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// 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(*threads));
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");
}
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