// SPDX-License-Identifier: GPL-2.0-only
#include <linux/atomic.h>
#include <linux/percpu.h>
#include <linux/wait.h>
#include <linux/lockdep.h>
#include <linux/percpu-rwsem.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/sched/task.h>
#include <linux/errno.h>

int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
                        const char *name, struct lock_class_key *key)
{
        sem->read_count = alloc_percpu(int);
        if (unlikely(!sem->read_count))
                return -ENOMEM;

        rcu_sync_init(&sem->rss);
        rcuwait_init(&sem->writer);
        init_waitqueue_head(&sem->waiters);
        atomic_set(&sem->block, 0);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
        debug_check_no_locks_freed((void *)sem, sizeof(*sem));
        lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
        return 0;
}
EXPORT_SYMBOL_GPL(__percpu_init_rwsem);

void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
{
        /*
         * XXX: temporary kludge. The error path in alloc_super()
         * assumes that percpu_free_rwsem() is safe after kzalloc().
         */
        if (!sem->read_count)
                return;

        rcu_sync_dtor(&sem->rss);
        free_percpu(sem->read_count);
        sem->read_count = NULL; /* catch use after free bugs */
}
EXPORT_SYMBOL_GPL(percpu_free_rwsem);

static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
        this_cpu_inc(*sem->read_count);

        /*
         * Due to having preemption disabled the decrement happens on
         * the same CPU as the increment, avoiding the
         * increment-on-one-CPU-and-decrement-on-another problem.
         *
         * If the reader misses the writer's assignment of sem->block, then the
         * writer is guaranteed to see the reader's increment.
         *
         * Conversely, any readers that increment their sem->read_count after
         * the writer looks are guaranteed to see the sem->block value, which
         * in turn means that they are guaranteed to immediately decrement
         * their sem->read_count, so that it doesn't matter that the writer
         * missed them.
         */

        smp_mb(); /* A matches D */

        /*
         * If !sem->block the critical section starts here, matched by the
         * release in percpu_up_write().
         */
        if (likely(!atomic_read_acquire(&sem->block)))
                return true;

        this_cpu_dec(*sem->read_count);

        /* Prod writer to re-evaluate readers_active_check() */
        rcuwait_wake_up(&sem->writer);

        return false;
}

static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem)
{
        if (atomic_read(&sem->block))
                return false;

        return atomic_xchg(&sem->block, 1) == 0;
}

static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader)
{
        if (reader) {
                bool ret;

                preempt_disable();
                ret = __percpu_down_read_trylock(sem);
                preempt_enable();

                return ret;
        }
        return __percpu_down_write_trylock(sem);
}

/*
 * The return value of wait_queue_entry::func means:
 *
 *  <0 - error, wakeup is terminated and the error is returned
 *   0 - no wakeup, a next waiter is tried
 *  >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive.
 *
 * We use EXCLUSIVE for both readers and writers to preserve FIFO order,
 * and play games with the return value to allow waking multiple readers.
 *
 * Specifically, we wake readers until we've woken a single writer, or until a
 * trylock fails.
 */
static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
                                      unsigned int mode, int wake_flags,
                                      void *key)
{
        bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
        struct percpu_rw_semaphore *sem = key;
        struct task_struct *p;

        /* concurrent against percpu_down_write(), can get stolen */
        if (!__percpu_rwsem_trylock(sem, reader))
                return 1;

        p = get_task_struct(wq_entry->private);
        list_del_init(&wq_entry->entry);
        smp_store_release(&wq_entry->private, NULL);

        wake_up_process(p);
        put_task_struct(p);

        return !reader; /* wake (readers until) 1 writer */
}

static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
{
        DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
        bool wait;

        spin_lock_irq(&sem->waiters.lock);
        /*
         * Serialize against the wakeup in percpu_up_write(), if we fail
         * the trylock, the wakeup must see us on the list.
         */
        wait = !__percpu_rwsem_trylock(sem, reader);
        if (wait) {
                wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
                __add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
        }
        spin_unlock_irq(&sem->waiters.lock);

        while (wait) {
                set_current_state(TASK_UNINTERRUPTIBLE);
                if (!smp_load_acquire(&wq_entry.private))
                        break;
                schedule();
        }
        __set_current_state(TASK_RUNNING);
}

bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
{
        if (__percpu_down_read_trylock(sem))
                return true;

        if (try)
                return false;

        preempt_enable();
        percpu_rwsem_wait(sem, /* .reader = */ true);
        preempt_disable();

        return true;
}
EXPORT_SYMBOL_GPL(__percpu_down_read);

#define per_cpu_sum(var)                                                \
({                                                                      \
        typeof(var) __sum = 0;                                          \
        int cpu;                                                        \
        compiletime_assert_atomic_type(__sum);                          \
        for_each_possible_cpu(cpu)                                      \
                __sum += per_cpu(var, cpu);                             \
        __sum;                                                          \
})

/*
 * Return true if the modular sum of the sem->read_count per-CPU variable is
 * zero.  If this sum is zero, then it is stable due to the fact that if any
 * newly arriving readers increment a given counter, they will immediately
 * decrement that same counter.
 *
 * Assumes sem->block is set.
 */
static bool readers_active_check(struct percpu_rw_semaphore *sem)
{
        if (per_cpu_sum(*sem->read_count) != 0)
                return false;

        /*
         * If we observed the decrement; ensure we see the entire critical
         * section.
         */

        smp_mb(); /* C matches B */

        return true;
}

void percpu_down_write(struct percpu_rw_semaphore *sem)
{
        might_sleep();
        rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);

        /* Notify readers to take the slow path. */
        rcu_sync_enter(&sem->rss);

        /*
         * Try set sem->block; this provides writer-writer exclusion.
         * Having sem->block set makes new readers block.
         */
        if (!__percpu_down_write_trylock(sem))
                percpu_rwsem_wait(sem, /* .reader = */ false);

        /* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */

        /*
         * If they don't see our store of sem->block, then we are guaranteed to
         * see their sem->read_count increment, and therefore will wait for
         * them.
         */

        /* Wait for all active readers to complete. */
        rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL_GPL(percpu_down_write);

void percpu_up_write(struct percpu_rw_semaphore *sem)
{
        rwsem_release(&sem->dep_map, _RET_IP_);

        /*
         * Signal the writer is done, no fast path yet.
         *
         * One reason that we cannot just immediately flip to readers_fast is
         * that new readers might fail to see the results of this writer's
         * critical section.
         *
         * Therefore we force it through the slow path which guarantees an
         * acquire and thereby guarantees the critical section's consistency.
         */
        atomic_set_release(&sem->block, 0);

        /*
         * Prod any pending reader/writer to make progress.
         */
        __wake_up(&sem->waiters, TASK_NORMAL, 1, sem);

        /*
         * Once this completes (at least one RCU-sched grace period hence) the
         * reader fast path will be available again. Safe to use outside the
         * exclusive write lock because its counting.
         */
        rcu_sync_exit(&sem->rss);
}
EXPORT_SYMBOL_GPL(percpu_up_write);