doc/examples/urcu-flavors/mb.c

   1 // SPDX-FileCopyrightText: 2013 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   2 //
   3 // SPDX-License-Identifier: LGPL-2.1-or-later
   4
   5 #include <unistd.h>
   6 #include <stdlib.h>
   7 #include <stdio.h>
   8 #include <stdint.h>
   9 #include <inttypes.h>
  10
  11 #include <urcu/urcu-mb.h>       /* Memory barrier RCU flavor */
  12 #include <urcu/rculist.h>       /* List example */
  13 #include <urcu/compiler.h>      /* For CAA_ARRAY_SIZE */
  14
  15 /*
  16  * Example showing how to use the memory-barrier-based Userspace RCU
  17  * flavor.
  18  *
  19  * This is a mock-up example where updates and RCU traversals are
  20  * performed by the same thread to keep things simple on purpose.
  21  */
  22
  23 static CDS_LIST_HEAD(mylist);
  24
  25 struct mynode {
  26         uint64_t value;
  27         struct cds_list_head node;      /* linked-list chaining */
  28         struct rcu_head rcu_head;       /* for call_rcu() */
  29 };
  30
  31 static
  32 int add_node(uint64_t v)
  33 {
  34         struct mynode *node;
  35
  36         node = calloc(1, sizeof(*node));
  37         if (!node)
  38                 return -1;
  39         node->value = v;
  40         cds_list_add_rcu(&node->node, &mylist);
  41         return 0;
  42 }
  43
  44 static
  45 void rcu_free_node(struct rcu_head *rh)
  46 {
  47         struct mynode *node = caa_container_of(rh, struct mynode, rcu_head);
  48
  49         free(node);
  50 }
  51
  52 int main(void)
  53 {
  54         uint64_t values[] = { 42, 36, 24, };
  55         unsigned int i;
  56         int ret;
  57         struct mynode *node, *n;
  58
  59         /*
  60          * Each thread need using RCU read-side need to be explicitly
  61          * registered.
  62          */
  63         urcu_mb_register_thread();
  64
  65         /*
  66          * Adding nodes to the linked-list. Safe against concurrent
  67          * RCU traversals, require mutual exclusion with list updates.
  68          */
  69         for (i = 0; i < CAA_ARRAY_SIZE(values); i++) {
  70                 ret = add_node(values[i]);
  71                 if (ret)
  72                         goto end;
  73         }
  74
  75         /*
  76          * We need to explicitly mark RCU read-side critical sections
  77          * with rcu_read_lock() and rcu_read_unlock(). They can be
  78          * nested. Those are no-ops for the QSBR flavor.
  79          */
  80         urcu_mb_read_lock();
  81
  82         /*
  83          * RCU traversal of the linked list.
  84          */
  85         cds_list_for_each_entry_rcu(node, &mylist, node) {
  86                 printf("Value: %" PRIu64 "\n", node->value);
  87         }
  88         urcu_mb_read_unlock();
  89
  90         /*
  91          * Removing nodes from linked list. Safe against concurrent RCU
  92          * traversals, require mutual exclusion with list updates.
  93          */
  94         cds_list_for_each_entry_safe(node, n, &mylist, node) {
  95                 cds_list_del_rcu(&node->node);
  96                 /*
  97                  * call_rcu() will ensure that the handler
  98                  * "rcu_free_node" is executed after a grace period.
  99                  * call_rcu() can be called from RCU read-side critical
 100                  * sections.
 101                  */
 102                 urcu_mb_call_rcu(&node->rcu_head, rcu_free_node);
 103         }
 104
 105         /*
 106          * We can also wait for a quiescent state by calling
 107          * synchronize_rcu() rather than using call_rcu(). It is usually
 108          * a slower approach than call_rcu(), because the latter can
 109          * batch work. Moreover, call_rcu() can be called from a RCU
 110          * read-side critical section, but synchronize_rcu() should not.
 111          */
 112         urcu_mb_synchronize_rcu();
 113
 114         sleep(1);
 115
 116         /*
 117          * Waiting for previously called call_rcu handlers to complete
 118          * before program exits, or in library destructors, is a good
 119          * practice.
 120          */
 121         urcu_mb_barrier();
 122
 123 end:
 124         urcu_mb_unregister_thread();
 125         return ret;
 126 }