Editing Read-copy-update (section)

==Analogy with reader–writer locking==
Although RCU can be used in many different ways, a very common use of RCU is analogous to reader–writer locking. The following side-by-side code display shows how closely related reader–writer locking and RCU can be.<ref>{{cite web
  | last = McKenney
  | first = Paul E.
  | title = Using {RCU} in the {Linux} 2.5 Kernel
  | publisher = [[Linux Journal]]
  | date = October 2003
  | url = http://www.linuxjournal.com/article/6993
  | access-date = September 24, 2010 }}</ref>
<syntaxhighlight lang="c" highlight="10, 17-18,21,25, 33,36-38,43">
   /* reader-writer locking */              /* RCU */

 1 struct el {                            1 struct el {
 2   struct list_head lp;                 2   struct list_head lp;
 3   long key;                            3   long key;
 4   spinlock_t mutex;                    4   spinlock_t mutex;
 5   int data;                            5   int data;
 6   /* Other data fields */              6   /* Other data fields */
 7 };                                     7 };
 8 DEFINE_RWLOCK(listmutex);              8 DEFINE_SPINLOCK(listmutex);
 9 LIST_HEAD(head);                       9 LIST_HEAD(head);

 1 int search(long key, int *result)      1 int search(long key, int *result)
 2 {                                      2 {
 3   struct el *p;                        3   struct el *p;
 4                                        4
 5   read_lock(&listmutex);               5   rcu_read_lock();
 6   list_for_each_entry(p, &head, lp) {  6   list_for_each_entry_rcu(p, &head, lp) {
 7     if (p->key == key) {               7     if (p->key == key) {
 8       *result = p->data;               8       *result = p->data;
 9       read_unlock(&listmutex);         9       rcu_read_unlock();
10       return 1;                       10       return 1;
11     }                                 11     }
12   }                                   12   }
13   read_unlock(&listmutex);            13   rcu_read_unlock();
14   return 0;                           14   return 0;
15 }                                     15 }

 1 int delete(long key)                   1 int delete(long key)
 2 {                                      2 {
 3   struct el *p;                        3   struct el *p;
 4                                        4
 5   write_lock(&listmutex);              5   spin_lock(&listmutex);
 6   list_for_each_entry(p, &head, lp) {  6   list_for_each_entry(p, &head, lp) {
 7     if (p->key == key) {               7     if (p->key == key) {
 8       list_del(&p->lp);                8       list_del_rcu(&p->lp);
 9       write_unlock(&listmutex);        9       spin_unlock(&listmutex);
                                         10       synchronize_rcu();
10       kfree(p);                       11       kfree(p);
11       return 1;                       12       return 1;
12     }                                 13     }
13   }                                   14   }
14   write_unlock(&listmutex);           15   spin_unlock(&listmutex);
15   return 0;                           16   return 0;
16 }                                     17 }
</syntaxhighlight>
The differences between the two approaches are quite small.  Read-side locking moves to <code>rcu_read_lock</code> and <code>rcu_read_unlock</code>, update-side locking moves from a reader-writer lock to a simple spinlock, and a <code>synchronize_rcu</code> precedes the <code>kfree</code>.

However, there is one potential catch: the read-side and update-side critical sections can now run concurrently. In many cases, this will not be a problem, but it is necessary to check carefully regardless. For example, if multiple independent list updates must be seen as a single atomic update, converting to RCU will require special care.

Also, the presence of <code>synchronize_rcu</code> means that the RCU version of <code>delete</code> can now block. If this is a problem, <code>call_rcu</code> could be used like <code>call_rcu (kfree, p)</code> in place of <code>synchronize_rcu</code>.  This is especially useful in combination with reference counting.