1 Userspace RCU Implementation
2 by Mathieu Desnoyers and Paul E. McKenney
14 Usage of all urcu libraries
16 * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
17 before including the urcu.h or urcu-qsbr.h header. If your application
18 is distributed under another license, function calls will be generated
19 instead of inlines, so your application can link with the library.
20 * Linking with one of the libraries below is always necessary even for
21 LGPL and GPL applications.
26 * Link the application with "-lurcu".
27 * This is the preferred version of the library, both in terms of speed
28 and flexibility. Requires a signal, typically SIGUSR1. Can be
29 overridden with -DSIGURCU by modifying Makefile.build.inc.
34 * Compile code with "-DURCU_MB"
35 * Link with "-lurcu-mb".
36 * This version of the urcu library does not need to
37 reserve a signal number. URCU_MB uses full memory barriers for
38 readers. This eliminates the need for signals but results in slower
43 * #include <urcu-qsbr.h>
44 * Link with "-lurcu-qsbr".
45 * The QSBR flavor of RCU needs to have each reader thread executing
46 rcu_quiescent_state() periodically to progress. rcu_thread_online()
47 and rcu_thread_offline() can be used to mark long periods for which
48 the threads are not active. It provides the fastest read-side at the
49 expense of more intrusiveness in the application code.
53 Each thread that has reader critical sections (that uses
54 rcu_read_lock()/rcu_read_unlock() must first register to the URCU
55 library. This is done by calling rcu_register_thread().
59 Reader critical sections must be protected by locating them between
60 calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
61 rcu_dereference() may be called to read an RCU protected pointer.
65 rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
66 After, synchronize_rcu() must be called. When it returns, the old
67 values are not in usage anymore.
69 Being careful with signals
71 The liburcu library uses signals internally. The signal handler is
72 registered with the SA_RESTART flag. However, these signals may cause
73 some non-restartable system calls to fail with errno = EINTR. Care
74 should be taken to restart system calls manually if they fail with this
75 error. A list of non-restartable system calls may be found in
76 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
77 library do not require any signal.
79 Read-side critical sections are allowed in a signal handler with
80 liburcu and liburcu-mb. Be careful, however, to disable these signals
81 between thread creation and calls to rcu_register_thread(), because a
82 signal handler nesting on an unregistered thread would not be allowed to
85 Read-side critical sections are _not_ allowed in a signal handler with
86 liburcu-qsbr, unless signals are disabled explicitly around each
87 rcu_quiescent_state() calls, when threads are put offline and around
88 calls to synchronize_rcu(). Even then, we do not recommend it.
92 DEBUG_RCU is used to add internal debugging self-checks to the
93 RCU library. This define adds a performance penalty when enabled.
94 Can be enabled by uncommenting the corresponding line in
99 DEBUG_YIELD is used to add random delays in the code for testing