wfcqueue: implement mutex-free splice
[urcu.git] / README
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7ac06cef 1Userspace RCU Implementation
c97ae6eb 2by Mathieu Desnoyers and Paul E. McKenney
6991f61a 3
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4BUILDING
5--------
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7 ./bootstrap (skip if using tarball)
8 ./configure
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9 make
10 make install
e197ac6f 11 ldconfig
9ca52251 12
7d413817 13 Hints: Forcing 32-bit build:
c4c18179 14 * CFLAGS="-m32 -g -O2" ./configure
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15
16 Forcing 64-bit build:
c4c18179 17 * CFLAGS="-m64 -g -O2" ./configure
aa8c36e0 18
f39cd442 19 Forcing a 32-bit build with 386 backward compatibility:
58e9f838 20 * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
7d413817 21
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22 Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
23 * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
24
207e8061 25
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26ARCHITECTURES SUPPORTED
27-----------------------
28
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29Currently, Linux x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64,
30S390, S390x, ARM, Alpha, ia64 and Sparcv9 32/64 are supported. Tested on
31Linux, FreeBSD 8.2/9.0, and Cygwin. Should also work on: Android, NetBSD 5,
32OpenBSD, Darwin (more testing needed before claiming support for these OS).
c51e75e6 33
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34Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
35better.
3b36a2e9 36
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37The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
38supported, with the following exceptions:
39
40- gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
41 accesses to offsets in a TLS structure on 32-bit x86. These versions are
42 therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
43 The problem has been reported to the gcc community:
44 http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
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45- gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
46 See: http://kerneltrap.org/node/7507
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47- Alpha, ia64 and ARM architectures depend on gcc 4.x with atomic builtins
48 support. For ARM this was introduced with gcc 4.4:
49 http://gcc.gnu.org/gcc-4.4/changes.html
7bcbcfb2 50
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51For developers using the git tree:
52
53This source tree is based on the autotools suite from GNU to simplify
54portability. Here are some things you should have on your system in order to
55compile the git repository tree :
56
57- GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
58 (make sure your system wide "automake" points to a recent version!)
59- GNU Libtool >=2.2
60 (for more information, go to http://www.gnu.org/software/autoconf/)
61
62If you get the tree from the repository, you will need to use the "bootstrap"
63script in the root of the tree. It calls all the GNU tools needed to prepare the
64tree configuration.
65
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66Test scripts provided in the tests/ directory of the source tree depend
67on "bash" and the "seq" program.
68
3b38cfe1 69
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70API
71---
72
73See the relevant API documentation files in doc/. The APIs provided by
74Userspace RCU are, by prefix:
75
76- rcu_ : Read-Copy Update
77- cmm_ : Concurrent Memory Model
78- caa_ : Concurrent Architecture Abstraction
79- cds_ : Concurrent Data Structures
80- uatomic_: Userspace Atomic
81
82
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83QUICK START GUIDE
84-----------------
aa8c36e0 85
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86Usage of all urcu libraries
87
88 * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
89 before including the urcu.h or urcu-qsbr.h header. If your application
90 is distributed under another license, function calls will be generated
91 instead of inlines, so your application can link with the library.
92 * Linking with one of the libraries below is always necessary even for
93 LGPL and GPL applications.
94
95Usage of liburcu
96
97 * #include <urcu.h>
98 * Link the application with "-lurcu".
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99 * This is the preferred version of the library, in terms of
100 grace-period detection speed, read-side speed and flexibility.
101 Dynamically detects kernel support for sys_membarrier(). Falls back
102 on urcu-mb scheme if support is not present, which has slower
103 read-side.
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104
105Usage of liburcu-qsbr
106
107 * #include <urcu-qsbr.h>
108 * Link with "-lurcu-qsbr".
109 * The QSBR flavor of RCU needs to have each reader thread executing
110 rcu_quiescent_state() periodically to progress. rcu_thread_online()
111 and rcu_thread_offline() can be used to mark long periods for which
112 the threads are not active. It provides the fastest read-side at the
113 expense of more intrusiveness in the application code.
114
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115Usage of liburcu-mb
116
117 * #include <urcu.h>
118 * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
119 * Link with "-lurcu-mb".
120 * This version of the urcu library uses memory barriers on the writer
121 and reader sides. This results in faster grace-period detection, but
122 results in slower reads.
123
124Usage of liburcu-signal
125
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126 * #include <urcu.h>
127 * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
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128 * Link the application with "-lurcu-signal".
129 * Version of the library that requires a signal, typically SIGUSR1. Can
130 be overridden with -DSIGRCU by modifying Makefile.build.inc.
131
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132Usage of liburcu-bp
133
134 * #include <urcu-bp.h>
135 * Link with "-lurcu-bp".
136 * The BP library flavor stands for "bulletproof". It is specifically
137 designed to help tracing library to hook on applications without
02be5561 138 requiring to modify these applications. rcu_init(),
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139 rcu_register_thread() and rcu_unregister_thread() all become nops.
140 The state is dealt with by the library internally at the expense of
141 read-side and write-side performance.
142
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143Initialization
144
145 Each thread that has reader critical sections (that uses
146 rcu_read_lock()/rcu_read_unlock() must first register to the URCU
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147 library. This is done by calling rcu_register_thread(). Unregistration
148 must be performed before exiting the thread by using
149 rcu_unregister_thread().
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150
151Reading
152
153 Reader critical sections must be protected by locating them between
154 calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
155 rcu_dereference() may be called to read an RCU protected pointer.
156
157Writing
158
159 rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
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160 After, synchronize_rcu() must be called. When it returns, the old
161 values are not in usage anymore.
c97ae6eb 162
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163Usage of liburcu-defer
164
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165 * Follow instructions for either liburcu, liburcu-qsbr,
166 liburcu-mb, liburcu-signal, or liburcu-bp above.
167 The liburcu-defer functionality is pulled into each of
168 those library modules.
632dd6ba 169 * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
ec4e58a3 170 callbacks are executed in batch periodically after a grace period.
632dd6ba 171 Do _not_ use defer_rcu() within a read-side critical section, because
ec4e58a3 172 it may call synchronize_rcu() if the thread queue is full.
0376e7b2 173 This can lead to deadlock or worse.
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174 * Requires that rcu_defer_barrier() must be called in library destructor
175 if a library queues callbacks and is expected to be unloaded with
176 dlclose().
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177 * Its API is currently experimental. It may change in future library
178 releases.
ec4e58a3 179
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180Usage of urcu-call-rcu
181
182 * Follow instructions for either liburcu, liburcu-qsbr,
183 liburcu-mb, liburcu-signal, or liburcu-bp above.
184 The urcu-call-rcu functionality is provided for each of
185 these library modules.
186 * Provides the call_rcu() primitive to enqueue delayed callbacks
187 in a manner similar to defer_rcu(), but without ever delaying
188 for a grace period. On the other hand, call_rcu()'s best-case
189 overhead is not quite as good as that of defer_rcu().
190 * Provides call_rcu() to allow asynchronous handling of RCU
191 grace periods. A number of additional functions are provided
192 to manage the helper threads used by call_rcu(), but reasonable
193 defaults are used if these additional functions are not invoked.
37a9ce52 194 See rcu-api.txt in userspace-rcu documentation for more details.
26ba798a 195
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196Being careful with signals
197
0a1d290b 198 The liburcu library uses signals internally. The signal handler is
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199 registered with the SA_RESTART flag. However, these signals may cause
200 some non-restartable system calls to fail with errno = EINTR. Care
201 should be taken to restart system calls manually if they fail with this
202 error. A list of non-restartable system calls may be found in
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203 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
204 library do not require any signal.
c97ae6eb 205
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206 Read-side critical sections are allowed in a signal handler,
207 except those setup with sigaltstack(2), with liburcu and
208 liburcu-mb. Be careful, however, to disable these signals
7ac06cef 209 between thread creation and calls to rcu_register_thread(), because a
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210 signal handler nesting on an unregistered thread would not be
211 allowed to call rcu_read_lock().
cee02f0a 212
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213 Read-side critical sections are _not_ allowed in a signal handler with
214 liburcu-qsbr, unless signals are disabled explicitly around each
215 rcu_quiescent_state() calls, when threads are put offline and around
216 calls to synchronize_rcu(). Even then, we do not recommend it.
c97ae6eb 217
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218Interaction with mutexes
219
220 One must be careful to do not cause deadlocks due to interaction of
221 synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
222 is called with a mutex held, this mutex (or any mutex which has this
223 mutex in its dependency chain) should not be acquired from within a RCU
224 read-side critical section.
225
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226 This is especially important to understand in the context of the
227 QSBR flavor: a registered reader thread being "online" by
228 default should be considered as within a RCU read-side critical
229 section unless explicitly put "offline". Therefore, if
230 synchronize_rcu() is called with a mutex held, this mutex, as
231 well as any mutex which has this mutex in its dependency chain
232 should only be taken when the RCU reader thread is "offline"
233 (this can be performed by calling rcu_thread_offline()).
234
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235Usage of DEBUG_RCU
236
237 DEBUG_RCU is used to add internal debugging self-checks to the
0a1d290b 238 RCU library. This define adds a performance penalty when enabled.
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239 Can be enabled by uncommenting the corresponding line in
240 Makefile.build.inc.
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241
242Usage of DEBUG_YIELD
243
244 DEBUG_YIELD is used to add random delays in the code for testing
245 purposes.
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246
247SMP support
248
249 By default the library is configured to use synchronization primitives
250 adequate for SMP systems. On uniprocessor systems, support for SMP
251 systems can be disabled with:
252
253 ./configure --disable-smp-support
254
255 theoretically yielding slightly better performance.
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256
257Interaction with fork()
258
259 Special care must be taken for applications performing fork() without
260 any following exec(). This is caused by the fact that Linux only clones
261 the thread calling fork(), and thus never replicates any of the other
262 parent thread into the child process. Most liburcu implementations
263 require that all registrations (as reader, defer_rcu and call_rcu
264 threads) should be released before a fork() is performed, except for the
265 rather common scenario where fork() is immediately followed by exec() in
266 the child process. The only implementation not subject to that rule is
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267 liburcu-bp, which is designed to handle fork() by calling
268 rcu_bp_before_fork, rcu_bp_after_fork_parent and
269 rcu_bp_after_fork_child.
81ad2e19 270
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271 Applications that use call_rcu() and that fork() without
272 doing an immediate exec() must take special action. The parent
273 must invoke call_rcu_before_fork() before the fork() and
274 call_rcu_after_fork_parent() after the fork(). The child
275 process must invoke call_rcu_after_fork_child().
276 These three APIs are suitable for passing to pthread_atfork().
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277
278Thread Local Storage (TLS)
279
280 Userspace RCU can fall back on pthread_getspecific() to emulate
281 TLS variables on systems where it is not available. This behavior
282 can be forced by specifying --disable-compiler-tls as configure
283 argument.
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