#include <linux/atomic.h>
#include <linux/rwsem.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/errno.h>

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

        /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
        __init_rwsem(&brw->rw_sem, name, rwsem_key);
        atomic_set(&brw->write_ctr, 0);
        atomic_set(&brw->slow_read_ctr, 0);
        init_waitqueue_head(&brw->write_waitq);
        return 0;
}

void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
{
        free_percpu(brw->fast_read_ctr);
        brw->fast_read_ctr = NULL; /* catch use after free bugs */
}

/*
 * This is the fast-path for down_read/up_read, it only needs to ensure
 * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the
 * fast per-cpu counter. The writer uses synchronize_sched_expedited() to
 * serialize with the preempt-disabled section below.
 *
 * The nontrivial part is that we should guarantee acquire/release semantics
 * in case when
 *
 *      R_W: down_write() comes after up_read(), the writer should see all
 *           changes done by the reader
 * or
 *      W_R: down_read() comes after up_write(), the reader should see all
 *           changes done by the writer
 *
 * If this helper fails the callers rely on the normal rw_semaphore and
 * atomic_dec_and_test(), so in this case we have the necessary barriers.
 *
 * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or
 * __this_cpu_add() below can be reordered with any LOAD/STORE done by the
 * reader inside the critical section. See the comments in down_write and
 * up_write below.
 */
static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
{
        bool success = false;

        preempt_disable();
        if (likely(!atomic_read(&brw->write_ctr))) {
                __this_cpu_add(*brw->fast_read_ctr, val);
                success = true;
        }
        preempt_enable();

        return success;
}

/*
 * Like the normal down_read() this is not recursive, the writer can
 * come after the first percpu_down_read() and create the deadlock.
 *
 * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
 * percpu_up_read() does rwsem_release(). This pairs with the usage
 * of ->rw_sem in percpu_down/up_write().
 */
void percpu_down_read(struct percpu_rw_semaphore *brw)
{
        might_sleep();
        if (likely(update_fast_ctr(brw, +1))) {
                rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
                return;
        }

        down_read(&brw->rw_sem);
        atomic_inc(&brw->slow_read_ctr);
        /* avoid up_read()->rwsem_release() */
        __up_read(&brw->rw_sem);
}

void percpu_up_read(struct percpu_rw_semaphore *brw)
{
        rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);

        if (likely(update_fast_ctr(brw, -1)))
                return;

        /* false-positive is possible but harmless */
        if (atomic_dec_and_test(&brw->slow_read_ctr))
                wake_up_all(&brw->write_waitq);
}

static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
{
        unsigned int sum = 0;
        int cpu;

        for_each_possible_cpu(cpu) {
                sum += per_cpu(*brw->fast_read_ctr, cpu);
                per_cpu(*brw->fast_read_ctr, cpu) = 0;
        }

        return sum;
}

/*
 * A writer increments ->write_ctr to force the readers to switch to the
 * slow mode, note the atomic_read() check in update_fast_ctr().
 *
 * After that the readers can only inc/dec the slow ->slow_read_ctr counter,
 * ->fast_read_ctr is stable. Once the writer moves its sum into the slow
 * counter it represents the number of active readers.
 *
 * Finally the writer takes ->rw_sem for writing and blocks the new readers,
 * then waits until the slow counter becomes zero.
 */
void percpu_down_write(struct percpu_rw_semaphore *brw)
{
        /* tell update_fast_ctr() there is a pending writer */
        atomic_inc(&brw->write_ctr);
        /*
         * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read
         *    so that update_fast_ctr() can't succeed.
         *
         * 2. Ensures we see the result of every previous this_cpu_add() in
         *    update_fast_ctr().
         *
         * 3. Ensures that if any reader has exited its critical section via
         *    fast-path, it executes a full memory barrier before we return.
         *    See R_W case in the comment above update_fast_ctr().
         */
        synchronize_sched_expedited();

        /* exclude other writers, and block the new readers completely */
        down_write(&brw->rw_sem);

        /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
        atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);

        /* wait for all readers to complete their percpu_up_read() */
        wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
}

void percpu_up_write(struct percpu_rw_semaphore *brw)
{
        /* release the lock, but the readers can't use the fast-path */
        up_write(&brw->rw_sem);
        /*
         * Insert the barrier before the next fast-path in down_read,
         * see W_R case in the comment above update_fast_ctr().
         */
        synchronize_sched_expedited();
        /* the last writer unblocks update_fast_ctr() */
        atomic_dec(&brw->write_ctr);
}