diff options
Diffstat (limited to 'kernel/futex.c')
| -rw-r--r-- | kernel/futex.c | 798 |
1 files changed, 798 insertions, 0 deletions
diff --git a/kernel/futex.c b/kernel/futex.c new file mode 100644 index 000000000000..7b54a672d0ad --- /dev/null +++ b/kernel/futex.c @@ -0,0 +1,798 @@ +/* + * 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> + +#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 ¤t->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(¤t->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(¤t->mm->page_table_lock); + return 0; + } + spin_unlock(¤t->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(¤t->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(¤t->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(¤t->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(¤t->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(¤t->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(¤t->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(¤t->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(¤t->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(¤t->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 (signal < 0 || signal > _NSIG) + 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(¤t->mm->mmap_sem); + err = get_futex_key(uaddr, &q->key); + + if (unlikely(err != 0)) { + up_read(¤t->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(¤t->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); |