Headers: move arch_*.h to urcu/arch/*.h
[urcu.git] / README
1 Userspace RCU Implementation
2 by Mathieu Desnoyers and Paul E. McKenney
3
4 BUILDING
5 --------
6
7 ./bootstrap (skip if using tarball)
8 ./configure
9 make
10 make install
11
12 Hints: Forcing 32-bit build:
13 * CFLAGS="-m32 -g -O2" ./configure
14
15 Forcing 64-bit build:
16 * CFLAGS="-m64 -g -O2" ./configure
17
18 Forcing a 32-bit build with 386 backward compatibility:
19 * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
20
21 Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
22 * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
23
24 ARCHITECTURES SUPPORTED
25 -----------------------
26
27 Currently, x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64, S390, S390x,
28 ARM, Alpha, ia64 and Sparcv9 32/64 are supported. Only tested on Linux so
29 far, but should theoretically work on other operating systems.
30
31 ARM depends on running a Linux kernel 2.6.15 or better.
32
33 The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
34 supported, with the following exceptions:
35
36 - gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
37 accesses to offsets in a TLS structure on 32-bit x86. These versions are
38 therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
39 The problem has been reported to the gcc community:
40 http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
41 - Alpha, ia64 and ARM architectures depend on 4.x gcc with atomic builtins
42 support.
43
44
45 QUICK START GUIDE
46 -----------------
47
48 Usage of all urcu libraries
49
50 * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
51 before including the urcu.h or urcu-qsbr.h header. If your application
52 is distributed under another license, function calls will be generated
53 instead of inlines, so your application can link with the library.
54 * Linking with one of the libraries below is always necessary even for
55 LGPL and GPL applications.
56
57 Usage of liburcu
58
59 * #include <urcu.h>
60 * Link the application with "-lurcu".
61 * This is the preferred version of the library, in terms of
62 grace-period detection speed, read-side speed and flexibility.
63 Dynamically detects kernel support for sys_membarrier(). Falls back
64 on urcu-mb scheme if support is not present, which has slower
65 read-side.
66
67 Usage of liburcu-qsbr
68
69 * #include <urcu-qsbr.h>
70 * Link with "-lurcu-qsbr".
71 * The QSBR flavor of RCU needs to have each reader thread executing
72 rcu_quiescent_state() periodically to progress. rcu_thread_online()
73 and rcu_thread_offline() can be used to mark long periods for which
74 the threads are not active. It provides the fastest read-side at the
75 expense of more intrusiveness in the application code.
76
77 Usage of liburcu-mb
78
79 * #include <urcu.h>
80 * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
81 * Link with "-lurcu-mb".
82 * This version of the urcu library uses memory barriers on the writer
83 and reader sides. This results in faster grace-period detection, but
84 results in slower reads.
85
86 Usage of liburcu-signal
87
88 * #include <urcu.h>
89 * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
90 * Link the application with "-lurcu-signal".
91 * Version of the library that requires a signal, typically SIGUSR1. Can
92 be overridden with -DSIGRCU by modifying Makefile.build.inc.
93
94 Usage of liburcu-bp
95
96 * #include <urcu-bp.h>
97 * Link with "-lurcu-bp".
98 * The BP library flavor stands for "bulletproof". It is specifically
99 designed to help tracing library to hook on applications without
100 requiring to modify these applications. rcu_init(),
101 rcu_register_thread() and rcu_unregister_thread() all become nops.
102 The state is dealt with by the library internally at the expense of
103 read-side and write-side performance.
104
105 Initialization
106
107 Each thread that has reader critical sections (that uses
108 rcu_read_lock()/rcu_read_unlock() must first register to the URCU
109 library. This is done by calling rcu_register_thread(). Unregistration
110 must be performed before exiting the thread by using
111 rcu_unregister_thread().
112
113 Reading
114
115 Reader critical sections must be protected by locating them between
116 calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
117 rcu_dereference() may be called to read an RCU protected pointer.
118
119 Writing
120
121 rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
122 After, synchronize_rcu() must be called. When it returns, the old
123 values are not in usage anymore.
124
125 Usage of liburcu-defer
126
127 * Follow instructions for either liburcu, liburcu-qsbr,
128 liburcu-mb, liburcu-signal, or liburcu-bp above.
129 The liburcu-defer functionality is pulled into each of
130 those library modules.
131 * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
132 callbacks are executed in batch periodically after a grace period.
133 Do _not_ use defer_rcu() within a read-side critical section, because
134 it may call synchronize_rcu() if the thread queue is full.
135 This can lead to deadlock or worse.
136 * Requires that rcu_defer_barrier() must be called in library destructor
137 if a library queues callbacks and is expected to be unloaded with
138 dlclose().
139 * Its API is currently experimental. It may change in future library
140 releases.
141
142 Usage of urcu-call-rcu
143
144 * Follow instructions for either liburcu, liburcu-qsbr,
145 liburcu-mb, liburcu-signal, or liburcu-bp above.
146 The urcu-call-rcu functionality is provided for each of
147 these library modules.
148 * Provides the call_rcu() primitive to enqueue delayed callbacks
149 in a manner similar to defer_rcu(), but without ever delaying
150 for a grace period. On the other hand, call_rcu()'s best-case
151 overhead is not quite as good as that of defer_rcu().
152 * Provides call_rcu() to allow asynchronous handling of RCU
153 grace periods. A number of additional functions are provided
154 to manage the helper threads used by call_rcu(), but reasonable
155 defaults are used if these additional functions are not invoked.
156 See API.txt for more details.
157
158 Being careful with signals
159
160 The liburcu library uses signals internally. The signal handler is
161 registered with the SA_RESTART flag. However, these signals may cause
162 some non-restartable system calls to fail with errno = EINTR. Care
163 should be taken to restart system calls manually if they fail with this
164 error. A list of non-restartable system calls may be found in
165 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
166 library do not require any signal.
167
168 Read-side critical sections are allowed in a signal handler with
169 liburcu and liburcu-mb. Be careful, however, to disable these signals
170 between thread creation and calls to rcu_register_thread(), because a
171 signal handler nesting on an unregistered thread would not be allowed to
172 call rcu_read_lock().
173
174 Read-side critical sections are _not_ allowed in a signal handler with
175 liburcu-qsbr, unless signals are disabled explicitly around each
176 rcu_quiescent_state() calls, when threads are put offline and around
177 calls to synchronize_rcu(). Even then, we do not recommend it.
178
179 Interaction with mutexes
180
181 One must be careful to do not cause deadlocks due to interaction of
182 synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
183 is called with a mutex held, this mutex (or any mutex which has this
184 mutex in its dependency chain) should not be acquired from within a RCU
185 read-side critical section.
186
187 Usage of DEBUG_RCU
188
189 DEBUG_RCU is used to add internal debugging self-checks to the
190 RCU library. This define adds a performance penalty when enabled.
191 Can be enabled by uncommenting the corresponding line in
192 Makefile.build.inc.
193
194 Usage of DEBUG_YIELD
195
196 DEBUG_YIELD is used to add random delays in the code for testing
197 purposes.
198
199 SMP support
200
201 By default the library is configured to use synchronization primitives
202 adequate for SMP systems. On uniprocessor systems, support for SMP
203 systems can be disabled with:
204
205 ./configure --disable-smp-support
206
207 theoretically yielding slightly better performance.
208
209 Interaction with fork()
210
211 Special care must be taken for applications performing fork() without
212 any following exec(). This is caused by the fact that Linux only clones
213 the thread calling fork(), and thus never replicates any of the other
214 parent thread into the child process. Most liburcu implementations
215 require that all registrations (as reader, defer_rcu and call_rcu
216 threads) should be released before a fork() is performed, except for the
217 rather common scenario where fork() is immediately followed by exec() in
218 the child process. The only implementation not subject to that rule is
219 liburcu-bp, which is designed to handle fork() by calling
220 rcu_bp_before_fork, rcu_bp_after_fork_parent and
221 rcu_bp_after_fork_child.
222
223 Applications that use call_rcu() and that fork() without
224 doing an immediate exec() must take special action. The parent
225 must invoke call_rcu_before_fork() before the fork() and
226 call_rcu_after_fork_parent() after the fork(). The child
227 process must invoke call_rcu_after_fork_child().
228 These three APIs are suitable for passing to pthread_atfork().
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