1 Userspace RCU Implementation
2 by Mathieu Desnoyers and Paul E. McKenney
8 #force 32-bit build with: make -f Makefile32
9 #force 64-bit build with: make -f Makefile64
16 Usage of all urcu libraries
18 * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
19 before including the urcu.h or urcu-qsbr.h header. If your application
20 is distributed under another license, function calls will be generated
21 instead of inlines, so your application can link with the library.
22 * Linking with one of the libraries below is always necessary even for
23 LGPL and GPL applications.
28 * Link the application with "-lurcu".
29 * This is the preferred version of the library, both in terms of speed
30 and flexibility. Requires a signal, typically SIGUSR1. Can be
31 overridden with -DSIGURCU by modifying Makefile.build.inc.
36 * Compile any _LGPL_SOURCE code using this library with "-DURCU_MB".
37 * Link with "-lurcu-mb".
38 * This version of the urcu library does not need to
39 reserve a signal number. URCU_MB uses full memory barriers for
40 readers. This eliminates the need for signals but results in slower
45 * #include <urcu-qsbr.h>
46 * Link with "-lurcu-qsbr".
47 * The QSBR flavor of RCU needs to have each reader thread executing
48 rcu_quiescent_state() periodically to progress. rcu_thread_online()
49 and rcu_thread_offline() can be used to mark long periods for which
50 the threads are not active. It provides the fastest read-side at the
51 expense of more intrusiveness in the application code.
55 Each thread that has reader critical sections (that uses
56 rcu_read_lock()/rcu_read_unlock() must first register to the URCU
57 library. This is done by calling rcu_register_thread(). Unregistration
58 must be performed before exiting the thread by using
59 rcu_unregister_thread().
63 Reader critical sections must be protected by locating them between
64 calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
65 rcu_dereference() may be called to read an RCU protected pointer.
69 rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
70 After, synchronize_rcu() must be called. When it returns, the old
71 values are not in usage anymore.
73 Usage of liburcu-defer
75 * #include <urcu-defer.h>
76 * Link with "-lurcu-defer"
77 * Provides call_rcu() primitive to enqueue delayed callbacks. Queued
78 callbacks are executed in batch periodically after a grace period.
79 Do _not_ use call_rcu() within a read-side critical section, because
80 it may call synchronize_rcu() if the thread queue is full.
82 Being careful with signals
84 The liburcu library uses signals internally. The signal handler is
85 registered with the SA_RESTART flag. However, these signals may cause
86 some non-restartable system calls to fail with errno = EINTR. Care
87 should be taken to restart system calls manually if they fail with this
88 error. A list of non-restartable system calls may be found in
89 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
90 library do not require any signal.
92 Read-side critical sections are allowed in a signal handler with
93 liburcu and liburcu-mb. Be careful, however, to disable these signals
94 between thread creation and calls to rcu_register_thread(), because a
95 signal handler nesting on an unregistered thread would not be allowed to
98 Read-side critical sections are _not_ allowed in a signal handler with
99 liburcu-qsbr, unless signals are disabled explicitly around each
100 rcu_quiescent_state() calls, when threads are put offline and around
101 calls to synchronize_rcu(). Even then, we do not recommend it.
105 DEBUG_RCU is used to add internal debugging self-checks to the
106 RCU library. This define adds a performance penalty when enabled.
107 Can be enabled by uncommenting the corresponding line in
112 DEBUG_YIELD is used to add random delays in the code for testing