locking/rwsem: Make owner available even if !CONFIG_RWSEM_SPIN_ON_OWNER

[linux.git] / kernel / locking / rwsem-xadd.c

// SPDX-License-Identifier: GPL-2.0
/* rwsem.c: R/W semaphores: contention handling functions
 *
 * Written by David Howells (dhowells@redhat.com).
 * Derived from arch/i386/kernel/semaphore.c
 *
 * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
 * and Michel Lespinasse <walken@google.com>
 *
 * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
 * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
 */
#include <linux/rwsem.h>
#include <linux/init.h>
#include <linux/export.h>
#include <linux/sched/signal.h>
#include <linux/sched/rt.h>
#include <linux/sched/wake_q.h>
#include <linux/sched/debug.h>
#include <linux/osq_lock.h>

#include "rwsem.h"

/*
 * Guide to the rw_semaphore's count field for common values.
 * (32-bit case illustrated, similar for 64-bit)
 *
 * 0x0000000X   (1) X readers active or attempting lock, no writer waiting
 *                  X = #active_readers + #readers attempting to lock
 *                  (X*ACTIVE_BIAS)
 *
 * 0x00000000   rwsem is unlocked, and no one is waiting for the lock or
 *              attempting to read lock or write lock.
 *
 * 0xffff000X   (1) X readers active or attempting lock, with waiters for lock
 *                  X = #active readers + # readers attempting lock
 *                  (X*ACTIVE_BIAS + WAITING_BIAS)
 *              (2) 1 writer attempting lock, no waiters for lock
 *                  X-1 = #active readers + #readers attempting lock
 *                  ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
 *              (3) 1 writer active, no waiters for lock
 *                  X-1 = #active readers + #readers attempting lock
 *                  ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
 *
 * 0xffff0001   (1) 1 reader active or attempting lock, waiters for lock
 *                  (WAITING_BIAS + ACTIVE_BIAS)
 *              (2) 1 writer active or attempting lock, no waiters for lock
 *                  (ACTIVE_WRITE_BIAS)
 *
 * 0xffff0000   (1) There are writers or readers queued but none active
 *                  or in the process of attempting lock.
 *                  (WAITING_BIAS)
 *              Note: writer can attempt to steal lock for this count by adding
 *              ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
 *
 * 0xfffe0001   (1) 1 writer active, or attempting lock. Waiters on queue.
 *                  (ACTIVE_WRITE_BIAS + WAITING_BIAS)
 *
 * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
 *       the count becomes more than 0 for successful lock acquisition,
 *       i.e. the case where there are only readers or nobody has lock.
 *       (1st and 2nd case above).
 *
 *       Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
 *       checking the count becomes ACTIVE_WRITE_BIAS for successful lock
 *       acquisition (i.e. nobody else has lock or attempts lock).  If
 *       unsuccessful, in rwsem_down_write_failed, we'll check to see if there
 *       are only waiters but none active (5th case above), and attempt to
 *       steal the lock.
 *
 */

/*
 * Initialize an rwsem:
 */
void __init_rwsem(struct rw_semaphore *sem, const char *name,
                  struct lock_class_key *key)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
        /*
         * Make sure we are not reinitializing a held semaphore:
         */
        debug_check_no_locks_freed((void *)sem, sizeof(*sem));
        lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
        atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
        raw_spin_lock_init(&sem->wait_lock);
        INIT_LIST_HEAD(&sem->wait_list);
        sem->owner = NULL;
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
        osq_lock_init(&sem->osq);
#endif
}

EXPORT_SYMBOL(__init_rwsem);

enum rwsem_waiter_type {
        RWSEM_WAITING_FOR_WRITE,
        RWSEM_WAITING_FOR_READ
};

struct rwsem_waiter {
        struct list_head list;
        struct task_struct *task;
        enum rwsem_waiter_type type;
};

enum rwsem_wake_type {
        RWSEM_WAKE_ANY,         /* Wake whatever's at head of wait list */
        RWSEM_WAKE_READERS,     /* Wake readers only */
        RWSEM_WAKE_READ_OWNED   /* Waker thread holds the read lock */
};

/*
 * handle the lock release when processes blocked on it that can now run
 * - if we come here from up_xxxx(), then:
 *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
 *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
 * - there must be someone on the queue
 * - the wait_lock must be held by the caller
 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
 *   to actually wakeup the blocked task(s) and drop the reference count,
 *   preferably when the wait_lock is released
 * - woken process blocks are discarded from the list after having task zeroed
 * - writers are only marked woken if downgrading is false
 */
static void __rwsem_mark_wake(struct rw_semaphore *sem,
                              enum rwsem_wake_type wake_type,
                              struct wake_q_head *wake_q)
{
        struct rwsem_waiter *waiter, *tmp;
        long oldcount, woken = 0, adjustment = 0;
        struct list_head wlist;

        /*
         * Take a peek at the queue head waiter such that we can determine
         * the wakeup(s) to perform.
         */
        waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);

        if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
                if (wake_type == RWSEM_WAKE_ANY) {
                        /*
                         * Mark writer at the front of the queue for wakeup.
                         * Until the task is actually later awoken later by
                         * the caller, other writers are able to steal it.
                         * Readers, on the other hand, will block as they
                         * will notice the queued writer.
                         */
                        wake_q_add(wake_q, waiter->task);
                        lockevent_inc(rwsem_wake_writer);
                }

                return;
        }

        /*
         * Writers might steal the lock before we grant it to the next reader.
         * We prefer to do the first reader grant before counting readers
         * so we can bail out early if a writer stole the lock.
         */
        if (wake_type != RWSEM_WAKE_READ_OWNED) {
                adjustment = RWSEM_ACTIVE_READ_BIAS;
 try_reader_grant:
                oldcount = atomic_long_fetch_add(adjustment, &sem->count);
                if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
                        /*
                         * If the count is still less than RWSEM_WAITING_BIAS
                         * after removing the adjustment, it is assumed that
                         * a writer has stolen the lock. We have to undo our
                         * reader grant.
                         */
                        if (atomic_long_add_return(-adjustment, &sem->count) <
                            RWSEM_WAITING_BIAS)
                                return;

                        /* Last active locker left. Retry waking readers. */
                        goto try_reader_grant;
                }
                /*
                 * Set it to reader-owned to give spinners an early
                 * indication that readers now have the lock.
                 */
                __rwsem_set_reader_owned(sem, waiter->task);
        }

        /*
         * Grant an infinite number of read locks to the readers at the front
         * of the queue. We know that woken will be at least 1 as we accounted
         * for above. Note we increment the 'active part' of the count by the
         * number of readers before waking any processes up.
         *
         * We have to do wakeup in 2 passes to prevent the possibility that
         * the reader count may be decremented before it is incremented. It
         * is because the to-be-woken waiter may not have slept yet. So it
         * may see waiter->task got cleared, finish its critical section and
         * do an unlock before the reader count increment.
         *
         * 1) Collect the read-waiters in a separate list, count them and
         *    fully increment the reader count in rwsem.
         * 2) For each waiters in the new list, clear waiter->task and
         *    put them into wake_q to be woken up later.
         */
        list_for_each_entry(waiter, &sem->wait_list, list) {
                if (waiter->type == RWSEM_WAITING_FOR_WRITE)
                        break;

                woken++;
        }
        list_cut_before(&wlist, &sem->wait_list, &waiter->list);

        adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
        lockevent_cond_inc(rwsem_wake_reader, woken);
        if (list_empty(&sem->wait_list)) {
                /* hit end of list above */
                adjustment -= RWSEM_WAITING_BIAS;
        }

        if (adjustment)
                atomic_long_add(adjustment, &sem->count);

        /* 2nd pass */
        list_for_each_entry_safe(waiter, tmp, &wlist, list) {
                struct task_struct *tsk;

                tsk = waiter->task;
                get_task_struct(tsk);

                /*
                 * Ensure calling get_task_struct() before setting the reader
                 * waiter to nil such that rwsem_down_read_failed() cannot
                 * race with do_exit() by always holding a reference count
                 * to the task to wakeup.
                 */
                smp_store_release(&waiter->task, NULL);
                /*
                 * Ensure issuing the wakeup (either by us or someone else)
                 * after setting the reader waiter to nil.
                 */
                wake_q_add_safe(wake_q, tsk);
        }
}

/*
 * This function must be called with the sem->wait_lock held to prevent
 * race conditions between checking the rwsem wait list and setting the
 * sem->count accordingly.
 */
static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
{
        /*
         * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
         */
        if (count != RWSEM_WAITING_BIAS)
                return false;

        /*
         * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
         * are other tasks on the wait list, we need to add on WAITING_BIAS.
         */
        count = list_is_singular(&sem->wait_list) ?
                        RWSEM_ACTIVE_WRITE_BIAS :
                        RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;

        if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
                                                        == RWSEM_WAITING_BIAS) {
                rwsem_set_owner(sem);
                return true;
        }

        return false;
}

#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
 * Try to acquire write lock before the writer has been put on wait queue.
 */
static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
{
        long count = atomic_long_read(&sem->count);

        while (!count || count == RWSEM_WAITING_BIAS) {
                if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
                                        count + RWSEM_ACTIVE_WRITE_BIAS)) {
                        rwsem_set_owner(sem);
                        lockevent_inc(rwsem_opt_wlock);
                        return true;
                }
        }
        return false;
}

static inline bool owner_on_cpu(struct task_struct *owner)
{
        /*
         * As lock holder preemption issue, we both skip spinning if
         * task is not on cpu or its cpu is preempted
         */
        return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
}

static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
        struct task_struct *owner;
        bool ret = true;

        BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN));

        if (need_resched())
                return false;

        rcu_read_lock();
        owner = READ_ONCE(sem->owner);
        if (owner) {
                ret = is_rwsem_owner_spinnable(owner) &&
                      owner_on_cpu(owner);
        }
        rcu_read_unlock();
        return ret;
}

/*
 * Return true only if we can still spin on the owner field of the rwsem.
 */
static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
{
        struct task_struct *owner = READ_ONCE(sem->owner);

        if (!is_rwsem_owner_spinnable(owner))
                return false;

        rcu_read_lock();
        while (owner && (READ_ONCE(sem->owner) == owner)) {
                /*
                 * Ensure we emit the owner->on_cpu, dereference _after_
                 * checking sem->owner still matches owner, if that fails,
                 * owner might point to free()d memory, if it still matches,
                 * the rcu_read_lock() ensures the memory stays valid.
                 */
                barrier();

                /*
                 * abort spinning when need_resched or owner is not running or
                 * owner's cpu is preempted.
                 */
                if (need_resched() || !owner_on_cpu(owner)) {
                        rcu_read_unlock();
                        return false;
                }

                cpu_relax();
        }
        rcu_read_unlock();

        /*
         * If there is a new owner or the owner is not set, we continue
         * spinning.
         */
        return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
}

static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
        bool taken = false;

        preempt_disable();

        /* sem->wait_lock should not be held when doing optimistic spinning */
        if (!rwsem_can_spin_on_owner(sem))
                goto done;

        if (!osq_lock(&sem->osq))
                goto done;

        /*
         * Optimistically spin on the owner field and attempt to acquire the
         * lock whenever the owner changes. Spinning will be stopped when:
         *  1) the owning writer isn't running; or
         *  2) readers own the lock as we can't determine if they are
         *     actively running or not.
         */
        while (rwsem_spin_on_owner(sem)) {
                /*
                 * Try to acquire the lock
                 */
                if (rwsem_try_write_lock_unqueued(sem)) {
                        taken = true;
                        break;
                }

                /*
                 * 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 (!sem->owner && (need_resched() || rt_task(current)))
                        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(&sem->osq);
done:
        preempt_enable();
        lockevent_cond_inc(rwsem_opt_fail, !taken);
        return taken;
}

/*
 * Return true if the rwsem has active spinner
 */
static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
{
        return osq_is_locked(&sem->osq);
}

#else
static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
{
        return false;
}

static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
{
        return false;
}
#endif

/*
 * Wait for the read lock to be granted
 */
static inline struct rw_semaphore __sched *
__rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
{
        long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
        struct rwsem_waiter waiter;
        DEFINE_WAKE_Q(wake_q);

        waiter.task = current;
        waiter.type = RWSEM_WAITING_FOR_READ;

        raw_spin_lock_irq(&sem->wait_lock);
        if (list_empty(&sem->wait_list)) {
                /*
                 * In case the wait queue is empty and the lock isn't owned
                 * by a writer, this reader can exit the slowpath and return
                 * immediately as its RWSEM_ACTIVE_READ_BIAS has already
                 * been set in the count.
                 */
                if (atomic_long_read(&sem->count) >= 0) {
                        raw_spin_unlock_irq(&sem->wait_lock);
                        rwsem_set_reader_owned(sem);
                        lockevent_inc(rwsem_rlock_fast);
                        return sem;
                }
                adjustment += RWSEM_WAITING_BIAS;
        }
        list_add_tail(&waiter.list, &sem->wait_list);

        /* we're now waiting on the lock, but no longer actively locking */
        count = atomic_long_add_return(adjustment, &sem->count);

        /*
         * If there are no active locks, wake the front queued process(es).
         *
         * If there are no writers and we are first in the queue,
         * wake our own waiter to join the existing active readers !
         */
        if (count == RWSEM_WAITING_BIAS ||
            (count > RWSEM_WAITING_BIAS &&
             adjustment != -RWSEM_ACTIVE_READ_BIAS))
                __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

        raw_spin_unlock_irq(&sem->wait_lock);
        wake_up_q(&wake_q);

        /* wait to be given the lock */
        while (true) {
                set_current_state(state);
                if (!waiter.task)
                        break;
                if (signal_pending_state(state, current)) {
                        raw_spin_lock_irq(&sem->wait_lock);
                        if (waiter.task)
                                goto out_nolock;
                        raw_spin_unlock_irq(&sem->wait_lock);
                        break;
                }
                schedule();
                lockevent_inc(rwsem_sleep_reader);
        }

        __set_current_state(TASK_RUNNING);
        lockevent_inc(rwsem_rlock);
        return sem;
out_nolock:
        list_del(&waiter.list);
        if (list_empty(&sem->wait_list))
                atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
        raw_spin_unlock_irq(&sem->wait_lock);
        __set_current_state(TASK_RUNNING);
        lockevent_inc(rwsem_rlock_fail);
        return ERR_PTR(-EINTR);
}

__visible struct rw_semaphore * __sched
rwsem_down_read_failed(struct rw_semaphore *sem)
{
        return __rwsem_down_read_failed_common(sem, TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(rwsem_down_read_failed);

__visible struct rw_semaphore * __sched
rwsem_down_read_failed_killable(struct rw_semaphore *sem)
{
        return __rwsem_down_read_failed_common(sem, TASK_KILLABLE);
}
EXPORT_SYMBOL(rwsem_down_read_failed_killable);

/*
 * Wait until we successfully acquire the write lock
 */
static inline struct rw_semaphore *
__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
{
        long count;
        bool waiting = true; /* any queued threads before us */
        struct rwsem_waiter waiter;
        struct rw_semaphore *ret = sem;
        DEFINE_WAKE_Q(wake_q);

        /* undo write bias from down_write operation, stop active locking */
        count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);

        /* do optimistic spinning and steal lock if possible */
        if (rwsem_optimistic_spin(sem))
                return sem;

        /*
         * Optimistic spinning failed, proceed to the slowpath
         * and block until we can acquire the sem.
         */
        waiter.task = current;
        waiter.type = RWSEM_WAITING_FOR_WRITE;

        raw_spin_lock_irq(&sem->wait_lock);

        /* account for this before adding a new element to the list */
        if (list_empty(&sem->wait_list))
                waiting = false;

        list_add_tail(&waiter.list, &sem->wait_list);

        /* we're now waiting on the lock, but no longer actively locking */
        if (waiting) {
                count = atomic_long_read(&sem->count);

                /*
                 * If there were already threads queued before us and there are
                 * no active writers, the lock must be read owned; so we try to
                 * wake any read locks that were queued ahead of us.
                 */
                if (count > RWSEM_WAITING_BIAS) {
                        __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
                        /*
                         * The wakeup is normally called _after_ the wait_lock
                         * is released, but given that we are proactively waking
                         * readers we can deal with the wake_q overhead as it is
                         * similar to releasing and taking the wait_lock again
                         * for attempting rwsem_try_write_lock().
                         */
                        wake_up_q(&wake_q);

                        /*
                         * Reinitialize wake_q after use.
                         */
                        wake_q_init(&wake_q);
                }

        } else
                count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);

        /* wait until we successfully acquire the lock */
        set_current_state(state);
        while (true) {
                if (rwsem_try_write_lock(count, sem))
                        break;
                raw_spin_unlock_irq(&sem->wait_lock);

                /* Block until there are no active lockers. */
                do {
                        if (signal_pending_state(state, current))
                                goto out_nolock;

                        schedule();
                        lockevent_inc(rwsem_sleep_writer);
                        set_current_state(state);
                } while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);

                raw_spin_lock_irq(&sem->wait_lock);
        }
        __set_current_state(TASK_RUNNING);
        list_del(&waiter.list);
        raw_spin_unlock_irq(&sem->wait_lock);
        lockevent_inc(rwsem_wlock);

        return ret;

out_nolock:
        __set_current_state(TASK_RUNNING);
        raw_spin_lock_irq(&sem->wait_lock);
        list_del(&waiter.list);
        if (list_empty(&sem->wait_list))
                atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
        else
                __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
        raw_spin_unlock_irq(&sem->wait_lock);
        wake_up_q(&wake_q);
        lockevent_inc(rwsem_wlock_fail);

        return ERR_PTR(-EINTR);
}

__visible struct rw_semaphore * __sched
rwsem_down_write_failed(struct rw_semaphore *sem)
{
        return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(rwsem_down_write_failed);

__visible struct rw_semaphore * __sched
rwsem_down_write_failed_killable(struct rw_semaphore *sem)
{
        return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
}
EXPORT_SYMBOL(rwsem_down_write_failed_killable);

/*
 * handle waking up a waiter on the semaphore
 * - up_read/up_write has decremented the active part of count if we come here
 */
__visible
struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
{
        unsigned long flags;
        DEFINE_WAKE_Q(wake_q);

        /*
        * __rwsem_down_write_failed_common(sem)
        *   rwsem_optimistic_spin(sem)
        *     osq_unlock(sem->osq)
        *   ...
        *   atomic_long_add_return(&sem->count)
        *
        *      - VS -
        *
        *              __up_write()
        *                if (atomic_long_sub_return_release(&sem->count) < 0)
        *                  rwsem_wake(sem)
        *                    osq_is_locked(&sem->osq)
        *
        * And __up_write() must observe !osq_is_locked() when it observes the
        * atomic_long_add_return() in order to not miss a wakeup.
        *
        * This boils down to:
        *
        * [S.rel] X = 1                [RmW] r0 = (Y += 0)
        *         MB                         RMB
        * [RmW]   Y += 1               [L]   r1 = X
        *
        * exists (r0=1 /\ r1=0)
        */
        smp_rmb();

        /*
         * If a spinner is present, it is not necessary to do the wakeup.
         * Try to do wakeup only if the trylock succeeds to minimize
         * spinlock contention which may introduce too much delay in the
         * unlock operation.
         *
         *    spinning writer           up_write/up_read caller
         *    ---------------           -----------------------
         * [S]   osq_unlock()           [L]   osq
         *       MB                           RMB
         * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
         *
         * Here, it is important to make sure that there won't be a missed
         * wakeup while the rwsem is free and the only spinning writer goes
         * to sleep without taking the rwsem. Even when the spinning writer
         * is just going to break out of the waiting loop, it will still do
         * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
         * rwsem_has_spinner() is true, it will guarantee at least one
         * trylock attempt on the rwsem later on.
         */
        if (rwsem_has_spinner(sem)) {
                /*
                 * The smp_rmb() here is to make sure that the spinner
                 * state is consulted before reading the wait_lock.
                 */
                smp_rmb();
                if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
                        return sem;
                goto locked;
        }
        raw_spin_lock_irqsave(&sem->wait_lock, flags);
locked:

        if (!list_empty(&sem->wait_list))
                __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        wake_up_q(&wake_q);

        return sem;
}
EXPORT_SYMBOL(rwsem_wake);

/*
 * downgrade a write lock into a read lock
 * - caller incremented waiting part of count and discovered it still negative
 * - just wake up any readers at the front of the queue
 */
__visible
struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
        unsigned long flags;
        DEFINE_WAKE_Q(wake_q);

        raw_spin_lock_irqsave(&sem->wait_lock, flags);

        if (!list_empty(&sem->wait_list))
                __rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);

        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        wake_up_q(&wake_q);

        return sem;
}
EXPORT_SYMBOL(rwsem_downgrade_wake);