add missing comment to wfstack.c
[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 ARMv7l, 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 ARMv7l 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 ARMv7l 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 * #include <urcu-defer.h>
128 * Link with "-lurcu-defer", and also with one of the urcu library
129 (either urcu, urcu-bp, urcu-mb or urcu-qsbr).
130 * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
131 callbacks are executed in batch periodically after a grace period.
132 Do _not_ use defer_rcu() within a read-side critical section, because
133 it may call synchronize_rcu() if the thread queue is full.
134 * Provides defer_rcu_ratelimit() primitive, which acts just like
135 defer_rcu(), but takes an additional rate limiter callback forcing
136 synchronized callback execution of the limiter returns non-zero.
137 * Requires that rcu_defer_barrier() must be called in library destructor
138 if a library queues callbacks and is expected to be unloaded with
139 dlclose().
140 * Its API is currently experimental. It may change in future library
141 releases.
142
143 Being careful with signals
144
145 The liburcu library uses signals internally. The signal handler is
146 registered with the SA_RESTART flag. However, these signals may cause
147 some non-restartable system calls to fail with errno = EINTR. Care
148 should be taken to restart system calls manually if they fail with this
149 error. A list of non-restartable system calls may be found in
150 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
151 library do not require any signal.
152
153 Read-side critical sections are allowed in a signal handler with
154 liburcu and liburcu-mb. Be careful, however, to disable these signals
155 between thread creation and calls to rcu_register_thread(), because a
156 signal handler nesting on an unregistered thread would not be allowed to
157 call rcu_read_lock().
158
159 Read-side critical sections are _not_ allowed in a signal handler with
160 liburcu-qsbr, unless signals are disabled explicitly around each
161 rcu_quiescent_state() calls, when threads are put offline and around
162 calls to synchronize_rcu(). Even then, we do not recommend it.
163
164 Interaction with mutexes
165
166 One must be careful to do not cause deadlocks due to interaction of
167 synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
168 is called with a mutex held, this mutex (or any mutex which has this
169 mutex in its dependency chain) should not be acquired from within a RCU
170 read-side critical section.
171
172 Usage of DEBUG_RCU
173
174 DEBUG_RCU is used to add internal debugging self-checks to the
175 RCU library. This define adds a performance penalty when enabled.
176 Can be enabled by uncommenting the corresponding line in
177 Makefile.build.inc.
178
179 Usage of DEBUG_YIELD
180
181 DEBUG_YIELD is used to add random delays in the code for testing
182 purposes.
183
184 SMP support
185
186 By default the library is configured to use synchronization primitives
187 adequate for SMP systems. On uniprocessor systems, support for SMP
188 systems can be disabled with:
189
190 ./configure --disable-smp-support
191
192 theoretically yielding slightly better performance.
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