/* * 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 * * Robust futex support started by Ingo Molnar * (C) Copyright 2006 Red Hat Inc, All Rights Reserved * Thanks to Thomas Gleixner for suggestions, analysis and fixes. * * PI-futex support started by Ingo Molnar and Thomas Gleixner * Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com> * Copyright (C) 2006 Timesys Corp., Thomas Gleixner <tglx@timesys.com> * * 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> #include <asm/futex.h> #include "rtmutex_common.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 address; struct mm_struct *mm; int offset; } private; struct { unsigned long word; void *ptr; int offset; } both; }; /* * Priority Inheritance state: */ struct futex_pi_state { /* * list of 'owned' pi_state instances - these have to be * cleaned up in do_exit() if the task exits prematurely: */ struct list_head list; /* * The PI object: */ struct rt_mutex pi_mutex; struct task_struct *owner; atomic_t refcount; union futex_key key; }; /* * 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; /* Optional priority inheritance state: */ struct futex_pi_state *pi_state; struct task_struct *task; }; /* * 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(u32 __user *uaddr, union futex_key *key) { unsigned long address = (unsigned long)uaddr; 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 = address % PAGE_SIZE; if (unlikely((key->both.offset % sizeof(u32)) != 0)) return -EINVAL; address -= 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, address); 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.address = address; 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 = (((address - 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. */ err = get_user_pages(current, mm, address, 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(u32 *dest, u32 __user *from) { int ret; inc_preempt_count(); ret = __copy_from_user_inatomic(dest, from, sizeof(u32)); dec_preempt_count(); return ret ? -EFAULT : 0; } /* * Fault handling. Called with current->mm->mmap_sem held. */ static int futex_handle_fault(unsigned long address, int attempt) { struct vm_area_struct * vma; struct mm_struct *mm = current->mm; if (attempt >= 2 || !(vma = find_vma(mm, address)) || vma->vm_start > address || !(vma->vm_flags & VM_WRITE)) return -EFAULT; switch (handle_mm_fault(mm, vma, address, 1)) { case VM_FAULT_MINOR: current->min_flt++; break; case VM_FAULT_MAJOR: current->maj_flt++; break; default: return -EFAULT; } return 0; } /* * PI code: */ static int refill_pi_state_cache(void) { struct futex_pi_state *pi_state; if (likely(current->pi_state_cache)) return 0; pi_state = kmalloc(sizeof(*pi_state), GFP_KERNEL); if (!pi_state) return -ENOMEM; memset(pi_state, 0, sizeof(*pi_state)); INIT_LIST_HEAD(&pi_state->list); /* pi_mutex gets initialized later */ pi_state->owner = NULL; atomic_set(&pi_state->refcount, 1); current->pi_state_cache = pi_state; return 0; } static struct futex_pi_state * alloc_pi_state(void) { struct futex_pi_state *pi_state = current->pi_state_cache; WARN_ON(!pi_state); current->pi_state_cache = NULL; return pi_state; } static void free_pi_state(struct futex_pi_state *pi_state) { if (!atomic_dec_and_test(&pi_state->refcount)) return; /* * If pi_state->owner is NULL, the owner is most probably dying * and has cleaned up the pi_state already */ if (pi_state->owner) { spin_lock_irq(&pi_state->owner->pi_lock); list_del_init(&pi_state->list); spin_unlock_irq(&pi_state->owner->pi_lock); rt_mutex_proxy_unlock(&pi_state->pi_mutex, pi_state->owner); } if (current->pi_state_cache) kfree(pi_state); else { /* * pi_state->list is already empty. * clear pi_state->owner. * refcount is at 0 - put it back to 1. */ pi_state->owner = NULL; atomic_set(&pi_state->refcount, 1); current->pi_state_cache = pi_state; } } /* * Look up the task based on what TID userspace gave us. * We dont trust it. */ static struct task_struct * futex_find_get_task(pid_t pid) { struct task_struct *p; read_lock(&tasklist_lock); p = find_task_by_pid(pid); if (!p) goto out_unlock; if ((current->euid != p->euid) && (current->euid != p->uid)) { p = NULL; goto out_unlock; } if (p->state == EXIT_ZOMBIE || p->exit_state == EXIT_ZOMBIE) { p = NULL; goto out_unlock; } get_task_struct(p); out_unlock: read_unlock(&tasklist_lock); return p; } /* * This task is holding PI mutexes at exit time => bad. * Kernel cleans up PI-state, but userspace is likely hosed. * (Robust-futex cleanup is separate and might save the day for userspace.) */ void exit_pi_state_list(struct task_struct *curr) { struct list_head *next, *head = &curr->pi_state_list; struct futex_pi_state *pi_state; struct futex_hash_bucket *hb; union futex_key key; /* * We are a ZOMBIE and nobody can enqueue itself on * pi_state_list anymore, but we have to be careful * versus waiters unqueueing themselves: */ spin_lock_irq(&curr->pi_lock); while (!list_empty(head)) { next = head->next; pi_state = list_entry(next, struct futex_pi_state, list); key = pi_state->key; hb = hash_futex(&key); spin_unlock_irq(&curr->pi_lock); spin_lock(&hb->lock); spin_lock_irq(&curr->pi_lock); /* * We dropped the pi-lock, so re-check whether this * task still owns the PI-state: */ if (head->next != next) { spin_unlock(&hb->lock); continue; } WARN_ON(pi_state->owner != curr); WARN_ON(list_empty(&pi_state->list)); list_del_init(&pi_state->list); pi_state->owner = NULL; spin_unlock_irq(&curr->pi_lock); rt_mutex_unlock(&pi_state->pi_mutex); spin_unlock(&hb->lock); spin_lock_irq(&curr->pi_lock); } spin_unlock_irq(&curr->pi_lock); } static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, struct futex_q *me) { struct futex_pi_state *pi_state = NULL; struct futex_q *this, *next; struct list_head *head; struct task_struct *p; pid_t pid; head = &hb->chain; list_for_each_entry_safe(this, next, head, list) { if (match_futex(&this->key, &me->key)) { /* * Another waiter already exists - bump up * the refcount and return its pi_state: */ pi_state = this->pi_state; /* * Userspace might have messed up non PI and PI futexes */ if (unlikely(!pi_state)) return -EINVAL; WARN_ON(!atomic_read(&pi_state->refcount)); atomic_inc(&pi_state->refcount); me->pi_state = pi_state; return 0; } } /* * We are the first waiter - try to look up the real owner and attach * the new pi_state to it, but bail out when the owner died bit is set * and TID = 0: */ pid = uval & FUTEX_TID_MASK; if (!pid && (uval & FUTEX_OWNER_DIED)) return -ESRCH; p = futex_find_get_task(pid); if (!p) return -ESRCH; pi_state = alloc_pi_state(); /* * Initialize the pi_mutex in locked state and make 'p' * the owner of it: */ rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p); /* Store the key for possible exit cleanups: */ pi_state->key = me->key; spin_lock_irq(&p->pi_lock); WARN_ON(!list_empty(&pi_state->list)); list_add(&pi_state->list, &p->pi_state_list); pi_state->owner = p; spin_unlock_irq(&p->pi_lock); put_task_struct(p); me->pi_state = pi_state; return 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. * * A memory barrier is required here to prevent the following store * to lock_ptr from getting ahead of the wakeup. Clearing the lock * at the end of wake_up_all() does not prevent this store from * moving. */ wmb(); q->lock_ptr = NULL; } static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this) { struct task_struct *new_owner; struct futex_pi_state *pi_state = this->pi_state; u32 curval, newval; if (!pi_state) return -EINVAL; new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); /* * This happens when we have stolen the lock and the original * pending owner did not enqueue itself back on the rt_mutex. * Thats not a tragedy. We know that way, that a lock waiter * is on the fly. We make the futex_q waiter the pending owner. */ if (!new_owner) new_owner = this->task; /* * We pass it to the next owner. (The WAITERS bit is always * kept enabled while there is PI state around. We must also * preserve the owner died bit.) */ if (!(uval & FUTEX_OWNER_DIED)) { newval = FUTEX_WAITERS | new_owner->pid; inc_preempt_count(); curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval); dec_preempt_count(); if (curval == -EFAULT) return -EFAULT; if (curval != uval) return -EINVAL; } spin_lock_irq(&pi_state->owner->pi_lock); WARN_ON(list_empty(&pi_state->list)); list_del_init(&pi_state->list); spin_unlock_irq(&pi_state->owner->pi_lock); spin_lock_irq(&new_owner->pi_lock); WARN_ON(!list_empty(&pi_state->list)); list_add(&pi_state->list, &new_owner->pi_state_list); pi_state->owner = new_owner; spin_unlock_irq(&new_owner->pi_lock); rt_mutex_unlock(&pi_state->pi_mutex); return 0; } static int unlock_futex_pi(u32 __user *uaddr, u32 uval) { u32 oldval; /* * There is no waiter, so we unlock the futex. The owner died * bit has not to be preserved here. We are the owner: */ inc_preempt_count(); oldval = futex_atomic_cmpxchg_inatomic(uaddr, uval, 0); dec_preempt_count(); if (oldval == -EFAULT) return oldval; if (oldval != uval) return -EAGAIN; return 0; } /* * Express the locking dependencies for lockdep: */ static inline void double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2) { if (hb1 <= hb2) { spin_lock(&hb1->lock); if (hb1 < hb2) spin_lock_nested(&hb2->lock, SINGLE_DEPTH_NESTING); } else { /* hb1 > hb2 */ spin_lock(&hb2->lock); spin_lock_nested(&hb1->lock, SINGLE_DEPTH_NESTING); } } /* * Wake up all waiters hashed on the physical page that is mapped * to this virtual address: */ static int futex_wake(u32 __user *uaddr, int nr_wake) { struct futex_hash_bucket *hb; struct futex_q *this, *next; struct list_head *head; union futex_key key; int ret; down_read(¤t->mm->mmap_sem); ret = get_futex_key(uaddr, &key); if (unlikely(ret != 0)) goto out; hb = hash_futex(&key); spin_lock(&hb->lock); head = &hb->chain; list_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key)) { if (this->pi_state) { ret = -EINVAL; break; } wake_futex(this); if (++ret >= nr_wake) break; } } spin_unlock(&hb->lock); out: up_read(¤t->mm->mmap_sem); return ret; } /* * Wake up all waiters hashed on the physical page that is mapped * to this virtual address: */ static int futex_wake_op(u32 __user *uaddr1, u32 __user *uaddr2, int nr_wake, int nr_wake2, int op) { union futex_key key1, key2; struct futex_hash_bucket *hb1, *hb2; struct list_head *head; struct futex_q *this, *next; int ret, op_ret, attempt = 0; retryfull: 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; hb1 = hash_futex(&key1); hb2 = hash_futex(&key2); retry: double_lock_hb(hb1, hb2); op_ret = futex_atomic_op_inuser(op, uaddr2); if (unlikely(op_ret < 0)) { u32 dummy; spin_unlock(&hb1->lock); if (hb1 != hb2) spin_unlock(&hb2->lock); #ifndef CONFIG_MMU /* * we don't get EFAULT from MMU faults if we don't have an MMU, * but we might get them from range checking */ ret = op_ret; goto out; #endif if (unlikely(op_ret != -EFAULT)) { ret = op_ret; goto out; } /* * futex_atomic_op_inuser needs to both read and write * *(int __user *)uaddr2, but we can't modify it * non-atomically. Therefore, if get_user below is not * enough, we need to handle the fault ourselves, while * still holding the mmap_sem. */ if (attempt++) { if (futex_handle_fault((unsigned long)uaddr2, attempt)) goto out; goto retry; } /* * 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(dummy, uaddr2); if (ret) return ret; goto retryfull; } head = &hb1->chain; list_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key1)) { wake_futex(this); if (++ret >= nr_wake) break; } } if (op_ret > 0) { head = &hb2->chain; op_ret = 0; list_for_each_entry_safe(this, next, head, list) { if (match_futex (&this->key, &key2)) { wake_futex(this); if (++op_ret >= nr_wake2) break; } } ret += op_ret; } spin_unlock(&hb1->lock); if (hb1 != hb2) spin_unlock(&hb2->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(u32 __user *uaddr1, u32 __user *uaddr2, int nr_wake, int nr_requeue, u32 *cmpval) { union futex_key key1, key2; struct futex_hash_bucket *hb1, *hb2; 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; hb1 = hash_futex(&key1); hb2 = hash_futex(&key2); double_lock_hb(hb1, hb2); if (likely(cmpval != NULL)) { u32 curval; ret = get_futex_value_locked(&curval, uaddr1); if (unlikely(ret)) { spin_unlock(&hb1->lock); if (hb1 != hb2) spin_unlock(&hb2->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, uaddr1); if (!ret) goto retry; return ret; } if (curval != *cmpval) { ret = -EAGAIN; goto out_unlock; } } head1 = &hb1->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 { /* * If key1 and key2 hash to the same bucket, no need to * requeue. */ if (likely(head1 != &hb2->chain)) { list_move_tail(&this->list, &hb2->chain); this->lock_ptr = &hb2->lock; } this->key = key2; get_key_refs(&key2); drop_count++; if (ret - nr_wake >= nr_requeue) break; } } out_unlock: spin_unlock(&hb1->lock); if (hb1 != hb2) spin_unlock(&hb2->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 *hb; q->fd = fd; q->filp = filp; init_waitqueue_head(&q->waiters); get_key_refs(&q->key); hb = hash_futex(&q->key); q->lock_ptr = &hb->lock; spin_lock(&hb->lock); return hb; } static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *hb) { list_add_tail(&q->list, &hb->chain); q->task = current; spin_unlock(&hb->lock); } static inline void queue_unlock(struct futex_q *q, struct futex_hash_bucket *hb) { spin_unlock(&hb->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 *hb; hb = queue_lock(q, fd, filp); __queue_me(q, hb); } /* Return 1 if we were still queued (ie. 0 means we were woken) */ static int unqueue_me(struct futex_q *q) { spinlock_t *lock_ptr; int ret = 0; /* In the common case we don't take the spinlock, which is nice. */ retry: lock_ptr = q->lock_ptr; barrier(); 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); BUG_ON(q->pi_state); spin_unlock(lock_ptr); ret = 1; } drop_key_refs(&q->key); return ret; } /* * PI futexes can not be requeued and must remove themself from the * hash bucket. The hash bucket lock is held on entry and dropped here. */ static void unqueue_me_pi(struct futex_q *q, struct futex_hash_bucket *hb) { WARN_ON(list_empty(&q->list)); list_del(&q->list); BUG_ON(!q->pi_state); free_pi_state(q->pi_state); q->pi_state = NULL; spin_unlock(&hb->lock); drop_key_refs(&q->key); } static int futex_wait(u32 __user *uaddr, u32 val, unsigned long time) { struct task_struct *curr = current; DECLARE_WAITQUEUE(wait, curr); struct futex_hash_bucket *hb; struct futex_q q; u32 uval; int ret; q.pi_state = NULL; retry: down_read(&curr->mm->mmap_sem); ret = get_futex_key(uaddr, &q.key); if (unlikely(ret != 0)) goto out_release_sem; hb = 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(&uval, uaddr); if (unlikely(ret)) { queue_unlock(&q, hb); /* * If we would have faulted, release mmap_sem, fault it in and * start all over again. */ up_read(&curr->mm->mmap_sem); ret = get_user(uval, uaddr); if (!ret) goto retry; return ret; } ret = -EWOULDBLOCK; if (uval != val) goto out_unlock_release_sem; /* Only actually queue if *uaddr contained val. */ __queue_me(&q, hb); /* * Now the futex is queued and we have checked the data, we * don't want to hold mmap_sem while we sleep. */ up_read(&curr->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_unlock_release_sem: queue_unlock(&q, hb); out_release_sem: up_read(&curr->mm->mmap_sem); return ret; } /* * Userspace tried a 0 -> TID atomic transition of the futex value * and failed. The kernel side here does the whole locking operation: * if there are waiters then it will block, it does PI, etc. (Due to * races the kernel might see a 0 value of the futex too.) */ static int do_futex_lock_pi(u32 __user *uaddr, int detect, int trylock, struct hrtimer_sleeper *to) { struct task_struct *curr = current; struct futex_hash_bucket *hb; u32 uval, newval, curval; struct futex_q q; int ret, attempt = 0; if (refill_pi_state_cache()) return -ENOMEM; q.pi_state = NULL; retry: down_read(&curr->mm->mmap_sem); ret = get_futex_key(uaddr, &q.key); if (unlikely(ret != 0)) goto out_release_sem; hb = queue_lock(&q, -1, NULL); retry_locked: /* * To avoid races, we attempt to take the lock here again * (by doing a 0 -> TID atomic cmpxchg), while holding all * the locks. It will most likely not succeed. */ newval = current->pid; inc_preempt_count(); curval = futex_atomic_cmpxchg_inatomic(uaddr, 0, newval); dec_preempt_count(); if (unlikely(curval == -EFAULT)) goto uaddr_faulted; /* We own the lock already */ if (unlikely((curval & FUTEX_TID_MASK) == current->pid)) { if (!detect && 0) force_sig(SIGKILL, current); ret = -EDEADLK; goto out_unlock_release_sem; } /* * Surprise - we got the lock. Just return * to userspace: */ if (unlikely(!curval)) goto out_unlock_release_sem; uval = curval; newval = uval | FUTEX_WAITERS; inc_preempt_count(); curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval); dec_preempt_count(); if (unlikely(curval == -EFAULT)) goto uaddr_faulted; if (unlikely(curval != uval)) goto retry_locked; /* * We dont have the lock. Look up the PI state (or create it if * we are the first waiter): */ ret = lookup_pi_state(uval, hb, &q); if (unlikely(ret)) { /* * There were no waiters and the owner task lookup * failed. When the OWNER_DIED bit is set, then we * know that this is a robust futex and we actually * take the lock. This is safe as we are protected by * the hash bucket lock. We also set the waiters bit * unconditionally here, to simplify glibc handling of * multiple tasks racing to acquire the lock and * cleanup the problems which were left by the dead * owner. */ if (curval & FUTEX_OWNER_DIED) { uval = newval; newval = current->pid | FUTEX_OWNER_DIED | FUTEX_WAITERS; inc_preempt_count(); curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval); dec_preempt_count(); if (unlikely(curval == -EFAULT)) goto uaddr_faulted; if (unlikely(curval != uval)) goto retry_locked; ret = 0; } goto out_unlock_release_sem; } /* * Only actually queue now that the atomic ops are done: */ __queue_me(&q, hb); /* * Now the futex is queued and we have checked the data, we * don't want to hold mmap_sem while we sleep. */ up_read(&curr->mm->mmap_sem); WARN_ON(!q.pi_state); /* * Block on the PI mutex: */ if (!trylock) ret = rt_mutex_timed_lock(&q.pi_state->pi_mutex, to, 1); else { ret = rt_mutex_trylock(&q.pi_state->pi_mutex); /* Fixup the trylock return value: */ ret = ret ? 0 : -EWOULDBLOCK; } down_read(&curr->mm->mmap_sem); spin_lock(q.lock_ptr); /* * Got the lock. We might not be the anticipated owner if we * did a lock-steal - fix up the PI-state in that case. */ if (!ret && q.pi_state->owner != curr) { u32 newtid = current->pid | FUTEX_WAITERS; /* Owner died? */ if (q.pi_state->owner != NULL) { spin_lock_irq(&q.pi_state->owner->pi_lock); WARN_ON(list_empty(&q.pi_state->list)); list_del_init(&q.pi_state->list); spin_unlock_irq(&q.pi_state->owner->pi_lock); } else newtid |= FUTEX_OWNER_DIED; q.pi_state->owner = current; spin_lock_irq(¤t->pi_lock); WARN_ON(!list_empty(&q.pi_state->list)); list_add(&q.pi_state->list, ¤t->pi_state_list); spin_unlock_irq(¤t->pi_lock); /* Unqueue and drop the lock */ unqueue_me_pi(&q, hb); up_read(&curr->mm->mmap_sem); /* * We own it, so we have to replace the pending owner * TID. This must be atomic as we have preserve the * owner died bit here. */ ret = get_user(uval, uaddr); while (!ret) { newval = (uval & FUTEX_OWNER_DIED) | newtid; curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval); if (curval == -EFAULT) ret = -EFAULT; if (curval == uval) break; uval = curval; } } else { /* * Catch the rare case, where the lock was released * when we were on the way back before we locked * the hash bucket. */ if (ret && q.pi_state->owner == curr) { if (rt_mutex_trylock(&q.pi_state->pi_mutex)) ret = 0; } /* Unqueue and drop the lock */ unqueue_me_pi(&q, hb); up_read(&curr->mm->mmap_sem); } if (!detect && ret == -EDEADLK && 0) force_sig(SIGKILL, current); return ret; out_unlock_release_sem: queue_unlock(&q, hb); out_release_sem: up_read(&curr->mm->mmap_sem); return ret; uaddr_faulted: /* * We have to r/w *(int __user *)uaddr, but we can't modify it * non-atomically. Therefore, if get_user below is not * enough, we need to handle the fault ourselves, while * still holding the mmap_sem. */ if (attempt++) { if (futex_handle_fault((unsigned long)uaddr, attempt)) goto out_unlock_release_sem; goto retry_locked; } queue_unlock(&q, hb); up_read(&curr->mm->mmap_sem); ret = get_user(uval, uaddr); if (!ret && (uval != -EFAULT)) goto retry; return ret; } /* * Restart handler */ static long futex_lock_pi_restart(struct restart_block *restart) { struct hrtimer_sleeper timeout, *to = NULL; int ret; restart->fn = do_no_restart_syscall; if (restart->arg2 || restart->arg3) { to = &timeout; hrtimer_init(&to->timer, CLOCK_REALTIME, HRTIMER_ABS); hrtimer_init_sleeper(to, current); to->timer.expires.tv64 = ((u64)restart->arg1 << 32) | (u64) restart->arg0; } pr_debug("lock_pi restart: %p, %d (%d)\n", (u32 __user *)restart->arg0, current->pid); ret = do_futex_lock_pi((u32 __user *)restart->arg0, restart->arg1, 0, to); if (ret != -EINTR) return ret; restart->fn = futex_lock_pi_restart; /* The other values are filled in */ return -ERESTART_RESTARTBLOCK; } /* * Called from the syscall entry below. */ static int futex_lock_pi(u32 __user *uaddr, int detect, unsigned long sec, long nsec, int trylock) { struct hrtimer_sleeper timeout, *to = NULL; struct restart_block *restart; int ret; if (sec != MAX_SCHEDULE_TIMEOUT) { to = &timeout; hrtimer_init(&to->timer, CLOCK_REALTIME, HRTIMER_ABS); hrtimer_init_sleeper(to, current); to->timer.expires = ktime_set(sec, nsec); } ret = do_futex_lock_pi(uaddr, detect, trylock, to); if (ret != -EINTR) return ret; pr_debug("lock_pi interrupted: %p, %d (%d)\n", uaddr, current->pid); restart = ¤t_thread_info()->restart_block; restart->fn = futex_lock_pi_restart; restart->arg0 = (unsigned long) uaddr; restart->arg1 = detect; if (to) { restart->arg2 = to->timer.expires.tv64 & 0xFFFFFFFF; restart->arg3 = to->timer.expires.tv64 >> 32; } else restart->arg2 = restart->arg3 = 0; return -ERESTART_RESTARTBLOCK; } /* * Userspace attempted a TID -> 0 atomic transition, and failed. * This is the in-kernel slowpath: we look up the PI state (if any), * and do the rt-mutex unlock. */ static int futex_unlock_pi(u32 __user *uaddr) { struct futex_hash_bucket *hb; struct futex_q *this, *next; u32 uval; struct list_head *head; union futex_key key; int ret, attempt = 0; retry: if (get_user(uval, uaddr)) return -EFAULT; /* * We release only a lock we actually own: */ if ((uval & FUTEX_TID_MASK) != current->pid) return -EPERM; /* * First take all the futex related locks: */ down_read(¤t->mm->mmap_sem); ret = get_futex_key(uaddr, &key); if (unlikely(ret != 0)) goto out; hb = hash_futex(&key); spin_lock(&hb->lock); retry_locked: /* * To avoid races, try to do the TID -> 0 atomic transition * again. If it succeeds then we can return without waking * anyone else up: */ if (!(uval & FUTEX_OWNER_DIED)) { inc_preempt_count(); uval = futex_atomic_cmpxchg_inatomic(uaddr, current->pid, 0); dec_preempt_count(); } if (unlikely(uval == -EFAULT)) goto pi_faulted; /* * Rare case: we managed to release the lock atomically, * no need to wake anyone else up: */ if (unlikely(uval == current->pid)) goto out_unlock; /* * Ok, other tasks may need to be woken up - check waiters * and do the wakeup if necessary: */ head = &hb->chain; list_for_each_entry_safe(this, next, head, list) { if (!match_futex (&this->key, &key)) continue; ret = wake_futex_pi(uaddr, uval, this); /* * The atomic access to the futex value * generated a pagefault, so retry the * user-access and the wakeup: */ if (ret == -EFAULT) goto pi_faulted; goto out_unlock; } /* * No waiters - kernel unlocks the futex: */ if (!(uval & FUTEX_OWNER_DIED)) { ret = unlock_futex_pi(uaddr, uval); if (ret == -EFAULT) goto pi_faulted; } out_unlock: spin_unlock(&hb->lock); out: up_read(¤t->mm->mmap_sem); return ret; pi_faulted: /* * We have to r/w *(int __user *)uaddr, but we can't modify it * non-atomically. Therefore, if get_user below is not * enough, we need to handle the fault ourselves, while * still holding the mmap_sem. */ if (attempt++) { if (futex_handle_fault((unsigned long)uaddr, attempt)) goto out_unlock; goto retry_locked; } spin_unlock(&hb->lock); up_read(¤t->mm->mmap_sem); ret = get_user(uval, uaddr); if (!ret && (uval != -EFAULT)) goto retry; 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(u32 __user *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) { err = f_setown(filp, current->pid, 1); if (err < 0) { goto error; } filp->f_owner.signum = signal; } q = kmalloc(sizeof(*q), GFP_KERNEL); if (!q) { err = -ENOMEM; goto error; } q->pi_state = NULL; down_read(¤t->mm->mmap_sem); err = get_futex_key(uaddr, &q->key); if (unlikely(err != 0)) { up_read(¤t->mm->mmap_sem); kfree(q); goto error; } /* * 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; error: put_unused_fd(ret); put_filp(filp); ret = err; goto out; } /* * Support for robust futexes: the kernel cleans up held futexes at * thread exit time. * * Implementation: user-space maintains a per-thread list of locks it * is holding. Upon do_exit(), the kernel carefully walks this list, * and marks all locks that are owned by this thread with the * FUTEX_OWNER_DIED bit, and wakes up a waiter (if any). The list is * always manipulated with the lock held, so the list is private and * per-thread. Userspace also maintains a per-thread 'list_op_pending' * field, to allow the kernel to clean up if the thread dies after * acquiring the lock, but just before it could have added itself to * the list. There can only be one such pending lock. */ /** * sys_set_robust_list - set the robust-futex list head of a task * @head: pointer to the list-head * @len: length of the list-head, as userspace expects */ asmlinkage long sys_set_robust_list(struct robust_list_head __user *head, size_t len) { /* * The kernel knows only one size for now: */ if (unlikely(len != sizeof(*head))) return -EINVAL; current->robust_list = head; return 0; } /** * sys_get_robust_list - get the robust-futex list head of a task * @pid: pid of the process [zero for current task] * @head_ptr: pointer to a list-head pointer, the kernel fills it in * @len_ptr: pointer to a length field, the kernel fills in the header size */ asmlinkage long sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr, size_t __user *len_ptr) { struct robust_list_head *head; unsigned long ret; if (!pid) head = current->robust_list; else { struct task_struct *p; ret = -ESRCH; read_lock(&tasklist_lock); p = find_task_by_pid(pid); if (!p) goto err_unlock; ret = -EPERM; if ((current->euid != p->euid) && (current->euid != p->uid) && !capable(CAP_SYS_PTRACE)) goto err_unlock; head = p->robust_list; read_unlock(&tasklist_lock); } if (put_user(sizeof(*head), len_ptr)) return -EFAULT; return put_user(head, head_ptr); err_unlock: read_unlock(&tasklist_lock); return ret; } /* * Process a futex-list entry, check whether it's owned by the * dying task, and do notification if so: */ int handle_futex_death(u32 __user *uaddr, struct task_struct *curr, int pi) { u32 uval, nval, mval; retry: if (get_user(uval, uaddr)) return -1; if ((uval & FUTEX_TID_MASK) == curr->pid) { /* * Ok, this dying thread is truly holding a futex * of interest. Set the OWNER_DIED bit atomically * via cmpxchg, and if the value had FUTEX_WAITERS * set, wake up a waiter (if any). (We have to do a * futex_wake() even if OWNER_DIED is already set - * to handle the rare but possible case of recursive * thread-death.) The rest of the cleanup is done in * userspace. */ mval = (uval & FUTEX_WAITERS) | FUTEX_OWNER_DIED; nval = futex_atomic_cmpxchg_inatomic(uaddr, uval, mval); if (nval == -EFAULT) return -1; if (nval != uval) goto retry; /* * Wake robust non-PI futexes here. The wakeup of * PI futexes happens in exit_pi_state(): */ if (!pi) { if (uval & FUTEX_WAITERS) futex_wake(uaddr, 1); } } return 0; } /* * Fetch a robust-list pointer. Bit 0 signals PI futexes: */ static inline int fetch_robust_entry(struct robust_list __user **entry, struct robust_list __user **head, int *pi) { unsigned long uentry; if (get_user(uentry, (unsigned long *)head)) return -EFAULT; *entry = (void *)(uentry & ~1UL); *pi = uentry & 1; return 0; } /* * Walk curr->robust_list (very carefully, it's a userspace list!) * and mark any locks found there dead, and notify any waiters. * * We silently return on any sign of list-walking problem. */ void exit_robust_list(struct task_struct *curr) { struct robust_list_head __user *head = curr->robust_list; struct robust_list __user *entry, *pending; unsigned int limit = ROBUST_LIST_LIMIT, pi, pip; unsigned long futex_offset; /* * Fetch the list head (which was registered earlier, via * sys_set_robust_list()): */ if (fetch_robust_entry(&entry, &head->list.next, &pi)) return; /* * Fetch the relative futex offset: */ if (get_user(futex_offset, &head->futex_offset)) return; /* * Fetch any possibly pending lock-add first, and handle it * if it exists: */ if (fetch_robust_entry(&pending, &head->list_op_pending, &pip)) return; if (pending) handle_futex_death((void *)pending + futex_offset, curr, pip); while (entry != &head->list) { /* * A pending lock might already be on the list, so * don't process it twice: */ if (entry != pending) if (handle_futex_death((void *)entry + futex_offset, curr, pi)) return; /* * Fetch the next entry in the list: */ if (fetch_robust_entry(&entry, &entry->next, &pi)) return; /* * Avoid excessively long or circular lists: */ if (!--limit) break; cond_resched(); } } long do_futex(u32 __user *uaddr, int op, u32 val, unsigned long timeout, u32 __user *uaddr2, u32 val2, u32 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; case FUTEX_WAKE_OP: ret = futex_wake_op(uaddr, uaddr2, val, val2, val3); break; case FUTEX_LOCK_PI: ret = futex_lock_pi(uaddr, val, timeout, val2, 0); break; case FUTEX_UNLOCK_PI: ret = futex_unlock_pi(uaddr); break; case FUTEX_TRYLOCK_PI: ret = futex_lock_pi(uaddr, 0, timeout, val2, 1); break; default: ret = -ENOSYS; } return ret; } asmlinkage long sys_futex(u32 __user *uaddr, int op, u32 val, struct timespec __user *utime, u32 __user *uaddr2, u32 val3) { struct timespec t; unsigned long timeout = MAX_SCHEDULE_TIMEOUT; u32 val2 = 0; if (utime && (op == FUTEX_WAIT || op == FUTEX_LOCK_PI)) { if (copy_from_user(&t, utime, sizeof(t)) != 0) return -EFAULT; if (!timespec_valid(&t)) return -EINVAL; if (op == FUTEX_WAIT) timeout = timespec_to_jiffies(&t) + 1; else { timeout = t.tv_sec; val2 = t.tv_nsec; } } /* * requeue parameter in 'utime' if op == FUTEX_REQUEUE. */ if (op == FUTEX_REQUEUE || op == FUTEX_CMP_REQUEUE) val2 = (u32) (unsigned long) utime; return do_futex(uaddr, op, val, timeout, uaddr2, val2, val3); } static int futexfs_get_sb(struct file_system_type *fs_type, int flags, const char *dev_name, void *data, struct vfsmount *mnt) { return get_sb_pseudo(fs_type, "futex", NULL, 0xBAD1DEA, mnt); } 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);