cregit-Linux how code gets into the kernel

Release 4.10 tools/testing/radix-tree/regression1.c

 * 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; }


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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); }


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


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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; }


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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; }


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static pthread_t *threads;
void regression1_test(void) { int nr_threads; int i; long arg; /* Regression #1 */ printf("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); printf("regression test 1, done\n"); }


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