// SPDX-License-Identifier: GPL-2.0
/*
 * Regression1
 * Description:
 * Salman Qazi describes the following radix-tree bug:
 *
 * In the following case, we get can get a deadlock:
 *
 * 0.  The radix tree contains two items, one has the index 0.
 * 1.  The reader (in this case find_get_pages) takes the rcu_read_lock.
 * 2.  The reader acquires slot(s) for item(s) including the index 0 item.
 * 3.  The non-zero index item is deleted, and as a consequence the other item
 *     is moved to the root of the tree. The place where it used to be is queued
 *     for deletion after the readers finish.
 * 3b. The zero item is deleted, removing it from the direct slot, it remains in
 *     the rcu-delayed indirect node.
 * 4.  The reader looks at the index 0 slot, and finds that the page has 0 ref
 *     count
 * 5.  The reader looks at it again, hoping that the item will either be freed
 *     or the ref count will increase. This never happens, as the slot it is
 *     looking at will never be updated. Also, this slot can never be reclaimed
 *     because the reader is holding rcu_read_lock and is in an infinite loop.
 *
 * The fix is to re-use the same "indirect" pointer case that requires a slot
 * lookup retry into a general "retry the lookup" bit.
 *
 * Running:
 * This test should run to completion in a few seconds. The above bug would
 * cause it to hang indefinitely.
 *
 * Upstream commit:
 * Not yet
 */
#include <linux/kernel.h>
#include <linux/gfp.h>
#include <linux/slab.h>
#include <linux/radix-tree.h>
#include <linux/rcupdate.h>
#include <stdlib.h>
#include <pthread.h>
#include <stdio.h>
#include <assert.h>

#include "regression.h"

static RADIX_TREE(mt_tree, GFP_KERNEL);
static pthread_mutex_t mt_lock = PTHREAD_MUTEX_INITIALIZER;

struct page {
        pthread_mutex_t lock;
        struct rcu_head rcu;
        int count;
        unsigned long index;
};

static struct page *page_alloc(void)
{
        struct page *p;
        p = malloc(sizeof(struct page));
        p->count = 1;
        p->index = 1;
        pthread_mutex_init(&p->lock, NULL);

        return p;
}

static void page_rcu_free(struct rcu_head *rcu)
{
        struct page *p = container_of(rcu, struct page, rcu);
        assert(!p->count);
        pthread_mutex_destroy(&p->lock);
        free(p);
}

static void page_free(struct page *p)
{
        call_rcu(&p->rcu, page_rcu_free);
}

static unsigned find_get_pages(unsigned long start,
                            unsigned int nr_pages, struct page **pages)
{
        unsigned int i;
        unsigned int ret;
        unsigned int nr_found;

        rcu_read_lock();
restart:
        nr_found = radix_tree_gang_lookup_slot(&mt_tree,
                                (void ***)pages, NULL, start, nr_pages);
        ret = 0;
        for (i = 0; i < nr_found; i++) {
                struct page *page;
repeat:
                page = radix_tree_deref_slot((void **)pages[i]);
                if (unlikely(!page))
                        continue;

                if (radix_tree_exception(page)) {
                        if (radix_tree_deref_retry(page)) {
                                /*
                                 * Transient condition which can only trigger
                                 * when entry at index 0 moves out of or back
                                 * to root: none yet gotten, safe to restart.
                                 */
                                assert((start | i) == 0);
                                goto restart;
                        }
                        /*
                         * No exceptional entries are inserted in this test.
                         */
                        assert(0);
                }

                pthread_mutex_lock(&page->lock);
                if (!page->count) {
                        pthread_mutex_unlock(&page->lock);
                        goto repeat;
                }
                /* don't actually update page refcount */
                pthread_mutex_unlock(&page->lock);

                /* Has the page moved? */
                if (unlikely(page != *((void **)pages[i]))) {
                        goto repeat;
                }

                pages[ret] = page;
                ret++;
        }
        rcu_read_unlock();
        return ret;
}

static pthread_barrier_t worker_barrier;

static void *regression1_fn(void *arg)
{
        rcu_register_thread();

        if (pthread_barrier_wait(&worker_barrier) ==
                        PTHREAD_BARRIER_SERIAL_THREAD) {
                int j;

                for (j = 0; j < 1000000; j++) {
                        struct page *p;

                        p = page_alloc();
                        pthread_mutex_lock(&mt_lock);
                        radix_tree_insert(&mt_tree, 0, p);
                        pthread_mutex_unlock(&mt_lock);

                        p = page_alloc();
                        pthread_mutex_lock(&mt_lock);
                        radix_tree_insert(&mt_tree, 1, p);
                        pthread_mutex_unlock(&mt_lock);

                        pthread_mutex_lock(&mt_lock);
                        p = radix_tree_delete(&mt_tree, 1);
                        pthread_mutex_lock(&p->lock);
                        p->count--;
                        pthread_mutex_unlock(&p->lock);
                        pthread_mutex_unlock(&mt_lock);
                        page_free(p);

                        pthread_mutex_lock(&mt_lock);
                        p = radix_tree_delete(&mt_tree, 0);
                        pthread_mutex_lock(&p->lock);
                        p->count--;
                        pthread_mutex_unlock(&p->lock);
                        pthread_mutex_unlock(&mt_lock);
                        page_free(p);
                }
        } else {
                int j;

                for (j = 0; j < 100000000; j++) {
                        struct page *pages[10];

                        find_get_pages(0, 10, pages);
                }
        }

        rcu_unregister_thread();

        return NULL;
}

static pthread_t *threads;
void regression1_test(void)
{
        int nr_threads;
        int i;
        long arg;

        /* Regression #1 */
        printv(1, "running regression test 1, should finish in under a minute\n");
        nr_threads = 2;
        pthread_barrier_init(&worker_barrier, NULL, nr_threads);

        threads = malloc(nr_threads * sizeof(pthread_t *));

        for (i = 0; i < nr_threads; i++) {
                arg = i;
                if (pthread_create(&threads[i], NULL, regression1_fn, (void *)arg)) {
                        perror("pthread_create");
                        exit(1);
                }
        }

        for (i = 0; i < nr_threads; i++) {
                if (pthread_join(threads[i], NULL)) {
                        perror("pthread_join");
                        exit(1);
                }
        }

        free(threads);

        printv(1, "regression test 1, done\n");
}