// SPDX-License-Identifier: GPL-2.0-only

/*
 * RT-specific reader/writer semaphores and reader/writer locks
 *
 * down_write/write_lock()
 *  1) Lock rtmutex
 *  2) Remove the reader BIAS to force readers into the slow path
 *  3) Wait until all readers have left the critical section
 *  4) Mark it write locked
 *
 * up_write/write_unlock()
 *  1) Remove the write locked marker
 *  2) Set the reader BIAS, so readers can use the fast path again
 *  3) Unlock rtmutex, to release blocked readers
 *
 * down_read/read_lock()
 *  1) Try fast path acquisition (reader BIAS is set)
 *  2) Take tmutex::wait_lock, which protects the writelocked flag
 *  3) If !writelocked, acquire it for read
 *  4) If writelocked, block on tmutex
 *  5) unlock rtmutex, goto 1)
 *
 * up_read/read_unlock()
 *  1) Try fast path release (reader count != 1)
 *  2) Wake the writer waiting in down_write()/write_lock() #3
 *
 * down_read/read_lock()#3 has the consequence, that rw semaphores and rw
 * locks on RT are not writer fair, but writers, which should be avoided in
 * RT tasks (think mmap_sem), are subject to the rtmutex priority/DL
 * inheritance mechanism.
 *
 * It's possible to make the rw primitives writer fair by keeping a list of
 * active readers. A blocked writer would force all newly incoming readers
 * to block on the rtmutex, but the rtmutex would have to be proxy locked
 * for one reader after the other. We can't use multi-reader inheritance
 * because there is no way to support that with SCHED_DEADLINE.
 * Implementing the one by one reader boosting/handover mechanism is a
 * major surgery for a very dubious value.
 *
 * The risk of writer starvation is there, but the pathological use cases
 * which trigger it are not necessarily the typical RT workloads.
 *
 * Fast-path orderings:
 * The lock/unlock of readers can run in fast paths: lock and unlock are only
 * atomic ops, and there is no inner lock to provide ACQUIRE and RELEASE
 * semantics of rwbase_rt. Atomic ops should thus provide _acquire()
 * and _release() (or stronger).
 *
 * Common code shared between RT rw_semaphore and rwlock
 */

static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb)
{
        int r;

        /*
         * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
         * set.
         */
        for (r = atomic_read(&rwb->readers); r < 0;) {
                if (likely(atomic_try_cmpxchg_acquire(&rwb->readers, &r, r + 1)))
                        return 1;
        }
        return 0;
}

static int __sched __rwbase_read_lock(struct rwbase_rt *rwb,
                                      unsigned int state)
{
        struct rt_mutex_base *rtm = &rwb->rtmutex;
        DEFINE_WAKE_Q(wake_q);
        int ret;

        rwbase_pre_schedule();
        raw_spin_lock_irq(&rtm->wait_lock);

        /*
         * Call into the slow lock path with the rtmutex->wait_lock
         * held, so this can't result in the following race:
         *
         * Reader1              Reader2         Writer
         *                      down_read()
         *                                      down_write()
         *                                      rtmutex_lock(m)
         *                                      wait()
         * down_read()
         * unlock(m->wait_lock)
         *                      up_read()
         *                      wake(Writer)
         *                                      lock(m->wait_lock)
         *                                      sem->writelocked=true
         *                                      unlock(m->wait_lock)
         *
         *                                      up_write()
         *                                      sem->writelocked=false
         *                                      rtmutex_unlock(m)
         *                      down_read()
         *                                      down_write()
         *                                      rtmutex_lock(m)
         *                                      wait()
         * rtmutex_lock(m)
         *
         * That would put Reader1 behind the writer waiting on
         * Reader2 to call up_read(), which might be unbound.
         */

        trace_contention_begin(rwb, LCB_F_RT | LCB_F_READ);

        /*
         * For rwlocks this returns 0 unconditionally, so the below
         * !ret conditionals are optimized out.
         */
        ret = rwbase_rtmutex_slowlock_locked(rtm, state, &wake_q);

        /*
         * On success the rtmutex is held, so there can't be a writer
         * active. Increment the reader count and immediately drop the
         * rtmutex again.
         *
         * rtmutex->wait_lock has to be unlocked in any case of course.
         */
        if (!ret)
                atomic_inc(&rwb->readers);

        preempt_disable();
        raw_spin_unlock_irq(&rtm->wait_lock);
        wake_up_q(&wake_q);
        preempt_enable();

        if (!ret)
                rwbase_rtmutex_unlock(rtm);

        trace_contention_end(rwb, ret);
        rwbase_post_schedule();
        return ret;
}

static __always_inline int rwbase_read_lock(struct rwbase_rt *rwb,
                                            unsigned int state)
{
        lockdep_assert(!current->pi_blocked_on);

        if (rwbase_read_trylock(rwb))
                return 0;

        return __rwbase_read_lock(rwb, state);
}

static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
                                         unsigned int state)
{
        struct rt_mutex_base *rtm = &rwb->rtmutex;
        struct task_struct *owner;
        DEFINE_RT_WAKE_Q(wqh);

        raw_spin_lock_irq(&rtm->wait_lock);
        /*
         * Wake the writer, i.e. the rtmutex owner. It might release the
         * rtmutex concurrently in the fast path (due to a signal), but to
         * clean up rwb->readers it needs to acquire rtm->wait_lock. The
         * worst case which can happen is a spurious wakeup.
         */
        owner = rt_mutex_owner(rtm);
        if (owner)
                rt_mutex_wake_q_add_task(&wqh, owner, state);

        /* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
        preempt_disable();
        raw_spin_unlock_irq(&rtm->wait_lock);
        rt_mutex_wake_up_q(&wqh);
}

static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
                                               unsigned int state)
{
        if (trace_contended_release_enabled() && rt_mutex_owner(&rwb->rtmutex))
                trace_call__contended_release(rwb);
        /*
         * rwb->readers can only hit 0 when a writer is waiting for the
         * active readers to leave the critical section.
         *
         * dec_and_test() is fully ordered, provides RELEASE.
         */
        if (unlikely(atomic_dec_and_test(&rwb->readers)))
                __rwbase_read_unlock(rwb, state);
}

static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias,
                                         unsigned long flags)
        __releases(&rwb->rtmutex.wait_lock)
{
        struct rt_mutex_base *rtm = &rwb->rtmutex;

        /*
         * _release() is needed in case that reader is in fast path, pairing
         * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().
         */
        (void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);
        raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
        rwbase_rtmutex_unlock(rtm);
}

static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
{
        struct rt_mutex_base *rtm = &rwb->rtmutex;
        unsigned long flags;

        raw_spin_lock_irqsave(&rtm->wait_lock, flags);
        if (trace_contended_release_enabled() && rt_mutex_has_waiters(rtm))
                trace_call__contended_release(rwb);
        __rwbase_write_unlock(rwb, WRITER_BIAS, flags);
}

static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
{
        struct rt_mutex_base *rtm = &rwb->rtmutex;
        unsigned long flags;

        raw_spin_lock_irqsave(&rtm->wait_lock, flags);
        if (trace_contended_release_enabled() && rt_mutex_has_waiters(rtm))
                trace_call__contended_release(rwb);
        /* Release it and account current as reader */
        __rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
}

static inline bool __rwbase_write_trylock(struct rwbase_rt *rwb)
{
        /* Can do without CAS because we're serialized by wait_lock. */
        lockdep_assert_held(&rwb->rtmutex.wait_lock);

        /*
         * _acquire is needed in case the reader is in the fast path, pairing
         * with rwbase_read_unlock(), provides ACQUIRE.
         */
        if (!atomic_read_acquire(&rwb->readers)) {
                atomic_set(&rwb->readers, WRITER_BIAS);
                return 1;
        }

        return 0;
}

static int __sched rwbase_write_lock(struct rwbase_rt *rwb,
                                     unsigned int state)
{
        struct rt_mutex_base *rtm = &rwb->rtmutex;
        unsigned long flags;

        /* Take the rtmutex as a first step */
        if (rwbase_rtmutex_lock_state(rtm, state))
                return -EINTR;

        /* Force readers into slow path */
        atomic_sub(READER_BIAS, &rwb->readers);

        rwbase_pre_schedule();

        raw_spin_lock_irqsave(&rtm->wait_lock, flags);
        if (__rwbase_write_trylock(rwb))
                goto out_unlock;

        rwbase_set_and_save_current_state(state);
        trace_contention_begin(rwb, LCB_F_RT | LCB_F_WRITE);
        for (;;) {
                /* Optimized out for rwlocks */
                if (rwbase_signal_pending_state(state, current)) {
                        rwbase_restore_current_state();
                        __rwbase_write_unlock(rwb, 0, flags);
                        rwbase_post_schedule();
                        trace_contention_end(rwb, -EINTR);
                        return -EINTR;
                }

                if (__rwbase_write_trylock(rwb))
                        break;

                raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
                rwbase_schedule();
                raw_spin_lock_irqsave(&rtm->wait_lock, flags);

                set_current_state(state);
        }
        rwbase_restore_current_state();
        trace_contention_end(rwb, 0);

out_unlock:
        raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
        rwbase_post_schedule();
        return 0;
}

static inline int rwbase_write_trylock(struct rwbase_rt *rwb)
{
        struct rt_mutex_base *rtm = &rwb->rtmutex;
        unsigned long flags;

        if (!rwbase_rtmutex_trylock(rtm))
                return 0;

        atomic_sub(READER_BIAS, &rwb->readers);

        raw_spin_lock_irqsave(&rtm->wait_lock, flags);
        if (__rwbase_write_trylock(rwb)) {
                raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
                return 1;
        }
        __rwbase_write_unlock(rwb, 0, flags);
        return 0;
}