doc/examples/rcu-flavor-qsbr/rcu-flavor-qsbr.c

   1 /*
   2  * Copyright (C) 2013  Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   3  *
   4  * This library is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU Lesser General Public
   6  * License as published by the Free Software Foundation; either
   7  * version 2.1 of the License, or (at your option) any later version.
   8  *
   9  * This library is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * Lesser General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU Lesser General Public
  15  * License along with this library; if not, write to the Free Software
  16  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  17  */
  18
  19 #include <stdlib.h>
  20 #include <stdio.h>
  21 #include <stdint.h>
  22 #include <inttypes.h>
  23
  24 #include <urcu-qsbr.h>          /* QSBR RCU flavor */
  25 #include <urcu/rculist.h>       /* List example */
  26 #include <urcu/compiler.h>      /* For CAA_ARRAY_SIZE */
  27
  28 /*
  29  * This is a mock-up example where updates and RCU traversals are
  30  * performed by the same thread to keep things simple on purpose.
  31  */
  32
  33 static CDS_LIST_HEAD(mylist);
  34
  35 struct mynode {
  36         uint64_t value;
  37         struct cds_list_head node;      /* linked-list chaining */
  38         struct rcu_head rcu_head;       /* for call_rcu() */
  39 };
  40
  41 static
  42 int add_node(uint64_t v)
  43 {
  44         struct mynode *node;
  45
  46         node = calloc(sizeof(*node), 1);
  47         if (!node)
  48                 return -1;
  49         node->value = v;
  50         cds_list_add_rcu(&node->node, &mylist);
  51         return 0;
  52 }
  53
  54 static
  55 void rcu_free_node(struct rcu_head *rh)
  56 {
  57         struct mynode *node = caa_container_of(rh, struct mynode, rcu_head);
  58
  59         free(node);
  60 }
  61
  62 int main(int argc, char **argv)
  63 {
  64         uint64_t values[] = { 42, 36, 24, };
  65         unsigned int i;
  66         int ret;
  67         struct mynode *node, *n;
  68
  69         /*
  70          * Each thread need using RCU read-side need to be explicitly
  71          * registered.
  72          */
  73         rcu_register_thread();
  74
  75         /*
  76          * Adding nodes to the linked-list. Safe against concurrent
  77          * RCU traversals, require mutual exclusion with list updates.
  78          */
  79         for (i = 0; i < CAA_ARRAY_SIZE(values); i++) {
  80                 ret = add_node(values[i]);
  81                 if (ret)
  82                         goto end;
  83         }
  84
  85         /*
  86          * For all RCU flavors except QSBR, we need to explicitly mark
  87          * RCU read-side critical sections with rcu_read_lock() and
  88          * rcu_read_unlock(). They can be nested. Those are no-ops for
  89          * the QSBR flavor.
  90          */
  91         rcu_read_lock();
  92
  93         /*
  94          * RCU traversal of the linked list.
  95          */
  96         cds_list_for_each_entry_rcu(node, &mylist, node) {
  97                 printf("Value: %" PRIu64 "\n", node->value);
  98         }
  99         rcu_read_unlock();
 100
 101         /*
 102          * Removing nodes from linked list. Safe against concurrent RCU
 103          * traversals, require mutual exclusion with list updates.
 104          */
 105         cds_list_for_each_entry_safe(node, n, &mylist, node) {
 106                 cds_list_del_rcu(&node->node);
 107                 call_rcu(&node->rcu_head, rcu_free_node);
 108         }
 109
 110         /*
 111          * For QSBR flavor, we need to explicitly announce quiescent
 112          * states. Here is how it is done. This should be performed by
 113          * every online registered RCU threads in the program
 114          * periodically.
 115          */
 116         rcu_quiescent_state();
 117
 118         /*
 119          * For QSBR flavor, when a thread needs to be in a quiescent
 120          * state for a long period of time, we use rcu_thread_offline()
 121          * and rcu_thread_online().
 122          */
 123         rcu_thread_offline();
 124
 125         sleep(1);
 126
 127         rcu_thread_online();
 128
 129         /*
 130          * Waiting for previously called call_rcu handlers to complete
 131          * before program exits, or in library destructors, is a good
 132          * practice.
 133          */
 134         rcu_barrier();
 135
 136 end:
 137         rcu_unregister_thread();
 138         return ret;
 139 }