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1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Regression1
4 * Description:
5 * Salman Qazi describes the following radix-tree bug:
6 *
7 * In the following case, we get can get a deadlock:
8 *
9 * 0. The radix tree contains two items, one has the index 0.
10 * 1. The reader (in this case find_get_pages) takes the rcu_read_lock.
11 * 2. The reader acquires slot(s) for item(s) including the index 0 item.
12 * 3. The non-zero index item is deleted, and as a consequence the other item
13 * is moved to the root of the tree. The place where it used to be is queued
14 * for deletion after the readers finish.
15 * 3b. The zero item is deleted, removing it from the direct slot, it remains in
16 * the rcu-delayed indirect node.
17 * 4. The reader looks at the index 0 slot, and finds that the page has 0 ref
18 * count
19 * 5. The reader looks at it again, hoping that the item will either be freed
20 * or the ref count will increase. This never happens, as the slot it is
21 * looking at will never be updated. Also, this slot can never be reclaimed
22 * because the reader is holding rcu_read_lock and is in an infinite loop.
23 *
24 * The fix is to re-use the same "indirect" pointer case that requires a slot
25 * lookup retry into a general "retry the lookup" bit.
26 *
27 * Running:
28 * This test should run to completion in a few seconds. The above bug would
29 * cause it to hang indefinitely.
30 *
31 * Upstream commit:
32 * Not yet
33 */
34#include <linux/kernel.h>
35#include <linux/gfp.h>
36#include <linux/slab.h>
37#include <linux/radix-tree.h>
38#include <linux/rcupdate.h>
39#include <stdlib.h>
40#include <pthread.h>
41#include <stdio.h>
42#include <assert.h>
43
44#include "regression.h"
45
46static RADIX_TREE(mt_tree, GFP_KERNEL);
47
48struct page {
49 pthread_mutex_t lock;
50 struct rcu_head rcu;
51 int count;
52 unsigned long index;
53};
54
55static struct page *page_alloc(int index)
56{
57 struct page *p;
58 p = malloc(sizeof(struct page));
59 p->count = 1;
60 p->index = index;
61 pthread_mutex_init(&p->lock, NULL);
62
63 return p;
64}
65
66static void page_rcu_free(struct rcu_head *rcu)
67{
68 struct page *p = container_of(rcu, struct page, rcu);
69 assert(!p->count);
70 pthread_mutex_destroy(&p->lock);
71 free(p);
72}
73
74static void page_free(struct page *p)
75{
76 call_rcu(&p->rcu, page_rcu_free);
77}
78
79static unsigned find_get_pages(unsigned long start,
80 unsigned int nr_pages, struct page **pages)
81{
82 XA_STATE(xas, &mt_tree, start);
83 struct page *page;
84 unsigned int ret = 0;
85
86 rcu_read_lock();
87 xas_for_each(&xas, page, ULONG_MAX) {
88 if (xas_retry(&xas, page))
89 continue;
90
91 pthread_mutex_lock(&page->lock);
92 if (!page->count)
93 goto unlock;
94
95 /* don't actually update page refcount */
96 pthread_mutex_unlock(&page->lock);
97
98 /* Has the page moved? */
99 if (unlikely(page != xas_reload(&xas)))
100 goto put_page;
101
102 pages[ret] = page;
103 ret++;
104 continue;
105unlock:
106 pthread_mutex_unlock(&page->lock);
107put_page:
108 xas_reset(&xas);
109 }
110 rcu_read_unlock();
111 return ret;
112}
113
114static pthread_barrier_t worker_barrier;
115
116static void *regression1_fn(void *arg)
117{
118 rcu_register_thread();
119
120 if (pthread_barrier_wait(&worker_barrier) ==
121 PTHREAD_BARRIER_SERIAL_THREAD) {
122 int j;
123
124 for (j = 0; j < 1000000; j++) {
125 struct page *p;
126
127 p = page_alloc(0);
128 xa_lock(&mt_tree);
129 radix_tree_insert(&mt_tree, 0, p);
130 xa_unlock(&mt_tree);
131
132 p = page_alloc(1);
133 xa_lock(&mt_tree);
134 radix_tree_insert(&mt_tree, 1, p);
135 xa_unlock(&mt_tree);
136
137 xa_lock(&mt_tree);
138 p = radix_tree_delete(&mt_tree, 1);
139 pthread_mutex_lock(&p->lock);
140 p->count--;
141 pthread_mutex_unlock(&p->lock);
142 xa_unlock(&mt_tree);
143 page_free(p);
144
145 xa_lock(&mt_tree);
146 p = radix_tree_delete(&mt_tree, 0);
147 pthread_mutex_lock(&p->lock);
148 p->count--;
149 pthread_mutex_unlock(&p->lock);
150 xa_unlock(&mt_tree);
151 page_free(p);
152 }
153 } else {
154 int j;
155
156 for (j = 0; j < 100000000; j++) {
157 struct page *pages[10];
158
159 find_get_pages(0, 10, pages);
160 }
161 }
162
163 rcu_unregister_thread();
164
165 return NULL;
166}
167
168static pthread_t *threads;
169void regression1_test(void)
170{
171 int nr_threads;
172 int i;
173 long arg;
174
175 /* Regression #1 */
176 printv(1, "running regression test 1, should finish in under a minute\n");
177 nr_threads = 2;
178 pthread_barrier_init(&worker_barrier, NULL, nr_threads);
179
180 threads = malloc(nr_threads * sizeof(pthread_t *));
181
182 for (i = 0; i < nr_threads; i++) {
183 arg = i;
184 if (pthread_create(&threads[i], NULL, regression1_fn, (void *)arg)) {
185 perror("pthread_create");
186 exit(1);
187 }
188 }
189
190 for (i = 0; i < nr_threads; i++) {
191 if (pthread_join(threads[i], NULL)) {
192 perror("pthread_join");
193 exit(1);
194 }
195 }
196
197 free(threads);
198
199 printv(1, "regression test 1, done\n");
200}
1/*
2 * Regression1
3 * Description:
4 * Salman Qazi describes the following radix-tree bug:
5 *
6 * In the following case, we get can get a deadlock:
7 *
8 * 0. The radix tree contains two items, one has the index 0.
9 * 1. The reader (in this case find_get_pages) takes the rcu_read_lock.
10 * 2. The reader acquires slot(s) for item(s) including the index 0 item.
11 * 3. The non-zero index item is deleted, and as a consequence the other item
12 * is moved to the root of the tree. The place where it used to be is queued
13 * for deletion after the readers finish.
14 * 3b. The zero item is deleted, removing it from the direct slot, it remains in
15 * the rcu-delayed indirect node.
16 * 4. The reader looks at the index 0 slot, and finds that the page has 0 ref
17 * count
18 * 5. The reader looks at it again, hoping that the item will either be freed
19 * or the ref count will increase. This never happens, as the slot it is
20 * looking at will never be updated. Also, this slot can never be reclaimed
21 * because the reader is holding rcu_read_lock and is in an infinite loop.
22 *
23 * The fix is to re-use the same "indirect" pointer case that requires a slot
24 * lookup retry into a general "retry the lookup" bit.
25 *
26 * Running:
27 * This test should run to completion in a few seconds. The above bug would
28 * cause it to hang indefinitely.
29 *
30 * Upstream commit:
31 * Not yet
32 */
33#include <linux/kernel.h>
34#include <linux/gfp.h>
35#include <linux/slab.h>
36#include <linux/radix-tree.h>
37#include <linux/rcupdate.h>
38#include <stdlib.h>
39#include <pthread.h>
40#include <stdio.h>
41#include <assert.h>
42
43#include "regression.h"
44
45static RADIX_TREE(mt_tree, GFP_KERNEL);
46static pthread_mutex_t mt_lock;
47
48struct page {
49 pthread_mutex_t lock;
50 struct rcu_head rcu;
51 int count;
52 unsigned long index;
53};
54
55static struct page *page_alloc(void)
56{
57 struct page *p;
58 p = malloc(sizeof(struct page));
59 p->count = 1;
60 p->index = 1;
61 pthread_mutex_init(&p->lock, NULL);
62
63 return p;
64}
65
66static void page_rcu_free(struct rcu_head *rcu)
67{
68 struct page *p = container_of(rcu, struct page, rcu);
69 assert(!p->count);
70 pthread_mutex_destroy(&p->lock);
71 free(p);
72}
73
74static void page_free(struct page *p)
75{
76 call_rcu(&p->rcu, page_rcu_free);
77}
78
79static unsigned find_get_pages(unsigned long start,
80 unsigned int nr_pages, struct page **pages)
81{
82 unsigned int i;
83 unsigned int ret;
84 unsigned int nr_found;
85
86 rcu_read_lock();
87restart:
88 nr_found = radix_tree_gang_lookup_slot(&mt_tree,
89 (void ***)pages, NULL, start, nr_pages);
90 ret = 0;
91 for (i = 0; i < nr_found; i++) {
92 struct page *page;
93repeat:
94 page = radix_tree_deref_slot((void **)pages[i]);
95 if (unlikely(!page))
96 continue;
97
98 if (radix_tree_exception(page)) {
99 if (radix_tree_deref_retry(page)) {
100 /*
101 * Transient condition which can only trigger
102 * when entry at index 0 moves out of or back
103 * to root: none yet gotten, safe to restart.
104 */
105 assert((start | i) == 0);
106 goto restart;
107 }
108 /*
109 * No exceptional entries are inserted in this test.
110 */
111 assert(0);
112 }
113
114 pthread_mutex_lock(&page->lock);
115 if (!page->count) {
116 pthread_mutex_unlock(&page->lock);
117 goto repeat;
118 }
119 /* don't actually update page refcount */
120 pthread_mutex_unlock(&page->lock);
121
122 /* Has the page moved? */
123 if (unlikely(page != *((void **)pages[i]))) {
124 goto repeat;
125 }
126
127 pages[ret] = page;
128 ret++;
129 }
130 rcu_read_unlock();
131 return ret;
132}
133
134static pthread_barrier_t worker_barrier;
135
136static void *regression1_fn(void *arg)
137{
138 rcu_register_thread();
139
140 if (pthread_barrier_wait(&worker_barrier) ==
141 PTHREAD_BARRIER_SERIAL_THREAD) {
142 int j;
143
144 for (j = 0; j < 1000000; j++) {
145 struct page *p;
146
147 p = page_alloc();
148 pthread_mutex_lock(&mt_lock);
149 radix_tree_insert(&mt_tree, 0, p);
150 pthread_mutex_unlock(&mt_lock);
151
152 p = page_alloc();
153 pthread_mutex_lock(&mt_lock);
154 radix_tree_insert(&mt_tree, 1, p);
155 pthread_mutex_unlock(&mt_lock);
156
157 pthread_mutex_lock(&mt_lock);
158 p = radix_tree_delete(&mt_tree, 1);
159 pthread_mutex_lock(&p->lock);
160 p->count--;
161 pthread_mutex_unlock(&p->lock);
162 pthread_mutex_unlock(&mt_lock);
163 page_free(p);
164
165 pthread_mutex_lock(&mt_lock);
166 p = radix_tree_delete(&mt_tree, 0);
167 pthread_mutex_lock(&p->lock);
168 p->count--;
169 pthread_mutex_unlock(&p->lock);
170 pthread_mutex_unlock(&mt_lock);
171 page_free(p);
172 }
173 } else {
174 int j;
175
176 for (j = 0; j < 100000000; j++) {
177 struct page *pages[10];
178
179 find_get_pages(0, 10, pages);
180 }
181 }
182
183 rcu_unregister_thread();
184
185 return NULL;
186}
187
188static pthread_t *threads;
189void regression1_test(void)
190{
191 int nr_threads;
192 int i;
193 long arg;
194
195 /* Regression #1 */
196 printf("running regression test 1, should finish in under a minute\n");
197 nr_threads = 2;
198 pthread_barrier_init(&worker_barrier, NULL, nr_threads);
199
200 threads = malloc(nr_threads * sizeof(pthread_t *));
201
202 for (i = 0; i < nr_threads; i++) {
203 arg = i;
204 if (pthread_create(&threads[i], NULL, regression1_fn, (void *)arg)) {
205 perror("pthread_create");
206 exit(1);
207 }
208 }
209
210 for (i = 0; i < nr_threads; i++) {
211 if (pthread_join(threads[i], NULL)) {
212 perror("pthread_join");
213 exit(1);
214 }
215 }
216
217 free(threads);
218
219 printf("regression test 1, done\n");
220}