1 Userspace RCU Implementation
2 by Mathieu Desnoyers and Paul E. McKenney
7 ./bootstrap (skip if using tarball)
12 Hints: Forcing 32-bit build:
13 * CFLAGS="-m32 -g -O2" ./configure
16 * CFLAGS="-m64 -g -O2" ./configure
18 Forcing a 32-bit build with 386 backward compatibility:
19 * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
21 Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
22 * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
24 ARCHITECTURES SUPPORTED
25 -----------------------
27 Currently, x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64, S390, S390x
28 and Sparcv9 32/64 are supported. Only tested on Linux so far, but should
29 theoretically work on other operating systems.
31 Alpha and ia64 architectures are supported, but depend on 4.x gcc with atomic
34 The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
35 supported, with the following exceptions:
37 - gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
38 accesses to offsets in a TLS structure on 32-bit x86. These versions are
39 therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
40 The problem has been reported to the gcc community:
41 http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
46 Usage of all urcu libraries
48 * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
49 before including the urcu.h or urcu-qsbr.h header. If your application
50 is distributed under another license, function calls will be generated
51 instead of inlines, so your application can link with the library.
52 * Linking with one of the libraries below is always necessary even for
53 LGPL and GPL applications.
58 * Link the application with "-lurcu".
59 * This is the preferred version of the library, in terms of
60 grace-period detection speed, read-side speed and flexibility.
61 Dynamically detects kernel support for sys_membarrier(). Falls back
62 on urcu-mb scheme if support is not present, which has slower
67 * #include <urcu-qsbr.h>
68 * Link with "-lurcu-qsbr".
69 * The QSBR flavor of RCU needs to have each reader thread executing
70 rcu_quiescent_state() periodically to progress. rcu_thread_online()
71 and rcu_thread_offline() can be used to mark long periods for which
72 the threads are not active. It provides the fastest read-side at the
73 expense of more intrusiveness in the application code.
78 * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
79 * Link with "-lurcu-mb".
80 * This version of the urcu library uses memory barriers on the writer
81 and reader sides. This results in faster grace-period detection, but
82 results in slower reads.
84 Usage of liburcu-signal
87 * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
88 * Link the application with "-lurcu-signal".
89 * Version of the library that requires a signal, typically SIGUSR1. Can
90 be overridden with -DSIGRCU by modifying Makefile.build.inc.
94 * #include <urcu-bp.h>
95 * Link with "-lurcu-bp".
96 * The BP library flavor stands for "bulletproof". It is specifically
97 designed to help tracing library to hook on applications without
98 requiring to modify these applications. rcu_init(),
99 rcu_register_thread() and rcu_unregister_thread() all become nops.
100 The state is dealt with by the library internally at the expense of
101 read-side and write-side performance.
105 Each thread that has reader critical sections (that uses
106 rcu_read_lock()/rcu_read_unlock() must first register to the URCU
107 library. This is done by calling rcu_register_thread(). Unregistration
108 must be performed before exiting the thread by using
109 rcu_unregister_thread().
113 Reader critical sections must be protected by locating them between
114 calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
115 rcu_dereference() may be called to read an RCU protected pointer.
119 rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
120 After, synchronize_rcu() must be called. When it returns, the old
121 values are not in usage anymore.
123 Usage of liburcu-defer
125 * #include <urcu-defer.h>
126 * Link with "-lurcu-defer", and also with one of the urcu library
127 (either urcu, urcu-bp, urcu-mb or urcu-qsbr).
128 * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
129 callbacks are executed in batch periodically after a grace period.
130 Do _not_ use defer_rcu() within a read-side critical section, because
131 it may call synchronize_rcu() if the thread queue is full.
132 * Provides defer_rcu_ratelimit() primitive, which acts just like
133 defer_rcu(), but takes an additional rate limiter callback forcing
134 synchronized callback execution of the limiter returns non-zero.
135 * Requires that rcu_defer_barrier() must be called in library destructor
136 if a library queues callbacks and is expected to be unloaded with
138 * Its API is currently experimental. It may change in future library
141 Being careful with signals
143 The liburcu library uses signals internally. The signal handler is
144 registered with the SA_RESTART flag. However, these signals may cause
145 some non-restartable system calls to fail with errno = EINTR. Care
146 should be taken to restart system calls manually if they fail with this
147 error. A list of non-restartable system calls may be found in
148 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
149 library do not require any signal.
151 Read-side critical sections are allowed in a signal handler with
152 liburcu and liburcu-mb. Be careful, however, to disable these signals
153 between thread creation and calls to rcu_register_thread(), because a
154 signal handler nesting on an unregistered thread would not be allowed to
155 call rcu_read_lock().
157 Read-side critical sections are _not_ allowed in a signal handler with
158 liburcu-qsbr, unless signals are disabled explicitly around each
159 rcu_quiescent_state() calls, when threads are put offline and around
160 calls to synchronize_rcu(). Even then, we do not recommend it.
162 Interaction with mutexes
164 One must be careful to do not cause deadlocks due to interaction of
165 synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
166 is called with a mutex held, this mutex (or any mutex which has this
167 mutex in its dependency chain) should not be acquired from within a RCU
168 read-side critical section.
172 DEBUG_RCU is used to add internal debugging self-checks to the
173 RCU library. This define adds a performance penalty when enabled.
174 Can be enabled by uncommenting the corresponding line in
179 DEBUG_YIELD is used to add random delays in the code for testing
184 By default the library is configured to use synchronization primitives
185 adequate for SMP systems. On uniprocessor systems, support for SMP
186 systems can be disabled with:
188 ./configure --disable-smp-support
190 theoretically yielding slightly better performance.