ptrace: reintroduce __ptrace_detach() as a callee of ptrace_exit()

[linux-2.6/mini2440.git] / kernel / futex.c

/*
 *  Fast Userspace Mutexes (which I call "Futexes!").
 *  (C) Rusty Russell, IBM 2002
 *
 *  Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
 *  (C) Copyright 2003 Red Hat Inc, All Rights Reserved
 *
 *  Removed page pinning, fix privately mapped COW pages and other cleanups
 *  (C) Copyright 2003, 2004 Jamie Lokier
 *
 *  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>
 *
 *  PRIVATE futexes by Eric Dumazet
 *  Copyright (C) 2007 Eric Dumazet <dada1@cosmosbay.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 <linux/module.h>
#include <linux/magic.h>
#include <linux/pid.h>
#include <linux/nsproxy.h>

#include <asm/futex.h>

#include "rtmutex_common.h"

int __read_mostly futex_cmpxchg_enabled;

#define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)

/*
 * 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 plist_node_empty(&q->list) || q->lock_ptr == 0.
 * The order of wakup is always to make the first condition true, then
 * wake up q->waiter, then make the second condition true.
 */
struct futex_q {
        struct plist_node list;
        /* There can only be a single waiter */
        wait_queue_head_t waiter;

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

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

        /* Optional priority inheritance state: */
        struct futex_pi_state *pi_state;
        struct task_struct *task;

        /* Bitset for the optional bitmasked wakeup */
        u32 bitset;
};

/*
 * Hash buckets are shared by all the futex_keys that hash to the same
 * location.  Each key may have multiple futex_q structures, one for each task
 * waiting on a futex.
 */
struct futex_hash_bucket {
        spinlock_t lock;
        struct plist_head chain;
};

static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];

/*
 * 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);
}

/*
 * Take a reference to the resource addressed by a key.
 * Can be called while holding spinlocks.
 *
 */
static void get_futex_key_refs(union futex_key *key)
{
        if (!key->both.ptr)
                return;

        switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
        case FUT_OFF_INODE:
                atomic_inc(&key->shared.inode->i_count);
                break;
        case FUT_OFF_MMSHARED:
                atomic_inc(&key->private.mm->mm_count);
                break;
        }
}

/*
 * Drop a reference to the resource addressed by a key.
 * The hash bucket spinlock must not be held.
 */
static void drop_futex_key_refs(union futex_key *key)
{
        if (!key->both.ptr) {
                /* If we're here then we tried to put a key we failed to get */
                WARN_ON_ONCE(1);
                return;
        }

        switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
        case FUT_OFF_INODE:
                iput(key->shared.inode);
                break;
        case FUT_OFF_MMSHARED:
                mmdrop(key->private.mm);
                break;
        }
}

/**
 * get_futex_key - Get parameters which are the keys for a futex.
 * @uaddr: virtual address of the futex
 * @fshared: 0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
 * @key: address where result is stored.
 *
 * Returns a negative error code or 0
 * The key words are stored in *key on success.
 *
 * For shared mappings, it's (page->index, vma->vm_file->f_path.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.
 *
 * lock_page() might sleep, the caller should not hold a spinlock.
 */
static int get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
{
        unsigned long address = (unsigned long)uaddr;
        struct mm_struct *mm = current->mm;
        struct page *page;
        int err;

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

        /*
         * PROCESS_PRIVATE futexes are fast.
         * As the mm cannot disappear under us and the 'key' only needs
         * virtual address, we dont even have to find the underlying vma.
         * Note : We do have to check 'uaddr' is a valid user address,
         *        but access_ok() should be faster than find_vma()
         */
        if (!fshared) {
                if (unlikely(!access_ok(VERIFY_WRITE, uaddr, sizeof(u32))))
                        return -EFAULT;
                key->private.mm = mm;
                key->private.address = address;
                get_futex_key_refs(key);
                return 0;
        }

again:
        err = get_user_pages_fast(address, 1, 0, &page);
        if (err < 0)
                return err;

        lock_page(page);
        if (!page->mapping) {
                unlock_page(page);
                put_page(page);
                goto again;
        }

        /*
         * 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.
         */
        if (PageAnon(page)) {
                key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
                key->private.mm = mm;
                key->private.address = address;
        } else {
                key->both.offset |= FUT_OFF_INODE; /* inode-based key */
                key->shared.inode = page->mapping->host;
                key->shared.pgoff = page->index;
        }

        get_futex_key_refs(key);

        unlock_page(page);
        put_page(page);
        return 0;
}

static inline
void put_futex_key(int fshared, union futex_key *key)
{
        drop_futex_key_refs(key);
}

static u32 cmpxchg_futex_value_locked(u32 __user *uaddr, u32 uval, u32 newval)
{
        u32 curval;

        pagefault_disable();
        curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
        pagefault_enable();

        return curval;
}

static int get_futex_value_locked(u32 *dest, u32 __user *from)
{
        int ret;

        pagefault_disable();
        ret = __copy_from_user_inatomic(dest, from, sizeof(u32));
        pagefault_enable();

        return ret ? -EFAULT : 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 = kzalloc(sizeof(*pi_state), GFP_KERNEL);

        if (!pi_state)
                return -ENOMEM;

        INIT_LIST_HEAD(&pi_state->list);
        /* pi_mutex gets initialized later */
        pi_state->owner = NULL;
        atomic_set(&pi_state->refcount, 1);
        pi_state->key = FUTEX_KEY_INIT;

        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;
        const struct cred *cred = current_cred(), *pcred;

        rcu_read_lock();
        p = find_task_by_vpid(pid);
        if (!p) {
                p = ERR_PTR(-ESRCH);
        } else {
                pcred = __task_cred(p);
                if (cred->euid != pcred->euid &&
                    cred->euid != pcred->uid)
                        p = ERR_PTR(-ESRCH);
                else
                        get_task_struct(p);
        }

        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 = FUTEX_KEY_INIT;

        if (!futex_cmpxchg_enabled)
                return;
        /*
         * 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,
                union futex_key *key, struct futex_pi_state **ps)
{
        struct futex_pi_state *pi_state = NULL;
        struct futex_q *this, *next;
        struct plist_head *head;
        struct task_struct *p;
        pid_t pid = uval & FUTEX_TID_MASK;

        head = &hb->chain;

        plist_for_each_entry_safe(this, next, head, list) {
                if (match_futex(&this->key, 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));
                        WARN_ON(pid && pi_state->owner &&
                                pi_state->owner->pid != pid);

                        atomic_inc(&pi_state->refcount);
                        *ps = 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 TID = 0
         */
        if (!pid)
                return -ESRCH;
        p = futex_find_get_task(pid);
        if (IS_ERR(p))
                return PTR_ERR(p);

        /*
         * We need to look at the task state flags to figure out,
         * whether the task is exiting. To protect against the do_exit
         * change of the task flags, we do this protected by
         * p->pi_lock:
         */
        spin_lock_irq(&p->pi_lock);
        if (unlikely(p->flags & PF_EXITING)) {
                /*
                 * The task is on the way out. When PF_EXITPIDONE is
                 * set, we know that the task has finished the
                 * cleanup:
                 */
                int ret = (p->flags & PF_EXITPIDONE) ? -ESRCH : -EAGAIN;

                spin_unlock_irq(&p->pi_lock);
                put_task_struct(p);
                return ret;
        }

        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 = *key;

        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);

        *ps = 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)
{
        plist_del(&q->list, &q->list.plist);
        /*
         * The lock in wake_up_all() is a crucial memory barrier after the
         * plist_del() and also before assigning to q->lock_ptr.
         */
        wake_up(&q->waiter);
        /*
         * 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() does not prevent this store from moving.
         */
        smp_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;

        spin_lock(&pi_state->pi_mutex.wait_lock);
        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)) {
                int ret = 0;

                newval = FUTEX_WAITERS | task_pid_vnr(new_owner);

                curval = cmpxchg_futex_value_locked(uaddr, uval, newval);

                if (curval == -EFAULT)
                        ret = -EFAULT;
                else if (curval != uval)
                        ret = -EINVAL;
                if (ret) {
                        spin_unlock(&pi_state->pi_mutex.wait_lock);
                        return ret;
                }
        }

        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);

        spin_unlock(&pi_state->pi_mutex.wait_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:
         */
        oldval = cmpxchg_futex_value_locked(uaddr, uval, 0);

        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);
        }
}

static inline void
double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)
{
        spin_unlock(&hb1->lock);
        if (hb1 != hb2)
                spin_unlock(&hb2->lock);
}

/*
 * Wake up waiters matching bitset queued on this futex (uaddr).
 */
static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
{
        struct futex_hash_bucket *hb;
        struct futex_q *this, *next;
        struct plist_head *head;
        union futex_key key = FUTEX_KEY_INIT;
        int ret;

        if (!bitset)
                return -EINVAL;

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

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

        plist_for_each_entry_safe(this, next, head, list) {
                if (match_futex (&this->key, &key)) {
                        if (this->pi_state) {
                                ret = -EINVAL;
                                break;
                        }

                        /* Check if one of the bits is set in both bitsets */
                        if (!(this->bitset & bitset))
                                continue;

                        wake_futex(this);
                        if (++ret >= nr_wake)
                                break;
                }
        }

        spin_unlock(&hb->lock);
        put_futex_key(fshared, &key);
out:
        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, int fshared, u32 __user *uaddr2,
              int nr_wake, int nr_wake2, int op)
{
        union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
        struct futex_hash_bucket *hb1, *hb2;
        struct plist_head *head;
        struct futex_q *this, *next;
        int ret, op_ret;

retry:
        ret = get_futex_key(uaddr1, fshared, &key1);
        if (unlikely(ret != 0))
                goto out;
        ret = get_futex_key(uaddr2, fshared, &key2);
        if (unlikely(ret != 0))
                goto out_put_key1;

        hb1 = hash_futex(&key1);
        hb2 = hash_futex(&key2);

        double_lock_hb(hb1, hb2);
retry_private:
        op_ret = futex_atomic_op_inuser(op, uaddr2);
        if (unlikely(op_ret < 0)) {
                u32 dummy;

                double_unlock_hb(hb1, hb2);

#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_put_keys;
#endif

                if (unlikely(op_ret != -EFAULT)) {
                        ret = op_ret;
                        goto out_put_keys;
                }

                ret = get_user(dummy, uaddr2);
                if (ret)
                        goto out_put_keys;

                if (!fshared)
                        goto retry_private;

                put_futex_key(fshared, &key2);
                put_futex_key(fshared, &key1);
                goto retry;
        }

        head = &hb1->chain;

        plist_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;
                plist_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;
        }

        double_unlock_hb(hb1, hb2);
out_put_keys:
        put_futex_key(fshared, &key2);
out_put_key1:
        put_futex_key(fshared, &key1);
out:
        return ret;
}

/*
 * Requeue all waiters hashed on one physical page to another
 * physical page.
 */
static int futex_requeue(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
                         int nr_wake, int nr_requeue, u32 *cmpval)
{
        union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
        struct futex_hash_bucket *hb1, *hb2;
        struct plist_head *head1;
        struct futex_q *this, *next;
        int ret, drop_count = 0;

retry:
        ret = get_futex_key(uaddr1, fshared, &key1);
        if (unlikely(ret != 0))
                goto out;
        ret = get_futex_key(uaddr2, fshared, &key2);
        if (unlikely(ret != 0))
                goto out_put_key1;

        hb1 = hash_futex(&key1);
        hb2 = hash_futex(&key2);

retry_private:
        double_lock_hb(hb1, hb2);

        if (likely(cmpval != NULL)) {
                u32 curval;

                ret = get_futex_value_locked(&curval, uaddr1);

                if (unlikely(ret)) {
                        double_unlock_hb(hb1, hb2);

                        ret = get_user(curval, uaddr1);
                        if (ret)
                                goto out_put_keys;

                        if (!fshared)
                                goto retry_private;

                        put_futex_key(fshared, &key2);
                        put_futex_key(fshared, &key1);
                        goto retry;
                }
                if (curval != *cmpval) {
                        ret = -EAGAIN;
                        goto out_unlock;
                }
        }

        head1 = &hb1->chain;
        plist_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)) {
                                plist_del(&this->list, &hb1->chain);
                                plist_add(&this->list, &hb2->chain);
                                this->lock_ptr = &hb2->lock;
#ifdef CONFIG_DEBUG_PI_LIST
                                this->list.plist.lock = &hb2->lock;
#endif
                        }
                        this->key = key2;
                        get_futex_key_refs(&key2);
                        drop_count++;

                        if (ret - nr_wake >= nr_requeue)
                                break;
                }
        }

out_unlock:
        double_unlock_hb(hb1, hb2);

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

out_put_keys:
        put_futex_key(fshared, &key2);
out_put_key1:
        put_futex_key(fshared, &key1);
out:
        return ret;
}

/* The key must be already stored in q->key. */
static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
{
        struct futex_hash_bucket *hb;

        init_waitqueue_head(&q->waiter);

        get_futex_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)
{
        int prio;

        /*
         * The priority used to register this element is
         * - either the real thread-priority for the real-time threads
         * (i.e. threads with a priority lower than MAX_RT_PRIO)
         * - or MAX_RT_PRIO for non-RT threads.
         * Thus, all RT-threads are woken first in priority order, and
         * the others are woken last, in FIFO order.
         */
        prio = min(current->normal_prio, MAX_RT_PRIO);

        plist_node_init(&q->list, prio);
#ifdef CONFIG_DEBUG_PI_LIST
        q->list.plist.lock = &hb->lock;
#endif
        plist_add(&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_futex_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.
 */

/* 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 != NULL) {
                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(plist_node_empty(&q->list));
                plist_del(&q->list, &q->list.plist);

                BUG_ON(q->pi_state);

                spin_unlock(lock_ptr);
                ret = 1;
        }

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

/*
 * PI futexes can not be requeued and must remove themself from the
 * hash bucket. The hash bucket lock (i.e. lock_ptr) is held on entry
 * and dropped here.
 */
static void unqueue_me_pi(struct futex_q *q)
{
        WARN_ON(plist_node_empty(&q->list));
        plist_del(&q->list, &q->list.plist);

        BUG_ON(!q->pi_state);
        free_pi_state(q->pi_state);
        q->pi_state = NULL;

        spin_unlock(q->lock_ptr);

        drop_futex_key_refs(&q->key);
}

/*
 * Fixup the pi_state owner with the new owner.
 *
 * Must be called with hash bucket lock held and mm->sem held for non
 * private futexes.
 */
static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
                                struct task_struct *newowner, int fshared)
{
        u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
        struct futex_pi_state *pi_state = q->pi_state;
        struct task_struct *oldowner = pi_state->owner;
        u32 uval, curval, newval;
        int ret;

        /* Owner died? */
        if (!pi_state->owner)
                newtid |= FUTEX_OWNER_DIED;

        /*
         * We are here either because we stole the rtmutex from the
         * pending owner or we are the pending owner which failed to
         * get the rtmutex. We have to replace the pending owner TID
         * in the user space variable. This must be atomic as we have
         * to preserve the owner died bit here.
         *
         * Note: We write the user space value _before_ changing the pi_state
         * because we can fault here. Imagine swapped out pages or a fork
         * that marked all the anonymous memory readonly for cow.
         *
         * Modifying pi_state _before_ the user space value would
         * leave the pi_state in an inconsistent state when we fault
         * here, because we need to drop the hash bucket lock to
         * handle the fault. This might be observed in the PID check
         * in lookup_pi_state.
         */
retry:
        if (get_futex_value_locked(&uval, uaddr))
                goto handle_fault;

        while (1) {
                newval = (uval & FUTEX_OWNER_DIED) | newtid;

                curval = cmpxchg_futex_value_locked(uaddr, uval, newval);

                if (curval == -EFAULT)
                        goto handle_fault;
                if (curval == uval)
                        break;
                uval = curval;
        }

        /*
         * We fixed up user space. Now we need to fix the pi_state
         * itself.
         */
        if (pi_state->owner != NULL) {
                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);
        }

        pi_state->owner = newowner;

        spin_lock_irq(&newowner->pi_lock);
        WARN_ON(!list_empty(&pi_state->list));
        list_add(&pi_state->list, &newowner->pi_state_list);
        spin_unlock_irq(&newowner->pi_lock);
        return 0;

        /*
         * To handle the page fault we need to drop the hash bucket
         * lock here. That gives the other task (either the pending
         * owner itself or the task which stole the rtmutex) the
         * chance to try the fixup of the pi_state. So once we are
         * back from handling the fault we need to check the pi_state
         * after reacquiring the hash bucket lock and before trying to
         * do another fixup. When the fixup has been done already we
         * simply return.
         */
handle_fault:
        spin_unlock(q->lock_ptr);

        ret = get_user(uval, uaddr);

        spin_lock(q->lock_ptr);

        /*
         * Check if someone else fixed it for us:
         */
        if (pi_state->owner != oldowner)
                return 0;

        if (ret)
                return ret;

        goto retry;
}

/*
 * In case we must use restart_block to restart a futex_wait,
 * we encode in the 'flags' shared capability
 */
#define FLAGS_SHARED            0x01
#define FLAGS_CLOCKRT           0x02

static long futex_wait_restart(struct restart_block *restart);

static int futex_wait(u32 __user *uaddr, int fshared,
                      u32 val, ktime_t *abs_time, u32 bitset, int clockrt)
{
        struct task_struct *curr = current;
        struct restart_block *restart;
        DECLARE_WAITQUEUE(wait, curr);
        struct futex_hash_bucket *hb;
        struct futex_q q;
        u32 uval;
        int ret;
        struct hrtimer_sleeper t;
        int rem = 0;

        if (!bitset)
                return -EINVAL;

        q.pi_state = NULL;
        q.bitset = bitset;
retry:
        q.key = FUTEX_KEY_INIT;
        ret = get_futex_key(uaddr, fshared, &q.key);
        if (unlikely(ret != 0))
                goto out;

retry_private:
        hb = queue_lock(&q);

        /*
         * Access the page AFTER the hash-bucket is locked.
         * 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.
         *
         * For shared futexes, 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);

                ret = get_user(uval, uaddr);
                if (ret)
                        goto out_put_key;

                if (!fshared)
                        goto retry_private;

                put_futex_key(fshared, &q.key);
                goto retry;
        }
        ret = -EWOULDBLOCK;
        if (unlikely(uval != val)) {
                queue_unlock(&q, hb);
                goto out_put_key;
        }

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

        /*
         * 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.waiter, &wait);
        /*
         * !plist_node_empty() is safe here without any lock.
         * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
         */
        if (likely(!plist_node_empty(&q.list))) {
                if (!abs_time)
                        schedule();
                else {
                        hrtimer_init_on_stack(&t.timer,
                                              clockrt ? CLOCK_REALTIME :
                                              CLOCK_MONOTONIC,
                                              HRTIMER_MODE_ABS);
                        hrtimer_init_sleeper(&t, current);
                        hrtimer_set_expires_range_ns(&t.timer, *abs_time,
                                                     current->timer_slack_ns);

                        hrtimer_start_expires(&t.timer, HRTIMER_MODE_ABS);
                        if (!hrtimer_active(&t.timer))
                                t.task = NULL;

                        /*
                         * the timer could have already expired, in which
                         * case current would be flagged for rescheduling.
                         * Don't bother calling schedule.
                         */
                        if (likely(t.task))
                                schedule();

                        hrtimer_cancel(&t.timer);

                        /* Flag if a timeout occured */
                        rem = (t.task == NULL);

                        destroy_hrtimer_on_stack(&t.timer);
                }
        }
        __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. */
        ret = 0;
        if (!unqueue_me(&q))
                goto out_put_key;
        ret = -ETIMEDOUT;
        if (rem)
                goto out_put_key;

        /*
         * We expect signal_pending(current), but another thread may
         * have handled it for us already.
         */
        ret = -ERESTARTSYS;
        if (!abs_time)
                goto out_put_key;

        restart = &current_thread_info()->restart_block;
        restart->fn = futex_wait_restart;
        restart->futex.uaddr = (u32 *)uaddr;
        restart->futex.val = val;
        restart->futex.time = abs_time->tv64;
        restart->futex.bitset = bitset;
        restart->futex.flags = 0;

        if (fshared)
                restart->futex.flags |= FLAGS_SHARED;
        if (clockrt)
                restart->futex.flags |= FLAGS_CLOCKRT;

        ret = -ERESTART_RESTARTBLOCK;

out_put_key:
        put_futex_key(fshared, &q.key);
out:
        return ret;
}


static long futex_wait_restart(struct restart_block *restart)
{
        u32 __user *uaddr = (u32 __user *)restart->futex.uaddr;
        int fshared = 0;
        ktime_t t;

        t.tv64 = restart->futex.time;
        restart->fn = do_no_restart_syscall;
        if (restart->futex.flags & FLAGS_SHARED)
                fshared = 1;
        return (long)futex_wait(uaddr, fshared, restart->futex.val, &t,
                                restart->futex.bitset,
                                restart->futex.flags & FLAGS_CLOCKRT);
}


/*
 * 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 fshared,
                         int detect, ktime_t *time, 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, lock_taken, ownerdied = 0;

        if (refill_pi_state_cache())
                return -ENOMEM;

        if (time) {
                to = &timeout;
                hrtimer_init_on_stack(&to->timer, CLOCK_REALTIME,
                                      HRTIMER_MODE_ABS);
                hrtimer_init_sleeper(to, current);
                hrtimer_set_expires(&to->timer, *time);
        }

        q.pi_state = NULL;
retry:
        q.key = FUTEX_KEY_INIT;
        ret = get_futex_key(uaddr, fshared, &q.key);
        if (unlikely(ret != 0))
                goto out;

retry_private:
        hb = queue_lock(&q);

retry_locked:
        ret = lock_taken = 0;

        /*
         * 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 = task_pid_vnr(current);

        curval = cmpxchg_futex_value_locked(uaddr, 0, newval);

        if (unlikely(curval == -EFAULT))
                goto uaddr_faulted;

        /*
         * Detect deadlocks. In case of REQUEUE_PI this is a valid
         * situation and we return success to user space.
         */
        if (unlikely((curval & FUTEX_TID_MASK) == task_pid_vnr(current))) {
                ret = -EDEADLK;
                goto out_unlock_put_key;
        }

        /*
         * Surprise - we got the lock. Just return to userspace:
         */
        if (unlikely(!curval))
                goto out_unlock_put_key;

        uval = curval;

        /*
         * Set the WAITERS flag, so the owner will know it has someone
         * to wake at next unlock
         */
        newval = curval | FUTEX_WAITERS;

        /*
         * There are two cases, where a futex might have no owner (the
         * owner TID is 0): OWNER_DIED. We take over the futex in this
         * case. We also do an unconditional take over, when the owner
         * of the futex died.
         *
         * This is safe as we are protected by the hash bucket lock !
         */
        if (unlikely(ownerdied || !(curval & FUTEX_TID_MASK))) {
                /* Keep the OWNER_DIED bit */
                newval = (curval & ~FUTEX_TID_MASK) | task_pid_vnr(current);
                ownerdied = 0;
                lock_taken = 1;
        }

        curval = cmpxchg_futex_value_locked(uaddr, uval, newval);

        if (unlikely(curval == -EFAULT))
                goto uaddr_faulted;
        if (unlikely(curval != uval))
                goto retry_locked;

        /*
         * We took the lock due to owner died take over.
         */
        if (unlikely(lock_taken))
                goto out_unlock_put_key;

        /*
         * 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.key, &q.pi_state);

        if (unlikely(ret)) {
                switch (ret) {

                case -EAGAIN:
                        /*
                         * Task is exiting and we just wait for the
                         * exit to complete.
                         */
                        queue_unlock(&q, hb);
                        put_futex_key(fshared, &q.key);
                        cond_resched();
                        goto retry;

                case -ESRCH:
                        /*
                         * No owner found for this futex. Check if the
                         * OWNER_DIED bit is set to figure out whether
                         * this is a robust futex or not.
                         */
                        if (get_futex_value_locked(&curval, uaddr))
                                goto uaddr_faulted;

                        /*
                         * We simply start over in case of a robust
                         * futex. The code above will take the futex
                         * and return happy.
                         */
                        if (curval & FUTEX_OWNER_DIED) {
                                ownerdied = 1;
                                goto retry_locked;
                        }
                default:
                        goto out_unlock_put_key;
                }
        }

        /*
         * Only actually queue now that the atomic ops are done:
         */
        queue_me(&q, hb);

        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;
        }

        spin_lock(q.lock_ptr);

        if (!ret) {
                /*
                 * 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 (q.pi_state->owner != curr)
                        ret = fixup_pi_state_owner(uaddr, &q, curr, fshared);
        } else {
                /*
                 * Catch the rare case, where the lock was released
                 * when we were on the way back before we locked the
                 * hash bucket.
                 */
                if (q.pi_state->owner == curr) {
                        /*
                         * Try to get the rt_mutex now. This might
                         * fail as some other task acquired the
                         * rt_mutex after we removed ourself from the
                         * rt_mutex waiters list.
                         */
                        if (rt_mutex_trylock(&q.pi_state->pi_mutex))
                                ret = 0;
                        else {
                                /*
                                 * pi_state is incorrect, some other
                                 * task did a lock steal and we
                                 * returned due to timeout or signal
                                 * without taking the rt_mutex. Too
                                 * late. We can access the
                                 * rt_mutex_owner without locking, as
                                 * the other task is now blocked on
                                 * the hash bucket lock. Fix the state
                                 * up.
                                 */
                                struct task_struct *owner;
                                int res;

                                owner = rt_mutex_owner(&q.pi_state->pi_mutex);
                                res = fixup_pi_state_owner(uaddr, &q, owner,
                                                           fshared);

                                /* propagate -EFAULT, if the fixup failed */
                                if (res)
                                        ret = res;
                        }
                } else {
                        /*
                         * Paranoia check. If we did not take the lock
                         * in the trylock above, then we should not be
                         * the owner of the rtmutex, neither the real
                         * nor the pending one:
                         */
                        if (rt_mutex_owner(&q.pi_state->pi_mutex) == curr)
                                printk(KERN_ERR "futex_lock_pi: ret = %d "
                                       "pi-mutex: %p pi-state %p\n", ret,
                                       q.pi_state->pi_mutex.owner,
                                       q.pi_state->owner);
                }
        }

        /*
         * If fixup_pi_state_owner() faulted and was unable to handle the
         * fault, unlock it and return the fault to userspace.
         */
        if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current))
                rt_mutex_unlock(&q.pi_state->pi_mutex);

        /* Unqueue and drop the lock */
        unqueue_me_pi(&q);

        if (to)
                destroy_hrtimer_on_stack(&to->timer);
        return ret != -EINTR ? ret : -ERESTARTNOINTR;

out_unlock_put_key:
        queue_unlock(&q, hb);

out_put_key:
        put_futex_key(fshared, &q.key);
out:
        if (to)
                destroy_hrtimer_on_stack(&to->timer);
        return ret;

uaddr_faulted:
        /*
         * We have to r/w  *(int __user *)uaddr, and we have to modify it
         * atomically.  Therefore, if we continue to fault after get_user()
         * below, we need to handle the fault ourselves, while still holding
         * the mmap_sem.  This can occur if the uaddr is under contention as
         * we have to drop the mmap_sem in order to call get_user().
         */
        queue_unlock(&q, hb);

        ret = get_user(uval, uaddr);
        if (ret)
                goto out_put_key;

        if (!fshared)
                goto retry_private;

        put_futex_key(fshared, &q.key);
        goto retry;
}


/*
 * 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, int fshared)
{
        struct futex_hash_bucket *hb;
        struct futex_q *this, *next;
        u32 uval;
        struct plist_head *head;
        union futex_key key = FUTEX_KEY_INIT;
        int ret;

retry:
        if (get_user(uval, uaddr))
                return -EFAULT;
        /*
         * We release only a lock we actually own:
         */
        if ((uval & FUTEX_TID_MASK) != task_pid_vnr(current))
                return -EPERM;

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

        hb = hash_futex(&key);
        spin_lock(&hb->lock);

        /*
         * 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))
                uval = cmpxchg_futex_value_locked(uaddr, task_pid_vnr(current), 0);


        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 == task_pid_vnr(current)))
                goto out_unlock;

        /*
         * Ok, other tasks may need to be woken up - check waiters
         * and do the wakeup if necessary:
         */
        head = &hb->chain;

        plist_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);
        put_futex_key(fshared, &key);

out:
        return ret;

pi_faulted:
        /*
         * We have to r/w  *(int __user *)uaddr, and we have to modify it
         * atomically.  Therefore, if we continue to fault after get_user()
         * below, we need to handle the fault ourselves, while still holding
         * the mmap_sem.  This can occur if the uaddr is under contention as
         * we have to drop the mmap_sem in order to call get_user().
         */
        spin_unlock(&hb->lock);
        put_futex_key(fshared, &key);

        ret = get_user(uval, uaddr);
        if (!ret)
                goto retry;

        return ret;
}

/*
 * 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
 */
SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
                size_t, len)
{
        if (!futex_cmpxchg_enabled)
                return -ENOSYS;
        /*
         * 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
 */
SYSCALL_DEFINE3(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;
        const struct cred *cred = current_cred(), *pcred;

        if (!futex_cmpxchg_enabled)
                return -ENOSYS;

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

                ret = -ESRCH;
                rcu_read_lock();
                p = find_task_by_vpid(pid);
                if (!p)
                        goto err_unlock;
                ret = -EPERM;
                pcred = __task_cred(p);
                if (cred->euid != pcred->euid &&
                    cred->euid != pcred->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) == task_pid_vnr(curr)) {
                /*
                 * 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 && (uval & FUTEX_WAITERS))
                        futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);
        }
        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, *next_entry, *pending;
        unsigned int limit = ROBUST_LIST_LIMIT, pi, next_pi, pip;
        unsigned long futex_offset;
        int rc;

        if (!futex_cmpxchg_enabled)
                return;

        /*
         * 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;

        next_entry = NULL;      /* avoid warning with gcc */
        while (entry != &head->list) {
                /*
                 * Fetch the next entry in the list before calling
                 * handle_futex_death:
                 */
                rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
                /*
                 * 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;
                if (rc)
                        return;
                entry = next_entry;
                pi = next_pi;
                /*
                 * Avoid excessively long or circular lists:
                 */
                if (!--limit)
                        break;

                cond_resched();
        }

        if (pending)
                handle_futex_death((void __user *)pending + futex_offset,
                                   curr, pip);
}

long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
                u32 __user *uaddr2, u32 val2, u32 val3)
{
        int clockrt, ret = -ENOSYS;
        int cmd = op & FUTEX_CMD_MASK;
        int fshared = 0;

        if (!(op & FUTEX_PRIVATE_FLAG))
                fshared = 1;

        clockrt = op & FUTEX_CLOCK_REALTIME;
        if (clockrt && cmd != FUTEX_WAIT_BITSET)
                return -ENOSYS;

        switch (cmd) {
        case FUTEX_WAIT:
                val3 = FUTEX_BITSET_MATCH_ANY;
        case FUTEX_WAIT_BITSET:
                ret = futex_wait(uaddr, fshared, val, timeout, val3, clockrt);
                break;
        case FUTEX_WAKE:
                val3 = FUTEX_BITSET_MATCH_ANY;
        case FUTEX_WAKE_BITSET:
                ret = futex_wake(uaddr, fshared, val, val3);
                break;
        case FUTEX_REQUEUE:
                ret = futex_requeue(uaddr, fshared, uaddr2, val, val2, NULL);
                break;
        case FUTEX_CMP_REQUEUE:
                ret = futex_requeue(uaddr, fshared, uaddr2, val, val2, &val3);
                break;
        case FUTEX_WAKE_OP:
                ret = futex_wake_op(uaddr, fshared, uaddr2, val, val2, val3);
                break;
        case FUTEX_LOCK_PI:
                if (futex_cmpxchg_enabled)
                        ret = futex_lock_pi(uaddr, fshared, val, timeout, 0);
                break;
        case FUTEX_UNLOCK_PI:
                if (futex_cmpxchg_enabled)
                        ret = futex_unlock_pi(uaddr, fshared);
                break;
        case FUTEX_TRYLOCK_PI:
                if (futex_cmpxchg_enabled)
                        ret = futex_lock_pi(uaddr, fshared, 0, timeout, 1);
                break;
        default:
                ret = -ENOSYS;
        }
        return ret;
}


SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
                struct timespec __user *, utime, u32 __user *, uaddr2,
                u32, val3)
{
        struct timespec ts;
        ktime_t t, *tp = NULL;
        u32 val2 = 0;
        int cmd = op & FUTEX_CMD_MASK;

        if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI ||
                      cmd == FUTEX_WAIT_BITSET)) {
                if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
                        return -EFAULT;
                if (!timespec_valid(&ts))
                        return -EINVAL;

                t = timespec_to_ktime(ts);
                if (cmd == FUTEX_WAIT)
                        t = ktime_add_safe(ktime_get(), t);
                tp = &t;
        }
        /*
         * requeue parameter in 'utime' if cmd == FUTEX_REQUEUE.
         * number of waiters to wake in 'utime' if cmd == FUTEX_WAKE_OP.
         */
        if (cmd == FUTEX_REQUEUE || cmd == FUTEX_CMP_REQUEUE ||
            cmd == FUTEX_WAKE_OP)
                val2 = (u32) (unsigned long) utime;

        return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
}

static int __init futex_init(void)
{
        u32 curval;
        int i;

        /*
         * This will fail and we want it. Some arch implementations do
         * runtime detection of the futex_atomic_cmpxchg_inatomic()
         * functionality. We want to know that before we call in any
         * of the complex code paths. Also we want to prevent
         * registration of robust lists in that case. NULL is
         * guaranteed to fault and we get -EFAULT on functional
         * implementation, the non functional ones will return
         * -ENOSYS.
         */
        curval = cmpxchg_futex_value_locked(NULL, 0, 0);
        if (curval == -EFAULT)
                futex_cmpxchg_enabled = 1;

        for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
                plist_head_init(&futex_queues[i].chain, &futex_queues[i].lock);
                spin_lock_init(&futex_queues[i].lock);
        }

        return 0;
}
__initcall(futex_init);