doc/examples/urcu-flavors/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 <unistd.h>
  20 #include <stdlib.h>
  21 #include <stdio.h>
  22 #include <stdint.h>
  23 #include <inttypes.h>
  24
  25 #include <urcu-qsbr.h>          /* QSBR RCU flavor */
  26 #include <urcu/rculist.h>       /* List example */
  27 #include <urcu/compiler.h>      /* For CAA_ARRAY_SIZE */
  28
  29 /*
  30  * Example showing how to use the QSBR Userspace RCU flavor.
  31  *
  32  * This is a mock-up example where updates and RCU traversals are
  33  * performed by the same thread to keep things simple on purpose.
  34  */
  35
  36 static CDS_LIST_HEAD(mylist);
  37
  38 struct mynode {
  39         uint64_t value;
  40         struct cds_list_head node;      /* linked-list chaining */
  41         struct rcu_head rcu_head;       /* for call_rcu() */
  42 };
  43
  44 static
  45 int add_node(uint64_t v)
  46 {
  47         struct mynode *node;
  48
  49         node = calloc(sizeof(*node), 1);
  50         if (!node)
  51                 return -1;
  52         node->value = v;
  53         cds_list_add_rcu(&node->node, &mylist);
  54         return 0;
  55 }
  56
  57 static
  58 void rcu_free_node(struct rcu_head *rh)
  59 {
  60         struct mynode *node = caa_container_of(rh, struct mynode, rcu_head);
  61
  62         free(node);
  63 }
  64
  65 int main(int argc, char **argv)
  66 {
  67         uint64_t values[] = { 42, 36, 24, };
  68         unsigned int i;
  69         int ret;
  70         struct mynode *node, *n;
  71
  72         /*
  73          * Each thread need using RCU read-side need to be explicitly
  74          * registered.
  75          */
  76         rcu_register_thread();
  77
  78         /*
  79          * Adding nodes to the linked-list. Safe against concurrent
  80          * RCU traversals, require mutual exclusion with list updates.
  81          */
  82         for (i = 0; i < CAA_ARRAY_SIZE(values); i++) {
  83                 ret = add_node(values[i]);
  84                 if (ret)
  85                         goto end;
  86         }
  87
  88         /*
  89          * RCU traversal of the linked list.
  90          */
  91         cds_list_for_each_entry_rcu(node, &mylist, node) {
  92                 printf("Value: %" PRIu64 "\n", node->value);
  93         }
  94
  95         /*
  96          * Removing nodes from linked list. Safe against concurrent RCU
  97          * traversals, require mutual exclusion with list updates.
  98          */
  99         cds_list_for_each_entry_safe(node, n, &mylist, node) {
 100                 cds_list_del_rcu(&node->node);
 101                 /*
 102                  * call_rcu() will ensure that the handler
 103                  * "rcu_free_node" is executed after a grace period.
 104                  * call_rcu() can be called from RCU read-side critical
 105                  * sections.
 106                  */
 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          * We can also wait for a quiescent state by calling
 131          * synchronize_rcu() rather than using call_rcu(). It is usually
 132          * a slower approach than call_rcu(), because the latter can
 133          * batch work. Moreover, call_rcu() can be called from a RCU
 134          * read-side critical section, but synchronize_rcu() ensures the
 135          * caller thread is offline, thus acting as a quiescent state.
 136          */
 137         synchronize_rcu();
 138
 139         /*
 140          * Waiting for previously called call_rcu handlers to complete
 141          * before program exits, or in library destructors, is a good
 142          * practice.
 143          */
 144         rcu_barrier();
 145
 146 end:
 147         rcu_unregister_thread();
 148         return ret;
 149 }