/* * 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 ¤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 (!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(¤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);