#include <linux/atomic.h>
#include <linux/rwsem.h>
#include <linux/percpu.h>
#include <linux/lockdep.h>
#include <linux/percpu-rwsem.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/errno.h>

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

        /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
        rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
        __init_rwsem(&sem->rw_sem, name, rwsem_key);
        rcuwait_init(&sem->writer);
        sem->readers_block = 0;
        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);

int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
{
        /*
         * 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 readers_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 readers_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 !readers_block the critical section starts here, matched by the
         * release in percpu_up_write().
         */
        if (likely(!smp_load_acquire(&sem->readers_block)))
                return 1;

        /*
         * Per the above comment; we still have preemption disabled and
         * will thus decrement on the same CPU as we incremented.
         */
        __percpu_up_read(sem);

        if (try)
                return 0;

        /*
         * We either call schedule() in the wait, or we'll fall through
         * and reschedule on the preempt_enable() in percpu_down_read().
         */
        preempt_enable_no_resched();

        /*
         * Avoid lockdep for the down/up_read() we already have them.
         */
        __down_read(&sem->rw_sem);
        this_cpu_inc(*sem->read_count);
        __up_read(&sem->rw_sem);

        preempt_disable();
        return 1;
}
EXPORT_SYMBOL_GPL(__percpu_down_read);

void __percpu_up_read(struct percpu_rw_semaphore *sem)
{
        smp_mb(); /* B matches C */
        /*
         * In other words, if they see our decrement (presumably to aggregate
         * zero, as that is the only time it matters) they will also see our
         * critical section.
         */
        __this_cpu_dec(*sem->read_count);

        /* Prod writer to recheck readers_active */
        rcuwait_wake_up(&sem->writer);
}
EXPORT_SYMBOL_GPL(__percpu_up_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.
 */
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)
{
        /* Notify readers to take the slow path. */
        rcu_sync_enter(&sem->rss);

        down_write(&sem->rw_sem);

        /*
         * Notify new readers to block; up until now, and thus throughout the
         * longish rcu_sync_enter() above, new readers could still come in.
         */
        WRITE_ONCE(sem->readers_block, 1);

        smp_mb(); /* D matches A */

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

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

void percpu_up_write(struct percpu_rw_semaphore *sem)
{
        /*
         * 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.
         */
        smp_store_release(&sem->readers_block, 0);

        /*
         * Release the write lock, this will allow readers back in the game.
         */
        up_write(&sem->rw_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);