[PATCH] Add lockless helpers for remove_suid()

[linux-2.6] / 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
 *
 *  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 &current->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;

        rcu_read_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->exit_state != 0) {
                p = NULL;
                goto out_unlock;
        }
        get_task_struct(p);
out_unlock:
        rcu_read_unlock();

        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(&current->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(&current->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(&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;

        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)) {
                                ret = -EFAULT;
                                goto out;
                        }
                        goto retry;
                }

                /*
                 * 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(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(&current->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(&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;

        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(&current->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(&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 *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 futex_lock_pi(u32 __user *uaddr, int detect, unsigned long sec,
                         long nsec, int trylock)
{
        struct hrtimer_sleeper timeout, *to = NULL;
        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;

        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);
        }

        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(&current->pi_lock);
                WARN_ON(!list_empty(&q.pi_state->list));
                list_add(&q.pi_state->list, &current->pi_state_list);
                spin_unlock_irq(&current->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 != -EINTR ? ret : -ERESTARTNOINTR;

 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)) {
                        ret = -EFAULT;
                        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;
}

/*
 * 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(&current->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(&current->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)) {
                        ret = -EFAULT;
                        goto out_unlock;
                }
                goto retry_locked;
        }

        spin_unlock(&hb->lock);
        up_read(&current->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, task_pid(current), PIDTYPE_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(&current->mm->mmap_sem);
        err = get_futex_key(uaddr, &q->key);

        if (unlikely(err != 0)) {
                up_read(&current->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(&current->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 * __user *head_ptr,
                    size_t __user *len_ptr)
{
        struct robust_list_head __user *head;
        unsigned long ret;

        if (!pid)
                head = current->robust_list;
        else {
                struct task_struct *p;

                ret = -ESRCH;
                rcu_read_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;
                rcu_read_unlock();
        }

        if (put_user(sizeof(*head), len_ptr))
                return -EFAULT;
        return put_user(head, head_ptr);

err_unlock:
        rcu_read_unlock();

        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 * __user *head,
                                     int *pi)
{
        unsigned long uentry;

        if (get_user(uentry, (unsigned long __user *)head))
                return -EFAULT;

        *entry = (void __user *)(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 __user *)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 __user *)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);