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 <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  * Example showing how to use the QSBR Userspace RCU flavor.
  30  *
  31  * This is a mock-up example where updates and RCU traversals are
  32  * performed by the same thread to keep things simple on purpose.
  33  */
  34
  35 static CDS_LIST_HEAD(mylist);
  36
  37 struct mynode {
  38         uint64_t value;
  39         struct cds_list_head node;      /* linked-list chaining */
  40         struct rcu_head rcu_head;       /* for call_rcu() */
  41 };
  42
  43 static
  44 int add_node(uint64_t v)
  45 {
  46         struct mynode *node;
  47
  48         node = calloc(sizeof(*node), 1);
  49         if (!node)
  50                 return -1;
  51         node->value = v;
  52         cds_list_add_rcu(&node->node, &mylist);
  53         return 0;
  54 }
  55
  56 static
  57 void rcu_free_node(struct rcu_head *rh)
  58 {
  59         struct mynode *node = caa_container_of(rh, struct mynode, rcu_head);
  60
  61         free(node);
  62 }
  63
  64 int main(int argc, char **argv)
  65 {
  66         uint64_t values[] = { 42, 36, 24, };
  67         unsigned int i;
  68         int ret;
  69         struct mynode *node, *n;
  70
  71         /*
  72          * Each thread need using RCU read-side need to be explicitly
  73          * registered.
  74          */
  75         rcu_register_thread();
  76
  77         /*
  78          * Adding nodes to the linked-list. Safe against concurrent
  79          * RCU traversals, require mutual exclusion with list updates.
  80          */
  81         for (i = 0; i < CAA_ARRAY_SIZE(values); i++) {
  82                 ret = add_node(values[i]);
  83                 if (ret)
  84                         goto end;
  85         }
  86
  87         /*
  88          * For all RCU flavors except QSBR, we need to explicitly mark
  89          * RCU read-side critical sections with rcu_read_lock() and
  90          * rcu_read_unlock(). They can be nested. Those are no-ops for
  91          * the QSBR flavor.
  92          */
  93         rcu_read_lock();
  94
  95         /*
  96          * RCU traversal of the linked list.
  97          */
  98         cds_list_for_each_entry_rcu(node, &mylist, node) {
  99                 printf("Value: %" PRIu64 "\n", node->value);
 100         }
 101         rcu_read_unlock();
 102
 103         /*
 104          * Removing nodes from linked list. Safe against concurrent RCU
 105          * traversals, require mutual exclusion with list updates.
 106          */
 107         cds_list_for_each_entry_safe(node, n, &mylist, node) {
 108                 cds_list_del_rcu(&node->node);
 109                 call_rcu(&node->rcu_head, rcu_free_node);
 110         }
 111
 112         /*
 113          * For QSBR flavor, we need to explicitly announce quiescent
 114          * states. Here is how it is done. This should be performed by
 115          * every online registered RCU threads in the program
 116          * periodically.
 117          */
 118         rcu_quiescent_state();
 119
 120         /*
 121          * For QSBR flavor, when a thread needs to be in a quiescent
 122          * state for a long period of time, we use rcu_thread_offline()
 123          * and rcu_thread_online().
 124          */
 125         rcu_thread_offline();
 126
 127         sleep(1);
 128
 129         rcu_thread_online();
 130
 131         /*
 132          * Waiting for previously called call_rcu handlers to complete
 133          * before program exits, or in library destructors, is a good
 134          * practice.
 135          */
 136         rcu_barrier();
 137
 138 end:
 139         rcu_unregister_thread();
 140         return ret;
 141 }