Linux v2.6.13

[linux-2.6/next.git] / kernel / futex.c

/*
 *  Fast Userspace Mutexes (which I call "Futexes!").
 *  (C) Rusty Russell, IBM 2002
 *
 *  Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
 *  (C) Copyright 2003 Red Hat Inc, All Rights Reserved
 *
 *  Removed page pinning, fix privately mapped COW pages and other cleanups
 *  (C) Copyright 2003, 2004 Jamie Lokier
 *
 *  Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
 *  enough at me, Linus for the original (flawed) idea, Matthew
 *  Kirkwood for proof-of-concept implementation.
 *
 *  "The futexes are also cursed."
 *  "But they come in a choice of three flavours!"
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */
#include <linux/slab.h>
#include <linux/poll.h>
#include <linux/fs.h>
#include <linux/file.h>
#include <linux/jhash.h>
#include <linux/init.h>
#include <linux/futex.h>
#include <linux/mount.h>
#include <linux/pagemap.h>
#include <linux/syscalls.h>
#include <linux/signal.h>

#define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)

/*
 * Futexes are matched on equal values of this key.
 * The key type depends on whether it's a shared or private mapping.
 * Don't rearrange members without looking at hash_futex().
 *
 * offset is aligned to a multiple of sizeof(u32) (== 4) by definition.
 * We set bit 0 to indicate if it's an inode-based key.
 */
union futex_key {
        struct {
                unsigned long pgoff;
                struct inode *inode;
                int offset;
        } shared;
        struct {
                unsigned long uaddr;
                struct mm_struct *mm;
                int offset;
        } private;
        struct {
                unsigned long word;
                void *ptr;
                int offset;
        } both;
};

/*
 * We use this hashed waitqueue instead of a normal wait_queue_t, so
 * we can wake only the relevant ones (hashed queues may be shared).
 *
 * A futex_q has a woken state, just like tasks have TASK_RUNNING.
 * It is considered woken when list_empty(&q->list) || q->lock_ptr == 0.
 * The order of wakup is always to make the first condition true, then
 * wake up q->waiters, then make the second condition true.
 */
struct futex_q {
        struct list_head list;
        wait_queue_head_t waiters;

        /* Which hash list lock to use. */
        spinlock_t *lock_ptr;

        /* Key which the futex is hashed on. */
        union futex_key key;

        /* For fd, sigio sent using these. */
        int fd;
        struct file *filp;
};

/*
 * Split the global futex_lock into every hash list lock.
 */
struct futex_hash_bucket {
       spinlock_t              lock;
       struct list_head       chain;
};

static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];

/* Futex-fs vfsmount entry: */
static struct vfsmount *futex_mnt;

/*
 * We hash on the keys returned from get_futex_key (see below).
 */
static struct futex_hash_bucket *hash_futex(union futex_key *key)
{
        u32 hash = jhash2((u32*)&key->both.word,
                          (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
                          key->both.offset);
        return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];
}

/*
 * Return 1 if two futex_keys are equal, 0 otherwise.
 */
static inline int match_futex(union futex_key *key1, union futex_key *key2)
{
        return (key1->both.word == key2->both.word
                && key1->both.ptr == key2->both.ptr
                && key1->both.offset == key2->both.offset);
}

/*
 * Get parameters which are the keys for a futex.
 *
 * For shared mappings, it's (page->index, vma->vm_file->f_dentry->d_inode,
 * offset_within_page).  For private mappings, it's (uaddr, current->mm).
 * We can usually work out the index without swapping in the page.
 *
 * Returns: 0, or negative error code.
 * The key words are stored in *key on success.
 *
 * Should be called with &current->mm->mmap_sem but NOT any spinlocks.
 */
static int get_futex_key(unsigned long uaddr, union futex_key *key)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma;
        struct page *page;
        int err;

        /*
         * The futex address must be "naturally" aligned.
         */
        key->both.offset = uaddr % PAGE_SIZE;
        if (unlikely((key->both.offset % sizeof(u32)) != 0))
                return -EINVAL;
        uaddr -= key->both.offset;

        /*
         * The futex is hashed differently depending on whether
         * it's in a shared or private mapping.  So check vma first.
         */
        vma = find_extend_vma(mm, uaddr);
        if (unlikely(!vma))
                return -EFAULT;

        /*
         * Permissions.
         */
        if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ))
                return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES;

        /*
         * Private mappings are handled in a simple way.
         *
         * NOTE: When userspace waits on a MAP_SHARED mapping, even if
         * it's a read-only handle, it's expected that futexes attach to
         * the object not the particular process.  Therefore we use
         * VM_MAYSHARE here, not VM_SHARED which is restricted to shared
         * mappings of _writable_ handles.
         */
        if (likely(!(vma->vm_flags & VM_MAYSHARE))) {
                key->private.mm = mm;
                key->private.uaddr = uaddr;
                return 0;
        }

        /*
         * Linear file mappings are also simple.
         */
        key->shared.inode = vma->vm_file->f_dentry->d_inode;
        key->both.offset++; /* Bit 0 of offset indicates inode-based key. */
        if (likely(!(vma->vm_flags & VM_NONLINEAR))) {
                key->shared.pgoff = (((uaddr - vma->vm_start) >> PAGE_SHIFT)
                                     + vma->vm_pgoff);
                return 0;
        }

        /*
         * We could walk the page table to read the non-linear
         * pte, and get the page index without fetching the page
         * from swap.  But that's a lot of code to duplicate here
         * for a rare case, so we simply fetch the page.
         */

        /*
         * Do a quick atomic lookup first - this is the fastpath.
         */
        spin_lock(&current->mm->page_table_lock);
        page = follow_page(mm, uaddr, 0);
        if (likely(page != NULL)) {
                key->shared.pgoff =
                        page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
                spin_unlock(&current->mm->page_table_lock);
                return 0;
        }
        spin_unlock(&current->mm->page_table_lock);

        /*
         * Do it the general way.
         */
        err = get_user_pages(current, mm, uaddr, 1, 0, 0, &page, NULL);
        if (err >= 0) {
                key->shared.pgoff =
                        page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
                put_page(page);
                return 0;
        }
        return err;
}

/*
 * Take a reference to the resource addressed by a key.
 * Can be called while holding spinlocks.
 *
 * NOTE: mmap_sem MUST be held between get_futex_key() and calling this
 * function, if it is called at all.  mmap_sem keeps key->shared.inode valid.
 */
static inline void get_key_refs(union futex_key *key)
{
        if (key->both.ptr != 0) {
                if (key->both.offset & 1)
                        atomic_inc(&key->shared.inode->i_count);
                else
                        atomic_inc(&key->private.mm->mm_count);
        }
}

/*
 * Drop a reference to the resource addressed by a key.
 * The hash bucket spinlock must not be held.
 */
static void drop_key_refs(union futex_key *key)
{
        if (key->both.ptr != 0) {
                if (key->both.offset & 1)
                        iput(key->shared.inode);
                else
                        mmdrop(key->private.mm);
        }
}

static inline int get_futex_value_locked(int *dest, int __user *from)
{
        int ret;

        inc_preempt_count();
        ret = __copy_from_user_inatomic(dest, from, sizeof(int));
        dec_preempt_count();

        return ret ? -EFAULT : 0;
}

/*
 * The hash bucket lock must be held when this is called.
 * Afterwards, the futex_q must not be accessed.
 */
static void wake_futex(struct futex_q *q)
{
        list_del_init(&q->list);
        if (q->filp)
                send_sigio(&q->filp->f_owner, q->fd, POLL_IN);
        /*
         * The lock in wake_up_all() is a crucial memory barrier after the
         * list_del_init() and also before assigning to q->lock_ptr.
         */
        wake_up_all(&q->waiters);
        /*
         * The waiting task can free the futex_q as soon as this is written,
         * without taking any locks.  This must come last.
         */
        q->lock_ptr = NULL;
}

/*
 * Wake up all waiters hashed on the physical page that is mapped
 * to this virtual address:
 */
static int futex_wake(unsigned long uaddr, int nr_wake)
{
        union futex_key key;
        struct futex_hash_bucket *bh;
        struct list_head *head;
        struct futex_q *this, *next;
        int ret;

        down_read(&current->mm->mmap_sem);

        ret = get_futex_key(uaddr, &key);
        if (unlikely(ret != 0))
                goto out;

        bh = hash_futex(&key);
        spin_lock(&bh->lock);
        head = &bh->chain;

        list_for_each_entry_safe(this, next, head, list) {
                if (match_futex (&this->key, &key)) {
                        wake_futex(this);
                        if (++ret >= nr_wake)
                                break;
                }
        }

        spin_unlock(&bh->lock);
out:
        up_read(&current->mm->mmap_sem);
        return ret;
}

/*
 * Requeue all waiters hashed on one physical page to another
 * physical page.
 */
static int futex_requeue(unsigned long uaddr1, unsigned long uaddr2,
                         int nr_wake, int nr_requeue, int *valp)
{
        union futex_key key1, key2;
        struct futex_hash_bucket *bh1, *bh2;
        struct list_head *head1;
        struct futex_q *this, *next;
        int ret, drop_count = 0;

 retry:
        down_read(&current->mm->mmap_sem);

        ret = get_futex_key(uaddr1, &key1);
        if (unlikely(ret != 0))
                goto out;
        ret = get_futex_key(uaddr2, &key2);
        if (unlikely(ret != 0))
                goto out;

        bh1 = hash_futex(&key1);
        bh2 = hash_futex(&key2);

        if (bh1 < bh2)
                spin_lock(&bh1->lock);
        spin_lock(&bh2->lock);
        if (bh1 > bh2)
                spin_lock(&bh1->lock);

        if (likely(valp != NULL)) {
                int curval;

                ret = get_futex_value_locked(&curval, (int __user *)uaddr1);

                if (unlikely(ret)) {
                        spin_unlock(&bh1->lock);
                        if (bh1 != bh2)
                                spin_unlock(&bh2->lock);

                        /* If we would have faulted, release mmap_sem, fault
                         * it in and start all over again.
                         */
                        up_read(&current->mm->mmap_sem);

                        ret = get_user(curval, (int __user *)uaddr1);

                        if (!ret)
                                goto retry;

                        return ret;
                }
                if (curval != *valp) {
                        ret = -EAGAIN;
                        goto out_unlock;
                }
        }

        head1 = &bh1->chain;
        list_for_each_entry_safe(this, next, head1, list) {
                if (!match_futex (&this->key, &key1))
                        continue;
                if (++ret <= nr_wake) {
                        wake_futex(this);
                } else {
                        list_move_tail(&this->list, &bh2->chain);
                        this->lock_ptr = &bh2->lock;
                        this->key = key2;
                        get_key_refs(&key2);
                        drop_count++;

                        if (ret - nr_wake >= nr_requeue)
                                break;
                        /* Make sure to stop if key1 == key2 */
                        if (head1 == &bh2->chain && head1 != &next->list)
                                head1 = &this->list;
                }
        }

out_unlock:
        spin_unlock(&bh1->lock);
        if (bh1 != bh2)
                spin_unlock(&bh2->lock);

        /* drop_key_refs() must be called outside the spinlocks. */
        while (--drop_count >= 0)
                drop_key_refs(&key1);

out:
        up_read(&current->mm->mmap_sem);
        return ret;
}

/* The key must be already stored in q->key. */
static inline struct futex_hash_bucket *
queue_lock(struct futex_q *q, int fd, struct file *filp)
{
        struct futex_hash_bucket *bh;

        q->fd = fd;
        q->filp = filp;

        init_waitqueue_head(&q->waiters);

        get_key_refs(&q->key);
        bh = hash_futex(&q->key);
        q->lock_ptr = &bh->lock;

        spin_lock(&bh->lock);
        return bh;
}

static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *bh)
{
        list_add_tail(&q->list, &bh->chain);
        spin_unlock(&bh->lock);
}

static inline void
queue_unlock(struct futex_q *q, struct futex_hash_bucket *bh)
{
        spin_unlock(&bh->lock);
        drop_key_refs(&q->key);
}

/*
 * queue_me and unqueue_me must be called as a pair, each
 * exactly once.  They are called with the hashed spinlock held.
 */

/* The key must be already stored in q->key. */
static void queue_me(struct futex_q *q, int fd, struct file *filp)
{
        struct futex_hash_bucket *bh;
        bh = queue_lock(q, fd, filp);
        __queue_me(q, bh);
}

/* Return 1 if we were still queued (ie. 0 means we were woken) */
static int unqueue_me(struct futex_q *q)
{
        int ret = 0;
        spinlock_t *lock_ptr;

        /* In the common case we don't take the spinlock, which is nice. */
 retry:
        lock_ptr = q->lock_ptr;
        if (lock_ptr != 0) {
                spin_lock(lock_ptr);
                /*
                 * q->lock_ptr can change between reading it and
                 * spin_lock(), causing us to take the wrong lock.  This
                 * corrects the race condition.
                 *
                 * Reasoning goes like this: if we have the wrong lock,
                 * q->lock_ptr must have changed (maybe several times)
                 * between reading it and the spin_lock().  It can
                 * change again after the spin_lock() but only if it was
                 * already changed before the spin_lock().  It cannot,
                 * however, change back to the original value.  Therefore
                 * we can detect whether we acquired the correct lock.
                 */
                if (unlikely(lock_ptr != q->lock_ptr)) {
                        spin_unlock(lock_ptr);
                        goto retry;
                }
                WARN_ON(list_empty(&q->list));
                list_del(&q->list);
                spin_unlock(lock_ptr);
                ret = 1;
        }

        drop_key_refs(&q->key);
        return ret;
}

static int futex_wait(unsigned long uaddr, int val, unsigned long time)
{
        DECLARE_WAITQUEUE(wait, current);
        int ret, curval;
        struct futex_q q;
        struct futex_hash_bucket *bh;

 retry:
        down_read(&current->mm->mmap_sem);

        ret = get_futex_key(uaddr, &q.key);
        if (unlikely(ret != 0))
                goto out_release_sem;

        bh = queue_lock(&q, -1, NULL);

        /*
         * Access the page AFTER the futex is queued.
         * Order is important:
         *
         *   Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
         *   Userspace waker:  if (cond(var)) { var = new; futex_wake(&var); }
         *
         * The basic logical guarantee of a futex is that it blocks ONLY
         * if cond(var) is known to be true at the time of blocking, for
         * any cond.  If we queued after testing *uaddr, that would open
         * a race condition where we could block indefinitely with
         * cond(var) false, which would violate the guarantee.
         *
         * A consequence is that futex_wait() can return zero and absorb
         * a wakeup when *uaddr != val on entry to the syscall.  This is
         * rare, but normal.
         *
         * We hold the mmap semaphore, so the mapping cannot have changed
         * since we looked it up in get_futex_key.
         */

        ret = get_futex_value_locked(&curval, (int __user *)uaddr);

        if (unlikely(ret)) {
                queue_unlock(&q, bh);

                /* If we would have faulted, release mmap_sem, fault it in and
                 * start all over again.
                 */
                up_read(&current->mm->mmap_sem);

                ret = get_user(curval, (int __user *)uaddr);

                if (!ret)
                        goto retry;
                return ret;
        }
        if (curval != val) {
                ret = -EWOULDBLOCK;
                queue_unlock(&q, bh);
                goto out_release_sem;
        }

        /* Only actually queue if *uaddr contained val.  */
        __queue_me(&q, bh);

        /*
         * Now the futex is queued and we have checked the data, we
         * don't want to hold mmap_sem while we sleep.
         */     
        up_read(&current->mm->mmap_sem);

        /*
         * There might have been scheduling since the queue_me(), as we
         * cannot hold a spinlock across the get_user() in case it
         * faults, and we cannot just set TASK_INTERRUPTIBLE state when
         * queueing ourselves into the futex hash.  This code thus has to
         * rely on the futex_wake() code removing us from hash when it
         * wakes us up.
         */

        /* add_wait_queue is the barrier after __set_current_state. */
        __set_current_state(TASK_INTERRUPTIBLE);
        add_wait_queue(&q.waiters, &wait);
        /*
         * !list_empty() is safe here without any lock.
         * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
         */
        if (likely(!list_empty(&q.list)))
                time = schedule_timeout(time);
        __set_current_state(TASK_RUNNING);

        /*
         * NOTE: we don't remove ourselves from the waitqueue because
         * we are the only user of it.
         */

        /* If we were woken (and unqueued), we succeeded, whatever. */
        if (!unqueue_me(&q))
                return 0;
        if (time == 0)
                return -ETIMEDOUT;
        /* We expect signal_pending(current), but another thread may
         * have handled it for us already. */
        return -EINTR;

 out_release_sem:
        up_read(&current->mm->mmap_sem);
        return ret;
}

static int futex_close(struct inode *inode, struct file *filp)
{
        struct futex_q *q = filp->private_data;

        unqueue_me(q);
        kfree(q);
        return 0;
}

/* This is one-shot: once it's gone off you need a new fd */
static unsigned int futex_poll(struct file *filp,
                               struct poll_table_struct *wait)
{
        struct futex_q *q = filp->private_data;
        int ret = 0;

        poll_wait(filp, &q->waiters, wait);

        /*
         * list_empty() is safe here without any lock.
         * q->lock_ptr != 0 is not safe, because of ordering against wakeup.
         */
        if (list_empty(&q->list))
                ret = POLLIN | POLLRDNORM;

        return ret;
}

static struct file_operations futex_fops = {
        .release        = futex_close,
        .poll           = futex_poll,
};

/*
 * Signal allows caller to avoid the race which would occur if they
 * set the sigio stuff up afterwards.
 */
static int futex_fd(unsigned long uaddr, int signal)
{
        struct futex_q *q;
        struct file *filp;
        int ret, err;

        ret = -EINVAL;
        if (!valid_signal(signal))
                goto out;

        ret = get_unused_fd();
        if (ret < 0)
                goto out;
        filp = get_empty_filp();
        if (!filp) {
                put_unused_fd(ret);
                ret = -ENFILE;
                goto out;
        }
        filp->f_op = &futex_fops;
        filp->f_vfsmnt = mntget(futex_mnt);
        filp->f_dentry = dget(futex_mnt->mnt_root);
        filp->f_mapping = filp->f_dentry->d_inode->i_mapping;

        if (signal) {
                int err;
                err = f_setown(filp, current->pid, 1);
                if (err < 0) {
                        put_unused_fd(ret);
                        put_filp(filp);
                        ret = err;
                        goto out;
                }
                filp->f_owner.signum = signal;
        }

        q = kmalloc(sizeof(*q), GFP_KERNEL);
        if (!q) {
                put_unused_fd(ret);
                put_filp(filp);
                ret = -ENOMEM;
                goto out;
        }

        down_read(&current->mm->mmap_sem);
        err = get_futex_key(uaddr, &q->key);

        if (unlikely(err != 0)) {
                up_read(&current->mm->mmap_sem);
                put_unused_fd(ret);
                put_filp(filp);
                kfree(q);
                return err;
        }

        /*
         * queue_me() must be called before releasing mmap_sem, because
         * key->shared.inode needs to be referenced while holding it.
         */
        filp->private_data = q;

        queue_me(q, ret, filp);
        up_read(&current->mm->mmap_sem);

        /* Now we map fd to filp, so userspace can access it */
        fd_install(ret, filp);
out:
        return ret;
}

long do_futex(unsigned long uaddr, int op, int val, unsigned long timeout,
                unsigned long uaddr2, int val2, int val3)
{
        int ret;

        switch (op) {
        case FUTEX_WAIT:
                ret = futex_wait(uaddr, val, timeout);
                break;
        case FUTEX_WAKE:
                ret = futex_wake(uaddr, val);
                break;
        case FUTEX_FD:
                /* non-zero val means F_SETOWN(getpid()) & F_SETSIG(val) */
                ret = futex_fd(uaddr, val);
                break;
        case FUTEX_REQUEUE:
                ret = futex_requeue(uaddr, uaddr2, val, val2, NULL);
                break;
        case FUTEX_CMP_REQUEUE:
                ret = futex_requeue(uaddr, uaddr2, val, val2, &val3);
                break;
        default:
                ret = -ENOSYS;
        }
        return ret;
}


asmlinkage long sys_futex(u32 __user *uaddr, int op, int val,
                          struct timespec __user *utime, u32 __user *uaddr2,
                          int val3)
{
        struct timespec t;
        unsigned long timeout = MAX_SCHEDULE_TIMEOUT;
        int val2 = 0;

        if ((op == FUTEX_WAIT) && utime) {
                if (copy_from_user(&t, utime, sizeof(t)) != 0)
                        return -EFAULT;
                timeout = timespec_to_jiffies(&t) + 1;
        }
        /*
         * requeue parameter in 'utime' if op == FUTEX_REQUEUE.
         */
        if (op >= FUTEX_REQUEUE)
                val2 = (int) (unsigned long) utime;

        return do_futex((unsigned long)uaddr, op, val, timeout,
                        (unsigned long)uaddr2, val2, val3);
}

static struct super_block *
futexfs_get_sb(struct file_system_type *fs_type,
               int flags, const char *dev_name, void *data)
{
        return get_sb_pseudo(fs_type, "futex", NULL, 0xBAD1DEA);
}

static struct file_system_type futex_fs_type = {
        .name           = "futexfs",
        .get_sb         = futexfs_get_sb,
        .kill_sb        = kill_anon_super,
};

static int __init init(void)
{
        unsigned int i;

        register_filesystem(&futex_fs_type);
        futex_mnt = kern_mount(&futex_fs_type);

        for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
                INIT_LIST_HEAD(&futex_queues[i].chain);
                spin_lock_init(&futex_queues[i].lock);
        }
        return 0;
}
__initcall(init);