Linux 6.14-rc3

[linux.git] / kernel / futex / core.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 *  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 <[email protected]>
 *  Copyright (C) 2006 Timesys Corp., Thomas Gleixner <[email protected]>
 *
 *  PRIVATE futexes by Eric Dumazet
 *  Copyright (C) 2007 Eric Dumazet <[email protected]>
 *
 *  Requeue-PI support by Darren Hart <[email protected]>
 *  Copyright (C) IBM Corporation, 2009
 *  Thanks to Thomas Gleixner for conceptual design and careful reviews.
 *
 *  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!"
 */
#include <linux/compat.h>
#include <linux/jhash.h>
#include <linux/pagemap.h>
#include <linux/debugfs.h>
#include <linux/plist.h>
#include <linux/memblock.h>
#include <linux/fault-inject.h>
#include <linux/slab.h>

#include "futex.h"
#include "../locking/rtmutex_common.h"

/*
 * The base of the bucket array and its size are always used together
 * (after initialization only in futex_hash()), so ensure that they
 * reside in the same cacheline.
 */
static struct {
        struct futex_hash_bucket *queues;
        unsigned long            hashsize;
} __futex_data __read_mostly __aligned(2*sizeof(long));
#define futex_queues   (__futex_data.queues)
#define futex_hashsize (__futex_data.hashsize)


/*
 * Fault injections for futexes.
 */
#ifdef CONFIG_FAIL_FUTEX

static struct {
        struct fault_attr attr;

        bool ignore_private;
} fail_futex = {
        .attr = FAULT_ATTR_INITIALIZER,
        .ignore_private = false,
};

static int __init setup_fail_futex(char *str)
{
        return setup_fault_attr(&fail_futex.attr, str);
}
__setup("fail_futex=", setup_fail_futex);

bool should_fail_futex(bool fshared)
{
        if (fail_futex.ignore_private && !fshared)
                return false;

        return should_fail(&fail_futex.attr, 1);
}

#ifdef CONFIG_FAULT_INJECTION_DEBUG_FS

static int __init fail_futex_debugfs(void)
{
        umode_t mode = S_IFREG | S_IRUSR | S_IWUSR;
        struct dentry *dir;

        dir = fault_create_debugfs_attr("fail_futex", NULL,
                                        &fail_futex.attr);
        if (IS_ERR(dir))
                return PTR_ERR(dir);

        debugfs_create_bool("ignore-private", mode, dir,
                            &fail_futex.ignore_private);
        return 0;
}

late_initcall(fail_futex_debugfs);

#endif /* CONFIG_FAULT_INJECTION_DEBUG_FS */

#endif /* CONFIG_FAIL_FUTEX */

/**
 * futex_hash - Return the hash bucket in the global hash
 * @key:        Pointer to the futex key for which the hash is calculated
 *
 * We hash on the keys returned from get_futex_key (see below) and return the
 * corresponding hash bucket in the global hash.
 */
struct futex_hash_bucket *futex_hash(union futex_key *key)
{
        u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
                          key->both.offset);

        return &futex_queues[hash & (futex_hashsize - 1)];
}


/**
 * futex_setup_timer - set up the sleeping hrtimer.
 * @time:       ptr to the given timeout value
 * @timeout:    the hrtimer_sleeper structure to be set up
 * @flags:      futex flags
 * @range_ns:   optional range in ns
 *
 * Return: Initialized hrtimer_sleeper structure or NULL if no timeout
 *         value given
 */
struct hrtimer_sleeper *
futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
                  int flags, u64 range_ns)
{
        if (!time)
                return NULL;

        hrtimer_setup_sleeper_on_stack(timeout,
                                       (flags & FLAGS_CLOCKRT) ? CLOCK_REALTIME : CLOCK_MONOTONIC,
                                       HRTIMER_MODE_ABS);
        /*
         * If range_ns is 0, calling hrtimer_set_expires_range_ns() is
         * effectively the same as calling hrtimer_set_expires().
         */
        hrtimer_set_expires_range_ns(&timeout->timer, *time, range_ns);

        return timeout;
}

/*
 * Generate a machine wide unique identifier for this inode.
 *
 * This relies on u64 not wrapping in the life-time of the machine; which with
 * 1ns resolution means almost 585 years.
 *
 * This further relies on the fact that a well formed program will not unmap
 * the file while it has a (shared) futex waiting on it. This mapping will have
 * a file reference which pins the mount and inode.
 *
 * If for some reason an inode gets evicted and read back in again, it will get
 * a new sequence number and will _NOT_ match, even though it is the exact same
 * file.
 *
 * It is important that futex_match() will never have a false-positive, esp.
 * for PI futexes that can mess up the state. The above argues that false-negatives
 * are only possible for malformed programs.
 */
static u64 get_inode_sequence_number(struct inode *inode)
{
        static atomic64_t i_seq;
        u64 old;

        /* Does the inode already have a sequence number? */
        old = atomic64_read(&inode->i_sequence);
        if (likely(old))
                return old;

        for (;;) {
                u64 new = atomic64_inc_return(&i_seq);
                if (WARN_ON_ONCE(!new))
                        continue;

                old = 0;
                if (!atomic64_try_cmpxchg_relaxed(&inode->i_sequence, &old, new))
                        return old;
                return new;
        }
}

/**
 * get_futex_key() - Get parameters which are the keys for a futex
 * @uaddr:      virtual address of the futex
 * @flags:      FLAGS_*
 * @key:        address where result is stored.
 * @rw:         mapping needs to be read/write (values: FUTEX_READ,
 *              FUTEX_WRITE)
 *
 * Return: a negative error code or 0
 *
 * The key words are stored in @key on success.
 *
 * For shared mappings (when @fshared), the key is:
 *
 *   ( inode->i_sequence, page->index, offset_within_page )
 *
 * [ also see get_inode_sequence_number() ]
 *
 * For private mappings (or when !@fshared), the key is:
 *
 *   ( current->mm, address, 0 )
 *
 * This allows (cross process, where applicable) identification of the futex
 * without keeping the page pinned for the duration of the FUTEX_WAIT.
 *
 * lock_page() might sleep, the caller should not hold a spinlock.
 */
int get_futex_key(u32 __user *uaddr, unsigned int flags, union futex_key *key,
                  enum futex_access rw)
{
        unsigned long address = (unsigned long)uaddr;
        struct mm_struct *mm = current->mm;
        struct page *page;
        struct folio *folio;
        struct address_space *mapping;
        int err, ro = 0;
        bool fshared;

        fshared = flags & FLAGS_SHARED;

        /*
         * 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;

        if (unlikely(!access_ok(uaddr, sizeof(u32))))
                return -EFAULT;

        if (unlikely(should_fail_futex(fshared)))
                return -EFAULT;

        /*
         * 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) {
                /*
                 * On no-MMU, shared futexes are treated as private, therefore
                 * we must not include the current process in the key. Since
                 * there is only one address space, the address is a unique key
                 * on its own.
                 */
                if (IS_ENABLED(CONFIG_MMU))
                        key->private.mm = mm;
                else
                        key->private.mm = NULL;

                key->private.address = address;
                return 0;
        }

again:
        /* Ignore any VERIFY_READ mapping (futex common case) */
        if (unlikely(should_fail_futex(true)))
                return -EFAULT;

        err = get_user_pages_fast(address, 1, FOLL_WRITE, &page);
        /*
         * If write access is not required (eg. FUTEX_WAIT), try
         * and get read-only access.
         */
        if (err == -EFAULT && rw == FUTEX_READ) {
                err = get_user_pages_fast(address, 1, 0, &page);
                ro = 1;
        }
        if (err < 0)
                return err;
        else
                err = 0;

        /*
         * The treatment of mapping from this point on is critical. The folio
         * lock protects many things but in this context the folio lock
         * stabilizes mapping, prevents inode freeing in the shared
         * file-backed region case and guards against movement to swap cache.
         *
         * Strictly speaking the folio lock is not needed in all cases being
         * considered here and folio lock forces unnecessarily serialization.
         * From this point on, mapping will be re-verified if necessary and
         * folio lock will be acquired only if it is unavoidable
         *
         * Mapping checks require the folio so it is looked up now. For
         * anonymous pages, it does not matter if the folio is split
         * in the future as the key is based on the address. For
         * filesystem-backed pages, the precise page is required as the
         * index of the page determines the key.
         */
        folio = page_folio(page);
        mapping = READ_ONCE(folio->mapping);

        /*
         * If folio->mapping is NULL, then it cannot be an anonymous
         * page; but it might be the ZERO_PAGE or in the gate area or
         * in a special mapping (all cases which we are happy to fail);
         * or it may have been a good file page when get_user_pages_fast
         * found it, but truncated or holepunched or subjected to
         * invalidate_complete_page2 before we got the folio lock (also
         * cases which we are happy to fail).  And we hold a reference,
         * so refcount care in invalidate_inode_page's remove_mapping
         * prevents drop_caches from setting mapping to NULL beneath us.
         *
         * The case we do have to guard against is when memory pressure made
         * shmem_writepage move it from filecache to swapcache beneath us:
         * an unlikely race, but we do need to retry for folio->mapping.
         */
        if (unlikely(!mapping)) {
                int shmem_swizzled;

                /*
                 * Folio lock is required to identify which special case above
                 * applies. If this is really a shmem page then the folio lock
                 * will prevent unexpected transitions.
                 */
                folio_lock(folio);
                shmem_swizzled = folio_test_swapcache(folio) || folio->mapping;
                folio_unlock(folio);
                folio_put(folio);

                if (shmem_swizzled)
                        goto again;

                return -EFAULT;
        }

        /*
         * Private mappings are handled in a simple way.
         *
         * If the futex key is stored in anonymous memory, then the associated
         * object is the mm which is implicitly pinned by the calling process.
         *
         * 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 (folio_test_anon(folio)) {
                /*
                 * A RO anonymous page will never change and thus doesn't make
                 * sense for futex operations.
                 */
                if (unlikely(should_fail_futex(true)) || ro) {
                        err = -EFAULT;
                        goto out;
                }

                key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
                key->private.mm = mm;
                key->private.address = address;

        } else {
                struct inode *inode;

                /*
                 * The associated futex object in this case is the inode and
                 * the folio->mapping must be traversed. Ordinarily this should
                 * be stabilised under folio lock but it's not strictly
                 * necessary in this case as we just want to pin the inode, not
                 * update i_pages or anything like that.
                 *
                 * The RCU read lock is taken as the inode is finally freed
                 * under RCU. If the mapping still matches expectations then the
                 * mapping->host can be safely accessed as being a valid inode.
                 */
                rcu_read_lock();

                if (READ_ONCE(folio->mapping) != mapping) {
                        rcu_read_unlock();
                        folio_put(folio);

                        goto again;
                }

                inode = READ_ONCE(mapping->host);
                if (!inode) {
                        rcu_read_unlock();
                        folio_put(folio);

                        goto again;
                }

                key->both.offset |= FUT_OFF_INODE; /* inode-based key */
                key->shared.i_seq = get_inode_sequence_number(inode);
                key->shared.pgoff = page_pgoff(folio, page);
                rcu_read_unlock();
        }

out:
        folio_put(folio);
        return err;
}

/**
 * fault_in_user_writeable() - Fault in user address and verify RW access
 * @uaddr:      pointer to faulting user space address
 *
 * Slow path to fixup the fault we just took in the atomic write
 * access to @uaddr.
 *
 * We have no generic implementation of a non-destructive write to the
 * user address. We know that we faulted in the atomic pagefault
 * disabled section so we can as well avoid the #PF overhead by
 * calling get_user_pages() right away.
 */
int fault_in_user_writeable(u32 __user *uaddr)
{
        struct mm_struct *mm = current->mm;
        int ret;

        mmap_read_lock(mm);
        ret = fixup_user_fault(mm, (unsigned long)uaddr,
                               FAULT_FLAG_WRITE, NULL);
        mmap_read_unlock(mm);

        return ret < 0 ? ret : 0;
}

/**
 * futex_top_waiter() - Return the highest priority waiter on a futex
 * @hb:         the hash bucket the futex_q's reside in
 * @key:        the futex key (to distinguish it from other futex futex_q's)
 *
 * Must be called with the hb lock held.
 */
struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb, union futex_key *key)
{
        struct futex_q *this;

        plist_for_each_entry(this, &hb->chain, list) {
                if (futex_match(&this->key, key))
                        return this;
        }
        return NULL;
}

/**
 * wait_for_owner_exiting - Block until the owner has exited
 * @ret: owner's current futex lock status
 * @exiting:    Pointer to the exiting task
 *
 * Caller must hold a refcount on @exiting.
 */
void wait_for_owner_exiting(int ret, struct task_struct *exiting)
{
        if (ret != -EBUSY) {
                WARN_ON_ONCE(exiting);
                return;
        }

        if (WARN_ON_ONCE(ret == -EBUSY && !exiting))
                return;

        mutex_lock(&exiting->futex_exit_mutex);
        /*
         * No point in doing state checking here. If the waiter got here
         * while the task was in exec()->exec_futex_release() then it can
         * have any FUTEX_STATE_* value when the waiter has acquired the
         * mutex. OK, if running, EXITING or DEAD if it reached exit()
         * already. Highly unlikely and not a problem. Just one more round
         * through the futex maze.
         */
        mutex_unlock(&exiting->futex_exit_mutex);

        put_task_struct(exiting);
}

/**
 * __futex_unqueue() - Remove the futex_q from its futex_hash_bucket
 * @q:  The futex_q to unqueue
 *
 * The q->lock_ptr must not be NULL and must be held by the caller.
 */
void __futex_unqueue(struct futex_q *q)
{
        struct futex_hash_bucket *hb;

        if (WARN_ON_SMP(!q->lock_ptr) || WARN_ON(plist_node_empty(&q->list)))
                return;
        lockdep_assert_held(q->lock_ptr);

        hb = container_of(q->lock_ptr, struct futex_hash_bucket, lock);
        plist_del(&q->list, &hb->chain);
        futex_hb_waiters_dec(hb);
}

/* The key must be already stored in q->key. */
struct futex_hash_bucket *futex_q_lock(struct futex_q *q)
        __acquires(&hb->lock)
{
        struct futex_hash_bucket *hb;

        hb = futex_hash(&q->key);

        /*
         * Increment the counter before taking the lock so that
         * a potential waker won't miss a to-be-slept task that is
         * waiting for the spinlock. This is safe as all futex_q_lock()
         * users end up calling futex_queue(). Similarly, for housekeeping,
         * decrement the counter at futex_q_unlock() when some error has
         * occurred and we don't end up adding the task to the list.
         */
        futex_hb_waiters_inc(hb); /* implies smp_mb(); (A) */

        q->lock_ptr = &hb->lock;

        spin_lock(&hb->lock);
        return hb;
}

void futex_q_unlock(struct futex_hash_bucket *hb)
        __releases(&hb->lock)
{
        spin_unlock(&hb->lock);
        futex_hb_waiters_dec(hb);
}

void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,
                   struct task_struct *task)
{
        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);
        plist_add(&q->list, &hb->chain);
        q->task = task;
}

/**
 * futex_unqueue() - Remove the futex_q from its futex_hash_bucket
 * @q:  The futex_q to unqueue
 *
 * The q->lock_ptr must not be held by the caller. A call to futex_unqueue() must
 * be paired with exactly one earlier call to futex_queue().
 *
 * Return:
 *  - 1 - if the futex_q was still queued (and we removed unqueued it);
 *  - 0 - if the futex_q was already removed by the waking thread
 */
int futex_unqueue(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:
        /*
         * q->lock_ptr can change between this read and the following spin_lock.
         * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
         * optimizing lock_ptr out of the logic below.
         */
        lock_ptr = READ_ONCE(q->lock_ptr);
        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;
                }
                __futex_unqueue(q);

                BUG_ON(q->pi_state);

                spin_unlock(lock_ptr);
                ret = 1;
        }

        return ret;
}

/*
 * PI futexes can not be requeued and must remove themselves from the hash
 * bucket. The hash bucket lock (i.e. lock_ptr) is held.
 */
void futex_unqueue_pi(struct futex_q *q)
{
        /*
         * If the lock was not acquired (due to timeout or signal) then the
         * rt_waiter is removed before futex_q is. If this is observed by
         * an unlocker after dropping the rtmutex wait lock and before
         * acquiring the hash bucket lock, then the unlocker dequeues the
         * futex_q from the hash bucket list to guarantee consistent state
         * vs. userspace. Therefore the dequeue here must be conditional.
         */
        if (!plist_node_empty(&q->list))
                __futex_unqueue(q);

        BUG_ON(!q->pi_state);
        put_pi_state(q->pi_state);
        q->pi_state = NULL;
}

/* Constants for the pending_op argument of handle_futex_death */
#define HANDLE_DEATH_PENDING    true
#define HANDLE_DEATH_LIST       false

/*
 * Process a futex-list entry, check whether it's owned by the
 * dying task, and do notification if so:
 */
static int handle_futex_death(u32 __user *uaddr, struct task_struct *curr,
                              bool pi, bool pending_op)
{
        u32 uval, nval, mval;
        pid_t owner;
        int err;

        /* Futex address must be 32bit aligned */
        if ((((unsigned long)uaddr) % sizeof(*uaddr)) != 0)
                return -1;

retry:
        if (get_user(uval, uaddr))
                return -1;

        /*
         * Special case for regular (non PI) futexes. The unlock path in
         * user space has two race scenarios:
         *
         * 1. The unlock path releases the user space futex value and
         *    before it can execute the futex() syscall to wake up
         *    waiters it is killed.
         *
         * 2. A woken up waiter is killed before it can acquire the
         *    futex in user space.
         *
         * In the second case, the wake up notification could be generated
         * by the unlock path in user space after setting the futex value
         * to zero or by the kernel after setting the OWNER_DIED bit below.
         *
         * In both cases the TID validation below prevents a wakeup of
         * potential waiters which can cause these waiters to block
         * forever.
         *
         * In both cases the following conditions are met:
         *
         *      1) task->robust_list->list_op_pending != NULL
         *         @pending_op == true
         *      2) The owner part of user space futex value == 0
         *      3) Regular futex: @pi == false
         *
         * If these conditions are met, it is safe to attempt waking up a
         * potential waiter without touching the user space futex value and
         * trying to set the OWNER_DIED bit. If the futex value is zero,
         * the rest of the user space mutex state is consistent, so a woken
         * waiter will just take over the uncontended futex. Setting the
         * OWNER_DIED bit would create inconsistent state and malfunction
         * of the user space owner died handling. Otherwise, the OWNER_DIED
         * bit is already set, and the woken waiter is expected to deal with
         * this.
         */
        owner = uval & FUTEX_TID_MASK;

        if (pending_op && !pi && !owner) {
                futex_wake(uaddr, FLAGS_SIZE_32 | FLAGS_SHARED, 1,
                           FUTEX_BITSET_MATCH_ANY);
                return 0;
        }

        if (owner != task_pid_vnr(curr))
                return 0;

        /*
         * 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;

        /*
         * We are not holding a lock here, but we want to have
         * the pagefault_disable/enable() protection because
         * we want to handle the fault gracefully. If the
         * access fails we try to fault in the futex with R/W
         * verification via get_user_pages. get_user() above
         * does not guarantee R/W access. If that fails we
         * give up and leave the futex locked.
         */
        if ((err = futex_cmpxchg_value_locked(&nval, uaddr, uval, mval))) {
                switch (err) {
                case -EFAULT:
                        if (fault_in_user_writeable(uaddr))
                                return -1;
                        goto retry;

                case -EAGAIN:
                        cond_resched();
                        goto retry;

                default:
                        WARN_ON_ONCE(1);
                        return err;
                }
        }

        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, FLAGS_SIZE_32 | FLAGS_SHARED, 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,
                                     unsigned 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.
 */
static 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, pip;
        unsigned int next_pi;
        unsigned long futex_offset;
        int rc;

        /*
         * 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, HANDLE_DEATH_LIST))
                                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, HANDLE_DEATH_PENDING);
        }
}

#ifdef CONFIG_COMPAT
static void __user *futex_uaddr(struct robust_list __user *entry,
                                compat_long_t futex_offset)
{
        compat_uptr_t base = ptr_to_compat(entry);
        void __user *uaddr = compat_ptr(base + futex_offset);

        return uaddr;
}

/*
 * Fetch a robust-list pointer. Bit 0 signals PI futexes:
 */
static inline int
compat_fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
                   compat_uptr_t __user *head, unsigned int *pi)
{
        if (get_user(*uentry, head))
                return -EFAULT;

        *entry = compat_ptr((*uentry) & ~1);
        *pi = (unsigned int)(*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.
 */
static void compat_exit_robust_list(struct task_struct *curr)
{
        struct compat_robust_list_head __user *head = curr->compat_robust_list;
        struct robust_list __user *entry, *next_entry, *pending;
        unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
        unsigned int next_pi;
        compat_uptr_t uentry, next_uentry, upending;
        compat_long_t futex_offset;
        int rc;

        /*
         * Fetch the list head (which was registered earlier, via
         * sys_set_robust_list()):
         */
        if (compat_fetch_robust_entry(&uentry, &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 (compat_fetch_robust_entry(&upending, &pending,
                               &head->list_op_pending, &pip))
                return;

        next_entry = NULL;      /* avoid warning with gcc */
        while (entry != (struct robust_list __user *) &head->list) {
                /*
                 * Fetch the next entry in the list before calling
                 * handle_futex_death:
                 */
                rc = compat_fetch_robust_entry(&next_uentry, &next_entry,
                        (compat_uptr_t __user *)&entry->next, &next_pi);
                /*
                 * A pending lock might already be on the list, so
                 * dont process it twice:
                 */
                if (entry != pending) {
                        void __user *uaddr = futex_uaddr(entry, futex_offset);

                        if (handle_futex_death(uaddr, curr, pi,
                                               HANDLE_DEATH_LIST))
                                return;
                }
                if (rc)
                        return;
                uentry = next_uentry;
                entry = next_entry;
                pi = next_pi;
                /*
                 * Avoid excessively long or circular lists:
                 */
                if (!--limit)
                        break;

                cond_resched();
        }
        if (pending) {
                void __user *uaddr = futex_uaddr(pending, futex_offset);

                handle_futex_death(uaddr, curr, pip, HANDLE_DEATH_PENDING);
        }
}
#endif

#ifdef CONFIG_FUTEX_PI

/*
 * 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.)
 */
static 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;

        /*
         * We are a ZOMBIE and nobody can enqueue itself on
         * pi_state_list anymore, but we have to be careful
         * versus waiters unqueueing themselves:
         */
        raw_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 = futex_hash(&key);

                /*
                 * We can race against put_pi_state() removing itself from the
                 * list (a waiter going away). put_pi_state() will first
                 * decrement the reference count and then modify the list, so
                 * its possible to see the list entry but fail this reference
                 * acquire.
                 *
                 * In that case; drop the locks to let put_pi_state() make
                 * progress and retry the loop.
                 */
                if (!refcount_inc_not_zero(&pi_state->refcount)) {
                        raw_spin_unlock_irq(&curr->pi_lock);
                        cpu_relax();
                        raw_spin_lock_irq(&curr->pi_lock);
                        continue;
                }
                raw_spin_unlock_irq(&curr->pi_lock);

                spin_lock(&hb->lock);
                raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
                raw_spin_lock(&curr->pi_lock);
                /*
                 * We dropped the pi-lock, so re-check whether this
                 * task still owns the PI-state:
                 */
                if (head->next != next) {
                        /* retain curr->pi_lock for the loop invariant */
                        raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
                        spin_unlock(&hb->lock);
                        put_pi_state(pi_state);
                        continue;
                }

                WARN_ON(pi_state->owner != curr);
                WARN_ON(list_empty(&pi_state->list));
                list_del_init(&pi_state->list);
                pi_state->owner = NULL;

                raw_spin_unlock(&curr->pi_lock);
                raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
                spin_unlock(&hb->lock);

                rt_mutex_futex_unlock(&pi_state->pi_mutex);
                put_pi_state(pi_state);

                raw_spin_lock_irq(&curr->pi_lock);
        }
        raw_spin_unlock_irq(&curr->pi_lock);
}
#else
static inline void exit_pi_state_list(struct task_struct *curr) { }
#endif

static void futex_cleanup(struct task_struct *tsk)
{
        if (unlikely(tsk->robust_list)) {
                exit_robust_list(tsk);
                tsk->robust_list = NULL;
        }

#ifdef CONFIG_COMPAT
        if (unlikely(tsk->compat_robust_list)) {
                compat_exit_robust_list(tsk);
                tsk->compat_robust_list = NULL;
        }
#endif

        if (unlikely(!list_empty(&tsk->pi_state_list)))
                exit_pi_state_list(tsk);
}

/**
 * futex_exit_recursive - Set the tasks futex state to FUTEX_STATE_DEAD
 * @tsk:        task to set the state on
 *
 * Set the futex exit state of the task lockless. The futex waiter code
 * observes that state when a task is exiting and loops until the task has
 * actually finished the futex cleanup. The worst case for this is that the
 * waiter runs through the wait loop until the state becomes visible.
 *
 * This is called from the recursive fault handling path in make_task_dead().
 *
 * This is best effort. Either the futex exit code has run already or
 * not. If the OWNER_DIED bit has been set on the futex then the waiter can
 * take it over. If not, the problem is pushed back to user space. If the
 * futex exit code did not run yet, then an already queued waiter might
 * block forever, but there is nothing which can be done about that.
 */
void futex_exit_recursive(struct task_struct *tsk)
{
        /* If the state is FUTEX_STATE_EXITING then futex_exit_mutex is held */
        if (tsk->futex_state == FUTEX_STATE_EXITING)
                mutex_unlock(&tsk->futex_exit_mutex);
        tsk->futex_state = FUTEX_STATE_DEAD;
}

static void futex_cleanup_begin(struct task_struct *tsk)
{
        /*
         * Prevent various race issues against a concurrent incoming waiter
         * including live locks by forcing the waiter to block on
         * tsk->futex_exit_mutex when it observes FUTEX_STATE_EXITING in
         * attach_to_pi_owner().
         */
        mutex_lock(&tsk->futex_exit_mutex);

        /*
         * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
         *
         * This ensures that all subsequent checks of tsk->futex_state in
         * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
         * tsk->pi_lock held.
         *
         * It guarantees also that a pi_state which was queued right before
         * the state change under tsk->pi_lock by a concurrent waiter must
         * be observed in exit_pi_state_list().
         */
        raw_spin_lock_irq(&tsk->pi_lock);
        tsk->futex_state = FUTEX_STATE_EXITING;
        raw_spin_unlock_irq(&tsk->pi_lock);
}

static void futex_cleanup_end(struct task_struct *tsk, int state)
{
        /*
         * Lockless store. The only side effect is that an observer might
         * take another loop until it becomes visible.
         */
        tsk->futex_state = state;
        /*
         * Drop the exit protection. This unblocks waiters which observed
         * FUTEX_STATE_EXITING to reevaluate the state.
         */
        mutex_unlock(&tsk->futex_exit_mutex);
}

void futex_exec_release(struct task_struct *tsk)
{
        /*
         * The state handling is done for consistency, but in the case of
         * exec() there is no way to prevent further damage as the PID stays
         * the same. But for the unlikely and arguably buggy case that a
         * futex is held on exec(), this provides at least as much state
         * consistency protection which is possible.
         */
        futex_cleanup_begin(tsk);
        futex_cleanup(tsk);
        /*
         * Reset the state to FUTEX_STATE_OK. The task is alive and about
         * exec a new binary.
         */
        futex_cleanup_end(tsk, FUTEX_STATE_OK);
}

void futex_exit_release(struct task_struct *tsk)
{
        futex_cleanup_begin(tsk);
        futex_cleanup(tsk);
        futex_cleanup_end(tsk, FUTEX_STATE_DEAD);
}

static int __init futex_init(void)
{
        unsigned int futex_shift;
        unsigned long i;

#ifdef CONFIG_BASE_SMALL
        futex_hashsize = 16;
#else
        futex_hashsize = roundup_pow_of_two(256 * num_possible_cpus());
#endif

        futex_queues = alloc_large_system_hash("futex", sizeof(*futex_queues),
                                               futex_hashsize, 0, 0,
                                               &futex_shift, NULL,
                                               futex_hashsize, futex_hashsize);
        futex_hashsize = 1UL << futex_shift;

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

        return 0;
}
core_initcall(futex_init);