tools/testing/radix-tree/regression1.c

   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
  46 static RADIX_TREE(mt_tree, GFP_KERNEL);
  47 static pthread_mutex_t mt_lock = PTHREAD_MUTEX_INITIALIZER;
  48
  49 struct page {
  50         pthread_mutex_t lock;
  51         struct rcu_head rcu;
  52         int count;
  53         unsigned long index;
  54 };
  55
  56 static struct page *page_alloc(void)
  57 {
  58         struct page *p;
  59         p = malloc(sizeof(struct page));
  60         p->count = 1;
  61         p->index = 1;
  62         pthread_mutex_init(&p->lock, NULL);
  63
  64         return p;
  65 }
  66
  67 static void page_rcu_free(struct rcu_head *rcu)
  68 {
  69         struct page *p = container_of(rcu, struct page, rcu);
  70         assert(!p->count);
  71         pthread_mutex_destroy(&p->lock);
  72         free(p);
  73 }
  74
  75 static void page_free(struct page *p)
  76 {
  77         call_rcu(&p->rcu, page_rcu_free);
  78 }
  79
  80 static unsigned find_get_pages(unsigned long start,
  81                             unsigned int nr_pages, struct page **pages)
  82 {
  83         unsigned int i;
  84         unsigned int ret;
  85         unsigned int nr_found;
  86
  87         rcu_read_lock();
  88 restart:
  89         nr_found = radix_tree_gang_lookup_slot(&mt_tree,
  90                                 (void ***)pages, NULL, start, nr_pages);
  91         ret = 0;
  92         for (i = 0; i < nr_found; i++) {
  93                 struct page *page;
  94 repeat:
  95                 page = radix_tree_deref_slot((void **)pages[i]);
  96                 if (unlikely(!page))
  97                         continue;
  98
  99                 if (radix_tree_exception(page)) {
 100                         if (radix_tree_deref_retry(page)) {
 101                                 /*
 102                                  * Transient condition which can only trigger
 103                                  * when entry at index 0 moves out of or back
 104                                  * to root: none yet gotten, safe to restart.
 105                                  */
 106                                 assert((start | i) == 0);
 107                                 goto restart;
 108                         }
 109                         /*
 110                          * No exceptional entries are inserted in this test.
 111                          */
 112                         assert(0);
 113                 }
 114
 115                 pthread_mutex_lock(&page->lock);
 116                 if (!page->count) {
 117                         pthread_mutex_unlock(&page->lock);
 118                         goto repeat;
 119                 }
 120                 /* don't actually update page refcount */
 121                 pthread_mutex_unlock(&page->lock);
 122
 123                 /* Has the page moved? */
 124                 if (unlikely(page != *((void **)pages[i]))) {
 125                         goto repeat;
 126                 }
 127
 128                 pages[ret] = page;
 129                 ret++;
 130         }
 131         rcu_read_unlock();
 132         return ret;
 133 }
 134
 135 static pthread_barrier_t worker_barrier;
 136
 137 static void *regression1_fn(void *arg)
 138 {
 139         rcu_register_thread();
 140
 141         if (pthread_barrier_wait(&worker_barrier) ==
 142                         PTHREAD_BARRIER_SERIAL_THREAD) {
 143                 int j;
 144
 145                 for (j = 0; j < 1000000; j++) {
 146                         struct page *p;
 147
 148                         p = page_alloc();
 149                         pthread_mutex_lock(&mt_lock);
 150                         radix_tree_insert(&mt_tree, 0, p);
 151                         pthread_mutex_unlock(&mt_lock);
 152
 153                         p = page_alloc();
 154                         pthread_mutex_lock(&mt_lock);
 155                         radix_tree_insert(&mt_tree, 1, p);
 156                         pthread_mutex_unlock(&mt_lock);
 157
 158                         pthread_mutex_lock(&mt_lock);
 159                         p = radix_tree_delete(&mt_tree, 1);
 160                         pthread_mutex_lock(&p->lock);
 161                         p->count--;
 162                         pthread_mutex_unlock(&p->lock);
 163                         pthread_mutex_unlock(&mt_lock);
 164                         page_free(p);
 165
 166                         pthread_mutex_lock(&mt_lock);
 167                         p = radix_tree_delete(&mt_tree, 0);
 168                         pthread_mutex_lock(&p->lock);
 169                         p->count--;
 170                         pthread_mutex_unlock(&p->lock);
 171                         pthread_mutex_unlock(&mt_lock);
 172                         page_free(p);
 173                 }
 174         } else {
 175                 int j;
 176
 177                 for (j = 0; j < 100000000; j++) {
 178                         struct page *pages[10];
 179
 180                         find_get_pages(0, 10, pages);
 181                 }
 182         }
 183
 184         rcu_unregister_thread();
 185
 186         return NULL;
 187 }
 188
 189 static pthread_t *threads;
 190 void regression1_test(void)
 191 {
 192         int nr_threads;
 193         int i;
 194         long arg;
 195
 196         /* Regression #1 */
 197         printv(1, "running regression test 1, should finish in under a minute\n");
 198         nr_threads = 2;
 199         pthread_barrier_init(&worker_barrier, NULL, nr_threads);
 200
 201         threads = malloc(nr_threads * sizeof(pthread_t *));
 202
 203         for (i = 0; i < nr_threads; i++) {
 204                 arg = i;
 205                 if (pthread_create(&threads[i], NULL, regression1_fn, (void *)arg)) {
 206                         perror("pthread_create");
 207                         exit(1);
 208                 }
 209         }
 210
 211         for (i = 0; i < nr_threads; i++) {
 212                 if (pthread_join(threads[i], NULL)) {
 213                         perror("pthread_join");
 214                         exit(1);
 215                 }
 216         }
 217
 218         free(threads);
 219
 220         printv(1, "regression test 1, done\n");
 221 }