[urcu.git] / README
1 Userspace RCU Implementation
2 by Mathieu Desnoyers and Paul E. McKenney
5 --------
7 ./bootstrap (skip if using tarball)
8 ./configure
9 make
10 make install
11 ldconfig
13 Hints: Forcing 32-bit build:
14 * CFLAGS="-m32 -g -O2" ./configure
16 Forcing 64-bit build:
17 * CFLAGS="-m64 -g -O2" ./configure
19 Forcing a 32-bit build with 386 backward compatibility:
20 * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
22 Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
23 * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
27 -----------------------
29 Currently, Linux x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64,
30 S390, S390x, ARM, MIPS, Alpha, ia64 and Sparcv9 32/64 are supported.
31 Tested on Linux, FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64, and Cygwin.
32 Should also work on: Android, NetBSD 5, OpenBSD, Darwin (more testing
33 needed before claiming support for these OS).
35 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
36 better.
38 The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
39 supported, with the following exceptions:
41 - gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
42 accesses to offsets in a TLS structure on 32-bit x86. These versions are
43 therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
44 The problem has been reported to the gcc community:
45 http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
46 - gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
47 See: http://kerneltrap.org/node/7507
48 - Alpha, ia64 and ARM architectures depend on gcc 4.x with atomic builtins
49 support. For ARM this was introduced with gcc 4.4:
50 http://gcc.gnu.org/gcc-4.4/changes.html
52 Clang version 3.0 (based on LLVM 3.0) is supported.
54 Building on MacOS X (Darwin) requires a work-around for processor
55 detection:
56 # 32-bit
57 ./configure --build=i686-apple-darwin11
58 # 64-bit
59 ./configure --build=x86_64-apple-darwin11
61 For developers using the git tree:
63 This source tree is based on the autotools suite from GNU to simplify
64 portability. Here are some things you should have on your system in order to
65 compile the git repository tree :
67 - GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
68 (make sure your system wide "automake" points to a recent version!)
69 - GNU Libtool >=2.2
70 (for more information, go to http://www.gnu.org/software/autoconf/)
72 If you get the tree from the repository, you will need to use the "bootstrap"
73 script in the root of the tree. It calls all the GNU tools needed to prepare the
74 tree configuration.
76 Test scripts provided in the tests/ directory of the source tree depend
77 on "bash" and the "seq" program.
80 API
81 ---
83 See the relevant API documentation files in doc/. The APIs provided by
84 Userspace RCU are, by prefix:
86 - rcu_ : Read-Copy Update (see doc/rcu-api.txt)
87 - cmm_ : Concurrent Memory Model
88 - caa_ : Concurrent Architecture Abstraction
89 - cds_ : Concurrent Data Structures (see doc/cds-api.txt)
90 - uatomic_: Userspace Atomic (see doc/uatomic-api.txt)
94 -----------------
96 Usage of all urcu libraries
98 * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
99 before including the urcu.h or urcu-qsbr.h header. If your application
100 is distributed under another license, function calls will be generated
101 instead of inlines, so your application can link with the library.
102 * Linking with one of the libraries below is always necessary even for
103 LGPL and GPL applications.
105 Usage of liburcu
107 * #include <urcu.h>
108 * Link the application with "-lurcu".
109 * This is the preferred version of the library, in terms of
110 grace-period detection speed, read-side speed and flexibility.
111 Dynamically detects kernel support for sys_membarrier(). Falls back
112 on urcu-mb scheme if support is not present, which has slower
113 read-side.
115 Usage of liburcu-qsbr
117 * #include <urcu-qsbr.h>
118 * Link with "-lurcu-qsbr".
119 * The QSBR flavor of RCU needs to have each reader thread executing
120 rcu_quiescent_state() periodically to progress. rcu_thread_online()
121 and rcu_thread_offline() can be used to mark long periods for which
122 the threads are not active. It provides the fastest read-side at the
123 expense of more intrusiveness in the application code.
125 Usage of liburcu-mb
127 * #include <urcu.h>
128 * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
129 * Link with "-lurcu-mb".
130 * This version of the urcu library uses memory barriers on the writer
131 and reader sides. This results in faster grace-period detection, but
132 results in slower reads.
134 Usage of liburcu-signal
136 * #include <urcu.h>
137 * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
138 * Link the application with "-lurcu-signal".
139 * Version of the library that requires a signal, typically SIGUSR1. Can
140 be overridden with -DSIGRCU by modifying Makefile.build.inc.
142 Usage of liburcu-bp
144 * #include <urcu-bp.h>
145 * Link with "-lurcu-bp".
146 * The BP library flavor stands for "bulletproof". It is specifically
147 designed to help tracing library to hook on applications without
148 requiring to modify these applications. rcu_init(),
149 rcu_register_thread() and rcu_unregister_thread() all become nops.
150 The state is dealt with by the library internally at the expense of
151 read-side and write-side performance.
153 Initialization
155 Each thread that has reader critical sections (that uses
156 rcu_read_lock()/rcu_read_unlock() must first register to the URCU
157 library. This is done by calling rcu_register_thread(). Unregistration
158 must be performed before exiting the thread by using
159 rcu_unregister_thread().
161 Reading
163 Reader critical sections must be protected by locating them between
164 calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
165 rcu_dereference() may be called to read an RCU protected pointer.
167 Writing
169 rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
170 After, synchronize_rcu() must be called. When it returns, the old
171 values are not in usage anymore.
173 Usage of liburcu-defer
175 * Follow instructions for either liburcu, liburcu-qsbr,
176 liburcu-mb, liburcu-signal, or liburcu-bp above.
177 The liburcu-defer functionality is pulled into each of
178 those library modules.
179 * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
180 callbacks are executed in batch periodically after a grace period.
181 Do _not_ use defer_rcu() within a read-side critical section, because
182 it may call synchronize_rcu() if the thread queue is full.
183 This can lead to deadlock or worse.
184 * Requires that rcu_defer_barrier() must be called in library destructor
185 if a library queues callbacks and is expected to be unloaded with
186 dlclose().
187 * Its API is currently experimental. It may change in future library
188 releases.
190 Usage of urcu-call-rcu
192 * Follow instructions for either liburcu, liburcu-qsbr,
193 liburcu-mb, liburcu-signal, or liburcu-bp above.
194 The urcu-call-rcu functionality is provided for each of
195 these library modules.
196 * Provides the call_rcu() primitive to enqueue delayed callbacks
197 in a manner similar to defer_rcu(), but without ever delaying
198 for a grace period. On the other hand, call_rcu()'s best-case
199 overhead is not quite as good as that of defer_rcu().
200 * Provides call_rcu() to allow asynchronous handling of RCU
201 grace periods. A number of additional functions are provided
202 to manage the helper threads used by call_rcu(), but reasonable
203 defaults are used if these additional functions are not invoked.
204 See rcu-api.txt in userspace-rcu documentation for more details.
206 Being careful with signals
208 The liburcu library uses signals internally. The signal handler is
209 registered with the SA_RESTART flag. However, these signals may cause
210 some non-restartable system calls to fail with errno = EINTR. Care
211 should be taken to restart system calls manually if they fail with this
212 error. A list of non-restartable system calls may be found in
213 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
214 library do not require any signal.
216 Read-side critical sections are allowed in a signal handler,
217 except those setup with sigaltstack(2), with liburcu and
218 liburcu-mb. Be careful, however, to disable these signals
219 between thread creation and calls to rcu_register_thread(), because a
220 signal handler nesting on an unregistered thread would not be
221 allowed to call rcu_read_lock().
223 Read-side critical sections are _not_ allowed in a signal handler with
224 liburcu-qsbr, unless signals are disabled explicitly around each
225 rcu_quiescent_state() calls, when threads are put offline and around
226 calls to synchronize_rcu(). Even then, we do not recommend it.
228 Interaction with mutexes
230 One must be careful to do not cause deadlocks due to interaction of
231 synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
232 is called with a mutex held, this mutex (or any mutex which has this
233 mutex in its dependency chain) should not be acquired from within a RCU
234 read-side critical section.
236 This is especially important to understand in the context of the
237 QSBR flavor: a registered reader thread being "online" by
238 default should be considered as within a RCU read-side critical
239 section unless explicitly put "offline". Therefore, if
240 synchronize_rcu() is called with a mutex held, this mutex, as
241 well as any mutex which has this mutex in its dependency chain
242 should only be taken when the RCU reader thread is "offline"
243 (this can be performed by calling rcu_thread_offline()).
245 Interaction with fork()
247 Special care must be taken for applications performing fork() without
248 any following exec(). This is caused by the fact that Linux only clones
249 the thread calling fork(), and thus never replicates any of the other
250 parent thread into the child process. Most liburcu implementations
251 require that all registrations (as reader, defer_rcu and call_rcu
252 threads) should be released before a fork() is performed, except for the
253 rather common scenario where fork() is immediately followed by exec() in
254 the child process. The only implementation not subject to that rule is
255 liburcu-bp, which is designed to handle fork() by calling
256 rcu_bp_before_fork, rcu_bp_after_fork_parent and
257 rcu_bp_after_fork_child.
259 Applications that use call_rcu() and that fork() without
260 doing an immediate exec() must take special action. The parent
261 must invoke call_rcu_before_fork() before the fork() and
262 call_rcu_after_fork_parent() after the fork(). The child
263 process must invoke call_rcu_after_fork_child().
264 Even though these three APIs are suitable for passing to
265 pthread_atfork(), use of pthread_atfork() is *STRONGLY
266 DISCOURAGED* for programs calling the glibc memory allocator
267 (malloc(), calloc(), free(), ...) within call_rcu callbacks.
268 This is due to limitations in the way glibc memory allocator
269 handles calls to the memory allocator from concurrent threads
270 while the pthread_atfork() handlers are executing.
271 Combining e.g.:
272 * call to free() from callbacks executed within call_rcu worker
273 threads,
274 * executing call_rcu atfork handlers within the glibc pthread
275 atfork mechanism,
276 will sometimes trigger interesting process hangs. This usually
277 hangs on a memory allocator lock within glibc.
279 Thread Local Storage (TLS)
281 Userspace RCU can fall back on pthread_getspecific() to emulate
282 TLS variables on systems where it is not available. This behavior
283 can be forced by specifying --disable-compiler-tls as configure
284 argument.
286 Usage of DEBUG_RCU
288 DEBUG_RCU is used to add internal debugging self-checks to the
289 RCU library. This define adds a performance penalty when enabled.
290 Can be enabled by uncommenting the corresponding line in
291 Makefile.build.inc.
293 Usage of DEBUG_YIELD
295 DEBUG_YIELD is used to add random delays in the code for testing
296 purposes.
298 SMP support
300 By default the library is configured to use synchronization primitives
301 adequate for SMP systems. On uniprocessor systems, support for SMP
302 systems can be disabled with:
304 ./configure --disable-smp-support
306 theoretically yielding slightly better performance.
309 ------------
311 In addition to the usual "make check" target, Userspace RCU features
312 "make regtest" and "make bench" targets.
314 make check: Short tests, meant to be run when rebuilding or porting
315 Userspace RCU.
317 make regtest: Long (many hours) test, meant to be run when modifying
318 Userspace RCU or porting it to a new architecture or
319 operating system.
321 make bench: Long (many hours) benchmarks.
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