes_lock);
                        if (!btree_search(&TASK->futexes, paddr, &leaf)) {
                                /*
                                 * The futex is new to the current task.
                                 * Upgrade its reference count and put it to the
                                 * current task's B+tree of known futexes.
                                 */
                                futex->refcount++;
                                btree_insert(&TASK->futexes, paddr, futex,
                                    leaf);
                        }
                        mutex_unlock(&TASK->futexes_lock);
        
                        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);
                        
                        /*
                         * This is the first task referencing the futex.
                         * It can be directly inserted into its
                         * B+tree of known futexes.
                         */
                        mutex_lock(&TASK->futexes_lock);
                        btree_insert(&TASK->futexes, paddr, futex, NULL);
                        mutex_unlock(&TASK->futexes_lock);
                        
                        rwlock_write_unlock(&futex_ht_lock);
                }
        }
        
        return futex;
}

/** 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.
 */
size_t futex_ht_hash(unative_t *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(unative_t *key, size_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);
}

/** Remove references from futexes known to the current task. */
void futex_cleanup(void)
{
        link_t *cur;
        
        rwlock_write_lock(&futex_ht_lock);
        mutex_lock(&TASK->futexes_lock);

        for (cur = TASK->futexes.leaf_head.next;
            cur != &TASK->futexes.leaf_head; cur = cur->next) {
                btree_node_t *node;
                unsigned int i;
                
                node = list_get_instance(cur, btree_node_t, leaf_link);
                for (i = 0; i < node->keys; i++) {
                        futex_t *ftx;
                        uintptr_t paddr = node->key[i];
                        
                        ftx = (futex_t *) node->value[i];
                        if (--ftx->refcount == 0)
                                hash_table_remove(&futex_ht, &paddr, 1);
                }
        }
        
        mutex_unlock(&TASK->futexes_lock);
        rwlock_write_unlock(&futex_ht_lock);
}

/** @}
 */