[bcachefs-tools-debian] / linux / six.c

// SPDX-License-Identifier: GPL-2.0

#include <linux/export.h>
#include <linux/log2.h>
#include <linux/percpu.h>
#include <linux/preempt.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/sched/clock.h>
#include <linux/sched/rt.h>
#include <linux/six.h>
#include <linux/slab.h>

#include <trace/events/lock.h>

#ifdef DEBUG
#define EBUG_ON(cond)                   BUG_ON(cond)
#else
#define EBUG_ON(cond)                   do {} while (0)
#endif

#define six_acquire(l, t, r, ip)        lock_acquire(l, 0, t, r, 1, NULL, ip)
#define six_release(l, ip)              lock_release(l, ip)

static void do_six_unlock_type(struct six_lock *lock, enum six_lock_type type);

#define SIX_LOCK_HELD_read_OFFSET       0
#define SIX_LOCK_HELD_read              ~(~0U << 26)
#define SIX_LOCK_HELD_intent            (1U << 26)
#define SIX_LOCK_HELD_write             (1U << 27)
#define SIX_LOCK_WAITING_read           (1U << (28 + SIX_LOCK_read))
#define SIX_LOCK_WAITING_intent         (1U << (28 + SIX_LOCK_intent))
#define SIX_LOCK_WAITING_write          (1U << (28 + SIX_LOCK_write))
#define SIX_LOCK_NOSPIN                 (1U << 31)

struct six_lock_vals {
        /* Value we add to the lock in order to take the lock: */
        u32                     lock_val;

        /* If the lock has this value (used as a mask), taking the lock fails: */
        u32                     lock_fail;

        /* Mask that indicates lock is held for this type: */
        u32                     held_mask;

        /* Waitlist we wakeup when releasing the lock: */
        enum six_lock_type      unlock_wakeup;
};

static const struct six_lock_vals l[] = {
        [SIX_LOCK_read] = {
                .lock_val       = 1U << SIX_LOCK_HELD_read_OFFSET,
                .lock_fail      = SIX_LOCK_HELD_write,
                .held_mask      = SIX_LOCK_HELD_read,
                .unlock_wakeup  = SIX_LOCK_write,
        },
        [SIX_LOCK_intent] = {
                .lock_val       = SIX_LOCK_HELD_intent,
                .lock_fail      = SIX_LOCK_HELD_intent,
                .held_mask      = SIX_LOCK_HELD_intent,
                .unlock_wakeup  = SIX_LOCK_intent,
        },
        [SIX_LOCK_write] = {
                .lock_val       = SIX_LOCK_HELD_write,
                .lock_fail      = SIX_LOCK_HELD_read,
                .held_mask      = SIX_LOCK_HELD_write,
                .unlock_wakeup  = SIX_LOCK_read,
        },
};

static inline void six_set_bitmask(struct six_lock *lock, u32 mask)
{
        if ((atomic_read(&lock->state) & mask) != mask)
                atomic_or(mask, &lock->state);
}

static inline void six_clear_bitmask(struct six_lock *lock, u32 mask)
{
        if (atomic_read(&lock->state) & mask)
                atomic_and(~mask, &lock->state);
}

static inline void six_set_owner(struct six_lock *lock, enum six_lock_type type,
                                 u32 old, struct task_struct *owner)
{
        if (type != SIX_LOCK_intent)
                return;

        if (!(old & SIX_LOCK_HELD_intent)) {
                EBUG_ON(lock->owner);
                lock->owner = owner;
        } else {
                EBUG_ON(lock->owner != current);
        }
}

static inline unsigned pcpu_read_count(struct six_lock *lock)
{
        unsigned read_count = 0;
        int cpu;

        for_each_possible_cpu(cpu)
                read_count += *per_cpu_ptr(lock->readers, cpu);
        return read_count;
}

/*
 * __do_six_trylock() - main trylock routine
 *
 * Returns 1 on success, 0 on failure
 *
 * In percpu reader mode, a failed trylock may cause a spurious trylock failure
 * for anoter thread taking the competing lock type, and we may havve to do a
 * wakeup: when a wakeup is required, we return -1 - wakeup_type.
 */
static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
                            struct task_struct *task, bool try)
{
        int ret;
        u32 old;

        EBUG_ON(type == SIX_LOCK_write && lock->owner != task);
        EBUG_ON(type == SIX_LOCK_write &&
                (try != !(atomic_read(&lock->state) & SIX_LOCK_HELD_write)));

        /*
         * Percpu reader mode:
         *
         * The basic idea behind this algorithm is that you can implement a lock
         * between two threads without any atomics, just memory barriers:
         *
         * For two threads you'll need two variables, one variable for "thread a
         * has the lock" and another for "thread b has the lock".
         *
         * To take the lock, a thread sets its variable indicating that it holds
         * the lock, then issues a full memory barrier, then reads from the
         * other thread's variable to check if the other thread thinks it has
         * the lock. If we raced, we backoff and retry/sleep.
         *
         * Failure to take the lock may cause a spurious trylock failure in
         * another thread, because we temporarily set the lock to indicate that
         * we held it. This would be a problem for a thread in six_lock(), when
         * they are calling trylock after adding themself to the waitlist and
         * prior to sleeping.
         *
         * Therefore, if we fail to get the lock, and there were waiters of the
         * type we conflict with, we will have to issue a wakeup.
         *
         * Since we may be called under wait_lock (and by the wakeup code
         * itself), we return that the wakeup has to be done instead of doing it
         * here.
         */
        if (type == SIX_LOCK_read && lock->readers) {
                preempt_disable();
                this_cpu_inc(*lock->readers); /* signal that we own lock */

                smp_mb();

                old = atomic_read(&lock->state);
                ret = !(old & l[type].lock_fail);

                this_cpu_sub(*lock->readers, !ret);
                preempt_enable();

                if (!ret && (old & SIX_LOCK_WAITING_write))
                        ret = -1 - SIX_LOCK_write;
        } else if (type == SIX_LOCK_write && lock->readers) {
                if (try) {
                        atomic_add(SIX_LOCK_HELD_write, &lock->state);
                        smp_mb__after_atomic();
                }

                ret = !pcpu_read_count(lock);

                if (try && !ret) {
                        old = atomic_sub_return(SIX_LOCK_HELD_write, &lock->state);
                        if (old & SIX_LOCK_WAITING_read)
                                ret = -1 - SIX_LOCK_read;
                }
        } else {
                old = atomic_read(&lock->state);
                do {
                        ret = !(old & l[type].lock_fail);
                        if (!ret || (type == SIX_LOCK_write && !try)) {
                                smp_mb();
                                break;
                        }
                } while (!atomic_try_cmpxchg_acquire(&lock->state, &old, old + l[type].lock_val));

                EBUG_ON(ret && !(atomic_read(&lock->state) & l[type].held_mask));
        }

        if (ret > 0)
                six_set_owner(lock, type, old, task);

        EBUG_ON(type == SIX_LOCK_write && try && ret <= 0 &&
                (atomic_read(&lock->state) & SIX_LOCK_HELD_write));

        return ret;
}

static void __six_lock_wakeup(struct six_lock *lock, enum six_lock_type lock_type)
{
        struct six_lock_waiter *w, *next;
        struct task_struct *task;
        bool saw_one;
        int ret;
again:
        ret = 0;
        saw_one = false;
        raw_spin_lock(&lock->wait_lock);

        list_for_each_entry_safe(w, next, &lock->wait_list, list) {
                if (w->lock_want != lock_type)
                        continue;

                if (saw_one && lock_type != SIX_LOCK_read)
                        goto unlock;
                saw_one = true;

                ret = __do_six_trylock(lock, lock_type, w->task, false);
                if (ret <= 0)
                        goto unlock;

                __list_del(w->list.prev, w->list.next);
                task = w->task;
                /*
                 * Do no writes to @w besides setting lock_acquired - otherwise
                 * we would need a memory barrier:
                 */
                barrier();
                w->lock_acquired = true;
                wake_up_process(task);
        }

        six_clear_bitmask(lock, SIX_LOCK_WAITING_read << lock_type);
unlock:
        raw_spin_unlock(&lock->wait_lock);

        if (ret < 0) {
                lock_type = -ret - 1;
                goto again;
        }
}

__always_inline
static void six_lock_wakeup(struct six_lock *lock, u32 state,
                            enum six_lock_type lock_type)
{
        if (lock_type == SIX_LOCK_write && (state & SIX_LOCK_HELD_read))
                return;

        if (!(state & (SIX_LOCK_WAITING_read << lock_type)))
                return;

        __six_lock_wakeup(lock, lock_type);
}

__always_inline
static bool do_six_trylock(struct six_lock *lock, enum six_lock_type type, bool try)
{
        int ret;

        ret = __do_six_trylock(lock, type, current, try);
        if (ret < 0)
                __six_lock_wakeup(lock, -ret - 1);

        return ret > 0;
}

/**
 * six_trylock_ip - attempt to take a six lock without blocking
 * @lock:       lock to take
 * @type:       SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
 * @ip:         ip parameter for lockdep/lockstat, i.e. _THIS_IP_
 *
 * Return: true on success, false on failure.
 */
bool six_trylock_ip(struct six_lock *lock, enum six_lock_type type, unsigned long ip)
{
        if (!do_six_trylock(lock, type, true))
                return false;

        if (type != SIX_LOCK_write)
                six_acquire(&lock->dep_map, 1, type == SIX_LOCK_read, ip);
        return true;
}
EXPORT_SYMBOL_GPL(six_trylock_ip);

/**
 * six_relock_ip - attempt to re-take a lock that was held previously
 * @lock:       lock to take
 * @type:       SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
 * @seq:        lock sequence number obtained from six_lock_seq() while lock was
 *              held previously
 * @ip:         ip parameter for lockdep/lockstat, i.e. _THIS_IP_
 *
 * Return: true on success, false on failure.
 */
bool six_relock_ip(struct six_lock *lock, enum six_lock_type type,
                   unsigned seq, unsigned long ip)
{
        if (six_lock_seq(lock) != seq || !six_trylock_ip(lock, type, ip))
                return false;

        if (six_lock_seq(lock) != seq) {
                six_unlock_ip(lock, type, ip);
                return false;
        }

        return true;
}
EXPORT_SYMBOL_GPL(six_relock_ip);

#ifdef CONFIG_LOCK_SPIN_ON_OWNER

static inline bool six_can_spin_on_owner(struct six_lock *lock)
{
        struct task_struct *owner;
        bool ret;

        if (need_resched())
                return false;

        rcu_read_lock();
        owner = READ_ONCE(lock->owner);
        ret = !owner || owner_on_cpu(owner);
        rcu_read_unlock();

        return ret;
}

static inline bool six_spin_on_owner(struct six_lock *lock,
                                     struct task_struct *owner,
                                     u64 end_time)
{
        bool ret = true;
        unsigned loop = 0;

        rcu_read_lock();
        while (lock->owner == owner) {
                /*
                 * Ensure we emit the owner->on_cpu, dereference _after_
                 * checking lock->owner still matches owner. If that fails,
                 * owner might point to freed memory. If it still matches,
                 * the rcu_read_lock() ensures the memory stays valid.
                 */
                barrier();

                if (!owner_on_cpu(owner) || need_resched()) {
                        ret = false;
                        break;
                }

                if (!(++loop & 0xf) && (time_after64(sched_clock(), end_time))) {
                        six_set_bitmask(lock, SIX_LOCK_NOSPIN);
                        ret = false;
                        break;
                }

                cpu_relax();
        }
        rcu_read_unlock();

        return ret;
}

static inline bool six_optimistic_spin(struct six_lock *lock, enum six_lock_type type)
{
        struct task_struct *task = current;
        u64 end_time;

        if (type == SIX_LOCK_write)
                return false;

        preempt_disable();
        if (!six_can_spin_on_owner(lock))
                goto fail;

        if (!osq_lock(&lock->osq))
                goto fail;

        end_time = sched_clock() + 10 * NSEC_PER_USEC;

        while (1) {
                struct task_struct *owner;

                /*
                 * If there's an owner, wait for it to either
                 * release the lock or go to sleep.
                 */
                owner = READ_ONCE(lock->owner);
                if (owner && !six_spin_on_owner(lock, owner, end_time))
                        break;

                if (do_six_trylock(lock, type, false)) {
                        osq_unlock(&lock->osq);
                        preempt_enable();
                        return true;
                }

                /*
                 * When there's no owner, we might have preempted between the
                 * owner acquiring the lock and setting the owner field. If
                 * we're an RT task that will live-lock because we won't let
                 * the owner complete.
                 */
                if (!owner && (need_resched() || rt_task(task)))
                        break;

                /*
                 * The cpu_relax() call is a compiler barrier which forces
                 * everything in this loop to be re-loaded. We don't need
                 * memory barriers as we'll eventually observe the right
                 * values at the cost of a few extra spins.
                 */
                cpu_relax();
        }

        osq_unlock(&lock->osq);
fail:
        preempt_enable();

        /*
         * If we fell out of the spin path because of need_resched(),
         * reschedule now, before we try-lock again. This avoids getting
         * scheduled out right after we obtained the lock.
         */
        if (need_resched())
                schedule();

        return false;
}

#else /* CONFIG_LOCK_SPIN_ON_OWNER */

static inline bool six_optimistic_spin(struct six_lock *lock, enum six_lock_type type)
{
        return false;
}

#endif

noinline
static int six_lock_slowpath(struct six_lock *lock, enum six_lock_type type,
                             struct six_lock_waiter *wait,
                             six_lock_should_sleep_fn should_sleep_fn, void *p,
                             unsigned long ip)
{
        int ret = 0;

        if (type == SIX_LOCK_write) {
                EBUG_ON(atomic_read(&lock->state) & SIX_LOCK_HELD_write);
                atomic_add(SIX_LOCK_HELD_write, &lock->state);
                smp_mb__after_atomic();
        }

        trace_contention_begin(lock, 0);
        lock_contended(&lock->dep_map, ip);

        if (six_optimistic_spin(lock, type))
                goto out;

        wait->task              = current;
        wait->lock_want         = type;
        wait->lock_acquired     = false;

        raw_spin_lock(&lock->wait_lock);
        six_set_bitmask(lock, SIX_LOCK_WAITING_read << type);
        /*
         * Retry taking the lock after taking waitlist lock, in case we raced
         * with an unlock:
         */
        ret = __do_six_trylock(lock, type, current, false);
        if (ret <= 0) {
                wait->start_time = local_clock();

                if (!list_empty(&lock->wait_list)) {
                        struct six_lock_waiter *last =
                                list_last_entry(&lock->wait_list,
                                        struct six_lock_waiter, list);

                        if (time_before_eq64(wait->start_time, last->start_time))
                                wait->start_time = last->start_time + 1;
                }

                list_add_tail(&wait->list, &lock->wait_list);
        }
        raw_spin_unlock(&lock->wait_lock);

        if (unlikely(ret > 0)) {
                ret = 0;
                goto out;
        }

        if (unlikely(ret < 0)) {
                __six_lock_wakeup(lock, -ret - 1);
                ret = 0;
        }

        while (1) {
                set_current_state(TASK_UNINTERRUPTIBLE);

                if (wait->lock_acquired)
                        break;

                ret = should_sleep_fn ? should_sleep_fn(lock, p) : 0;
                if (unlikely(ret)) {
                        raw_spin_lock(&lock->wait_lock);
                        if (!wait->lock_acquired)
                                list_del(&wait->list);
                        raw_spin_unlock(&lock->wait_lock);

                        if (unlikely(wait->lock_acquired))
                                do_six_unlock_type(lock, type);
                        break;
                }

                schedule();
        }

        __set_current_state(TASK_RUNNING);
out:
        if (ret && type == SIX_LOCK_write) {
                six_clear_bitmask(lock, SIX_LOCK_HELD_write);
                six_lock_wakeup(lock, atomic_read(&lock->state), SIX_LOCK_read);
        }
        trace_contention_end(lock, 0);

        return ret;
}

/**
 * six_lock_ip_waiter - take a lock, with full waitlist interface
 * @lock:       lock to take
 * @type:       SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
 * @wait:       pointer to wait object, which will be added to lock's waitlist
 * @should_sleep_fn: callback run after adding to waitlist, immediately prior
 *              to scheduling
 * @p:          passed through to @should_sleep_fn
 * @ip:         ip parameter for lockdep/lockstat, i.e. _THIS_IP_
 *
 * This is the most general six_lock() variant, with parameters to support full
 * cycle detection for deadlock avoidance.
 *
 * The code calling this function must implement tracking of held locks, and the
 * @wait object should be embedded into the struct that tracks held locks -
 * which must also be accessible in a thread-safe way.
 *
 * @should_sleep_fn should invoke the cycle detector; it should walk each
 * lock's waiters, and for each waiter recursively walk their held locks.
 *
 * When this function must block, @wait will be added to @lock's waitlist before
 * calling trylock, and before calling @should_sleep_fn, and @wait will not be
 * removed from the lock waitlist until the lock has been successfully acquired,
 * or we abort.
 *
 * @wait.start_time will be monotonically increasing for any given waitlist, and
 * thus may be used as a loop cursor.
 *
 * Return: 0 on success, or the return code from @should_sleep_fn on failure.
 */
int six_lock_ip_waiter(struct six_lock *lock, enum six_lock_type type,
                       struct six_lock_waiter *wait,
                       six_lock_should_sleep_fn should_sleep_fn, void *p,
                       unsigned long ip)
{
        int ret;

        wait->start_time = 0;

        if (type != SIX_LOCK_write)
                six_acquire(&lock->dep_map, 0, type == SIX_LOCK_read, ip);

        ret = do_six_trylock(lock, type, true) ? 0
                : six_lock_slowpath(lock, type, wait, should_sleep_fn, p, ip);

        if (ret && type != SIX_LOCK_write)
                six_release(&lock->dep_map, ip);
        if (!ret)
                lock_acquired(&lock->dep_map, ip);

        return ret;
}
EXPORT_SYMBOL_GPL(six_lock_ip_waiter);

__always_inline
static void do_six_unlock_type(struct six_lock *lock, enum six_lock_type type)
{
        u32 state;

        if (type == SIX_LOCK_intent)
                lock->owner = NULL;

        if (type == SIX_LOCK_read &&
            lock->readers) {
                smp_mb(); /* unlock barrier */
                this_cpu_dec(*lock->readers);
                smp_mb(); /* between unlocking and checking for waiters */
                state = atomic_read(&lock->state);
        } else {
                u32 v = l[type].lock_val;

                if (type != SIX_LOCK_read)
                        v += atomic_read(&lock->state) & SIX_LOCK_NOSPIN;

                EBUG_ON(!(atomic_read(&lock->state) & l[type].held_mask));
                state = atomic_sub_return_release(v, &lock->state);
        }

        six_lock_wakeup(lock, state, l[type].unlock_wakeup);
}

/**
 * six_unlock_ip - drop a six lock
 * @lock:       lock to unlock
 * @type:       SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
 * @ip:         ip parameter for lockdep/lockstat, i.e. _THIS_IP_
 *
 * When a lock is held multiple times (because six_lock_incement()) was used),
 * this decrements the 'lock held' counter by one.
 *
 * For example:
 * six_lock_read(&foo->lock);                           read count 1
 * six_lock_increment(&foo->lock, SIX_LOCK_read);       read count 2
 * six_lock_unlock(&foo->lock, SIX_LOCK_read);          read count 1
 * six_lock_unlock(&foo->lock, SIX_LOCK_read);          read count 0
 */
void six_unlock_ip(struct six_lock *lock, enum six_lock_type type, unsigned long ip)
{
        EBUG_ON(type == SIX_LOCK_write &&
                !(atomic_read(&lock->state) & SIX_LOCK_HELD_intent));
        EBUG_ON((type == SIX_LOCK_write ||
                 type == SIX_LOCK_intent) &&
                lock->owner != current);

        if (type != SIX_LOCK_write)
                six_release(&lock->dep_map, ip);
        else
                lock->seq++;

        if (type == SIX_LOCK_intent &&
            lock->intent_lock_recurse) {
                --lock->intent_lock_recurse;
                return;
        }

        do_six_unlock_type(lock, type);
}
EXPORT_SYMBOL_GPL(six_unlock_ip);

/**
 * six_lock_downgrade - convert an intent lock to a read lock
 * @lock:       lock to dowgrade
 *
 * @lock will have read count incremented and intent count decremented
 */
void six_lock_downgrade(struct six_lock *lock)
{
        six_lock_increment(lock, SIX_LOCK_read);
        six_unlock_intent(lock);
}
EXPORT_SYMBOL_GPL(six_lock_downgrade);

/**
 * six_lock_tryupgrade - attempt to convert read lock to an intent lock
 * @lock:       lock to upgrade
 *
 * On success, @lock will have intent count incremented and read count
 * decremented
 *
 * Return: true on success, false on failure
 */
bool six_lock_tryupgrade(struct six_lock *lock)
{
        u32 old = atomic_read(&lock->state), new;

        do {
                new = old;

                if (new & SIX_LOCK_HELD_intent)
                        return false;

                if (!lock->readers) {
                        EBUG_ON(!(new & SIX_LOCK_HELD_read));
                        new -= l[SIX_LOCK_read].lock_val;
                }

                new |= SIX_LOCK_HELD_intent;
        } while (!atomic_try_cmpxchg_acquire(&lock->state, &old, new));

        if (lock->readers)
                this_cpu_dec(*lock->readers);

        six_set_owner(lock, SIX_LOCK_intent, old, current);

        return true;
}
EXPORT_SYMBOL_GPL(six_lock_tryupgrade);

/**
 * six_trylock_convert - attempt to convert a held lock from one type to another
 * @lock:       lock to upgrade
 * @from:       SIX_LOCK_read or SIX_LOCK_intent
 * @to:         SIX_LOCK_read or SIX_LOCK_intent
 *
 * On success, @lock will have intent count incremented and read count
 * decremented
 *
 * Return: true on success, false on failure
 */
bool six_trylock_convert(struct six_lock *lock,
                         enum six_lock_type from,
                         enum six_lock_type to)
{
        EBUG_ON(to == SIX_LOCK_write || from == SIX_LOCK_write);

        if (to == from)
                return true;

        if (to == SIX_LOCK_read) {
                six_lock_downgrade(lock);
                return true;
        } else {
                return six_lock_tryupgrade(lock);
        }
}
EXPORT_SYMBOL_GPL(six_trylock_convert);

/**
 * six_lock_increment - increase held lock count on a lock that is already held
 * @lock:       lock to increment
 * @type:       SIX_LOCK_read or SIX_LOCK_intent
 *
 * @lock must already be held, with a lock type that is greater than or equal to
 * @type
 *
 * A corresponding six_unlock_type() call will be required for @lock to be fully
 * unlocked.
 */
void six_lock_increment(struct six_lock *lock, enum six_lock_type type)
{
        six_acquire(&lock->dep_map, 0, type == SIX_LOCK_read, _RET_IP_);

        /* XXX: assert already locked, and that we don't overflow: */

        switch (type) {
        case SIX_LOCK_read:
                if (lock->readers) {
                        this_cpu_inc(*lock->readers);
                } else {
                        EBUG_ON(!(atomic_read(&lock->state) &
                                  (SIX_LOCK_HELD_read|
                                   SIX_LOCK_HELD_intent)));
                        atomic_add(l[type].lock_val, &lock->state);
                }
                break;
        case SIX_LOCK_intent:
                EBUG_ON(!(atomic_read(&lock->state) & SIX_LOCK_HELD_intent));
                lock->intent_lock_recurse++;
                break;
        case SIX_LOCK_write:
                BUG();
                break;
        }
}
EXPORT_SYMBOL_GPL(six_lock_increment);

/**
 * six_lock_wakeup_all - wake up all waiters on @lock
 * @lock:       lock to wake up waiters for
 *
 * Wakeing up waiters will cause them to re-run should_sleep_fn, which may then
 * abort the lock operation.
 *
 * This function is never needed in a bug-free program; it's only useful in
 * debug code, e.g. to determine if a cycle detector is at fault.
 */
void six_lock_wakeup_all(struct six_lock *lock)
{
        u32 state = atomic_read(&lock->state);
        struct six_lock_waiter *w;

        six_lock_wakeup(lock, state, SIX_LOCK_read);
        six_lock_wakeup(lock, state, SIX_LOCK_intent);
        six_lock_wakeup(lock, state, SIX_LOCK_write);

        raw_spin_lock(&lock->wait_lock);
        list_for_each_entry(w, &lock->wait_list, list)
                wake_up_process(w->task);
        raw_spin_unlock(&lock->wait_lock);
}
EXPORT_SYMBOL_GPL(six_lock_wakeup_all);

/**
 * six_lock_counts - return held lock counts, for each lock type
 * @lock:       lock to return counters for
 *
 * Return: the number of times a lock is held for read, intent and write.
 */
struct six_lock_count six_lock_counts(struct six_lock *lock)
{
        struct six_lock_count ret;

        ret.n[SIX_LOCK_read]    = !lock->readers
                ? atomic_read(&lock->state) & SIX_LOCK_HELD_read
                : pcpu_read_count(lock);
        ret.n[SIX_LOCK_intent]  = !!(atomic_read(&lock->state) & SIX_LOCK_HELD_intent) +
                lock->intent_lock_recurse;
        ret.n[SIX_LOCK_write]   = !!(atomic_read(&lock->state) & SIX_LOCK_HELD_write);

        return ret;
}
EXPORT_SYMBOL_GPL(six_lock_counts);

/**
 * six_lock_readers_add - directly manipulate reader count of a lock
 * @lock:       lock to add/subtract readers for
 * @nr:         reader count to add/subtract
 *
 * When an upper layer is implementing lock reentrency, we may have both read
 * and intent locks on the same lock.
 *
 * When we need to take a write lock, the read locks will cause self-deadlock,
 * because six locks themselves do not track which read locks are held by the
 * current thread and which are held by a different thread - it does no
 * per-thread tracking of held locks.
 *
 * The upper layer that is tracking held locks may however, if trylock() has
 * failed, count up its own read locks, subtract them, take the write lock, and
 * then re-add them.
 *
 * As in any other situation when taking a write lock, @lock must be held for
 * intent one (or more) times, so @lock will never be left unlocked.
 */
void six_lock_readers_add(struct six_lock *lock, int nr)
{
        if (lock->readers) {
                this_cpu_add(*lock->readers, nr);
        } else {
                EBUG_ON((int) (atomic_read(&lock->state) & SIX_LOCK_HELD_read) + nr < 0);
                /* reader count starts at bit 0 */
                atomic_add(nr, &lock->state);
        }
}
EXPORT_SYMBOL_GPL(six_lock_readers_add);

/**
 * six_lock_exit - release resources held by a lock prior to freeing
 * @lock:       lock to exit
 *
 * When a lock was initialized in percpu mode (SIX_OLCK_INIT_PCPU), this is
 * required to free the percpu read counts.
 */
void six_lock_exit(struct six_lock *lock)
{
        WARN_ON(lock->readers && pcpu_read_count(lock));
        WARN_ON(atomic_read(&lock->state) & SIX_LOCK_HELD_read);

        free_percpu(lock->readers);
        lock->readers = NULL;
}
EXPORT_SYMBOL_GPL(six_lock_exit);

void __six_lock_init(struct six_lock *lock, const char *name,
                     struct lock_class_key *key, enum six_lock_init_flags flags)
{
        atomic_set(&lock->state, 0);
        raw_spin_lock_init(&lock->wait_lock);
        INIT_LIST_HEAD(&lock->wait_list);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
        debug_check_no_locks_freed((void *) lock, sizeof(*lock));
        lockdep_init_map(&lock->dep_map, name, key, 0);
#endif

        /*
         * Don't assume that we have real percpu variables available in
         * userspace:
         */
#ifdef __KERNEL__
        if (flags & SIX_LOCK_INIT_PCPU) {
                /*
                 * We don't return an error here on memory allocation failure
                 * since percpu is an optimization, and locks will work with the
                 * same semantics in non-percpu mode: callers can check for
                 * failure if they wish by checking lock->readers, but generally
                 * will not want to treat it as an error.
                 */
                lock->readers = alloc_percpu(unsigned);
        }
#endif
}
EXPORT_SYMBOL_GPL(__six_lock_init);