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[urcu.git] / README.md
1 Userspace RCU Implementation
2 ============================
3
4 by Mathieu Desnoyers and Paul E. McKenney
5
6
7 Building
8 --------
9
10 ./bootstrap # skip if using tarball
11 ./configure
12 make
13 make install
14 ldconfig
15
16 Hints:
17
18 - Forcing 32-bit build:
19
20 CFLAGS="-m32 -g -O2" ./configure
21
22 - Forcing 64-bit build:
23
24 CFLAGS="-m64 -g -O2" ./configure
25
26 - Forcing a 32-bit build with 386 backward compatibility:
27
28 CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
29
30 - Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
31
32 CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
33
34
35 Architectures supported
36 -----------------------
37
38 Currently, the following architectures are supported:
39
40 - Linux x86 (i386, i486, i586, i686)
41 - x86 64-bit
42 - PowerPC 32/64
43 - S390, S390x
44 - ARM 32/64
45 - MIPS
46 - NIOS2
47 - Alpha
48 - ia64
49 - Sparcv9 32/64
50 - Tilera
51 - hppa/PA-RISC
52
53 Tested on Linux, FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64, and Cygwin.
54 Should also work on:
55
56 - Android
57 - NetBSD 5
58 - OpenBSD
59 - Darwin
60
61 (more testing needed before claiming support for these OS).
62
63 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
64 better.
65
66 The GCC compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
67 supported, with the following exceptions:
68
69 - GCC 3.3 and 3.4 have a bug that prevents them from generating volatile
70 accesses to offsets in a TLS structure on 32-bit x86. These versions are
71 therefore not compatible with `liburcu` on x86 32-bit
72 (i386, i486, i586, i686).
73 The problem has been reported to the GCC community:
74 http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
75 - GCC 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
76 See http://kerneltrap.org/node/7507
77 - Alpha, ia64 and ARM architectures depend on GCC 4.x with atomic builtins
78 support. For ARM this was introduced with GCC 4.4:
79 http://gcc.gnu.org/gcc-4.4/changes.html.
80
81 Clang version 3.0 (based on LLVM 3.0) is supported.
82
83 Building on MacOS X (Darwin) requires a work-around for processor
84 detection:
85
86 - 32-bit:
87
88 ./configure --build=i686-apple-darwin11
89
90 - 64-bit:
91
92 ./configure --build=x86_64-apple-darwin11
93
94 For developers using the Git tree:
95
96 This source tree is based on the autotools suite from GNU to simplify
97 portability. Here are some things you should have on your system in order to
98 compile the git repository tree :
99
100 - GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
101 (make sure your system wide `automake` points to a recent version!)
102 - GNU Libtool >=2.2
103 (for more information, go to http://www.gnu.org/software/autoconf/)
104
105 If you get the tree from the repository, you will need to use the `bootstrap`
106 script in the root of the tree. It calls all the GNU tools needed to prepare
107 the tree configuration.
108
109 Test scripts provided in the `tests/` directory of the source tree depend
110 on `bash` and the `seq` program.
111
112
113 API
114 ---
115
116 See the relevant API documentation files in `doc/`. The APIs provided by
117 Userspace RCU are, by prefix:
118
119 - `rcu_`: Read-Copy Update (see [`doc/rcu-api.md`](doc/rcu-api.md))
120 - `cmm_`: Concurrent Memory Model
121 - `caa_`: Concurrent Architecture Abstraction
122 - `cds_`: Concurrent Data Structures
123 (see [`doc/cds-api.md`](doc/cds-api.md))
124 - `uatomic_`: Userspace Atomic
125 (see [`doc/uatomic-api.md`](doc/uatomic-api.md))
126
127
128 Quick start guide
129 -----------------
130
131 ### Usage of all urcu libraries:
132
133 - Define `_LGPL_SOURCE` (only) if your code is LGPL or GPL compatible
134 before including the `urcu.h` or `urcu-qsbr.h` header. If your application
135 is distributed under another license, function calls will be generated
136 instead of inlines, so your application can link with the library.
137 - Linking with one of the libraries below is always necessary even for
138 LGPL and GPL applications.
139 - Define `URCU_INLINE_SMALL_FUNCTIONS` before including Userspace RCU
140 headers if you want Userspace RCU to inline small functions (10
141 lines or less) into the application. It can be used by applications
142 distributed under any kind of license, and does *not* make the
143 application a derived work of Userspace RCU.
144
145 Those small inlined functions are guaranteed to match the library
146 content as long as the library major version is unchanged.
147 Therefore, the application *must* be compiled with headers matching
148 the library major version number. Applications using
149 `URCU_INLINE_SMALL_FUNCTIONS` may be unable to use debugging
150 features of Userspace RCU without being recompiled.
151
152
153 ### Usage of `liburcu`
154
155 1. `#include <urcu.h>`
156 2. Link the application with `-lurcu`
157
158 This is the preferred version of the library, in terms of
159 grace-period detection speed, read-side speed and flexibility.
160 Dynamically detects kernel support for `sys_membarrier()`. Falls back
161 on `urcu-mb` scheme if support is not present, which has slower
162 read-side. Use the --disable-sys-membarrier-fallback configure option
163 to disable the fall back, thus requiring `sys_membarrier()` to be
164 available. This gives a small speedup when `sys_membarrier()` is
165 supported by the kernel, and aborts in the library constructor if not
166 supported.
167
168
169 ### Usage of `liburcu-qsbr`
170
171 1. `#include <urcu-qsbr.h>`
172 2. Link with `-lurcu-qsbr`
173
174 The QSBR flavor of RCU needs to have each reader thread executing
175 `rcu_quiescent_state()` periodically to progress. `rcu_thread_online()`
176 and `rcu_thread_offline()` can be used to mark long periods for which
177 the threads are not active. It provides the fastest read-side at the
178 expense of more intrusiveness in the application code.
179
180
181 ### Usage of `liburcu-mb`
182
183 1. `#include <urcu.h>`
184 2. Compile any `_LGPL_SOURCE` code using this library with `-DRCU_MB`
185 3. Link with `-lurcu-mb`
186
187 This version of the urcu library uses memory barriers on the writer
188 and reader sides. This results in faster grace-period detection, but
189 results in slower reads.
190
191
192 ### Usage of `liburcu-signal`
193
194 1. `#include <urcu.h>`
195 2. Compile any `_LGPL_SOURCE` code using this library with `-DRCU_SIGNAL`
196 3. Link the application with `-lurcu-signal`
197
198 Version of the library that requires a signal, typically `SIGUSR1`. Can
199 be overridden with `-DSIGRCU` by modifying `Makefile.build.inc`.
200
201
202 ### Usage of `liburcu-bp`
203
204 1. `#include <urcu-bp.h>`
205 2. Link with `-lurcu-bp`
206
207 The BP library flavor stands for "bulletproof". It is specifically
208 designed to help tracing library to hook on applications without
209 requiring to modify these applications. `rcu_init()`,
210 `rcu_register_thread()` and `rcu_unregister_thread()` all become nops.
211 The state is dealt with by the library internally at the expense of
212 read-side and write-side performance.
213
214
215 ### Initialization
216
217 Each thread that has reader critical sections (that uses
218 `rcu_read_lock()`/`rcu_read_unlock()` must first register to the URCU
219 library. This is done by calling `rcu_register_thread()`. Unregistration
220 must be performed before exiting the thread by using
221 `rcu_unregister_thread()`.
222
223
224 ### Reading
225
226 Reader critical sections must be protected by locating them between
227 calls to `rcu_read_lock()` and `rcu_read_unlock()`. Inside that lock,
228 `rcu_dereference()` may be called to read an RCU protected pointer.
229
230
231 ### Writing
232
233 `rcu_assign_pointer()` and `rcu_xchg_pointer()` may be called anywhere.
234 After, `synchronize_rcu()` must be called. When it returns, the old
235 values are not in usage anymore.
236
237
238 ### Usage of `liburcu-defer`
239
240 - Follow instructions for either `liburcu`, `liburcu-qsbr`,
241 `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
242 The `liburcu-defer` functionality is pulled into each of
243 those library modules.
244 - Provides `defer_rcu()` primitive to enqueue delayed callbacks. Queued
245 callbacks are executed in batch periodically after a grace period.
246 Do _not_ use `defer_rcu()` within a read-side critical section, because
247 it may call `synchronize_rcu()` if the thread queue is full.
248 This can lead to deadlock or worse.
249 - Requires that `rcu_defer_barrier()` must be called in library destructor
250 if a library queues callbacks and is expected to be unloaded with
251 `dlclose()`.
252
253 Its API is currently experimental. It may change in future library releases.
254
255
256 ### Usage of `urcu-call-rcu`
257
258 - Follow instructions for either `liburcu`, `liburcu-qsbr`,
259 `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
260 The `urcu-call-rcu` functionality is pulled into each of
261 those library modules.
262 - Provides the `call_rcu()` primitive to enqueue delayed callbacks
263 in a manner similar to `defer_rcu()`, but without ever delaying
264 for a grace period. On the other hand, `call_rcu()`'s best-case
265 overhead is not quite as good as that of `defer_rcu()`.
266 - Provides `call_rcu()` to allow asynchronous handling of RCU
267 grace periods. A number of additional functions are provided
268 to manage the helper threads used by `call_rcu()`, but reasonable
269 defaults are used if these additional functions are not invoked.
270 See [`doc/rcu-api.md`](doc/rcu-api.md) in userspace-rcu documentation
271 for more details.
272
273
274 ### Being careful with signals
275
276 The `liburcu` library uses signals internally. The signal handler is
277 registered with the `SA_RESTART` flag. However, these signals may cause
278 some non-restartable system calls to fail with `errno = EINTR`. Care
279 should be taken to restart system calls manually if they fail with this
280 error. A list of non-restartable system calls may be found in
281 `signal(7)`. The `liburcu-mb` and `liburcu-qsbr` versions of the Userspace RCU
282 library do not require any signal.
283
284 Read-side critical sections are allowed in a signal handler,
285 except those setup with `sigaltstack(2)`, with `liburcu` and
286 `liburcu-mb`. Be careful, however, to disable these signals
287 between thread creation and calls to `rcu_register_thread()`, because a
288 signal handler nesting on an unregistered thread would not be
289 allowed to call `rcu_read_lock()`.
290
291 Read-side critical sections are _not_ allowed in a signal handler with
292 `liburcu-qsbr`, unless signals are disabled explicitly around each
293 `rcu_quiescent_state()` calls, when threads are put offline and around
294 calls to `synchronize_rcu()`. Even then, we do not recommend it.
295
296
297 ### Interaction with mutexes
298
299 One must be careful to do not cause deadlocks due to interaction of
300 `synchronize_rcu()` and RCU read-side with mutexes. If `synchronize_rcu()`
301 is called with a mutex held, this mutex (or any mutex which has this
302 mutex in its dependency chain) should not be acquired from within a RCU
303 read-side critical section.
304
305 This is especially important to understand in the context of the
306 QSBR flavor: a registered reader thread being "online" by
307 default should be considered as within a RCU read-side critical
308 section unless explicitly put "offline". Therefore, if
309 `synchronize_rcu()` is called with a mutex held, this mutex, as
310 well as any mutex which has this mutex in its dependency chain
311 should only be taken when the RCU reader thread is "offline"
312 (this can be performed by calling `rcu_thread_offline()`).
313
314
315 ### Interaction with `fork()`
316
317 Special care must be taken for applications performing `fork()` without
318 any following `exec()`. This is caused by the fact that Linux only clones
319 the thread calling `fork()`, and thus never replicates any of the other
320 parent thread into the child process. Most `liburcu` implementations
321 require that all registrations (as reader, `defer_rcu` and `call_rcu`
322 threads) should be released before a `fork()` is performed, except for the
323 rather common scenario where `fork()` is immediately followed by `exec()` in
324 the child process. The only implementation not subject to that rule is
325 `liburcu-bp`, which is designed to handle `fork()` by calling
326 `rcu_bp_before_fork`, `rcu_bp_after_fork_parent` and
327 `rcu_bp_after_fork_child`.
328
329 Applications that use `call_rcu()` and that `fork()` without
330 doing an immediate `exec()` must take special action. The parent
331 must invoke `call_rcu_before_fork()` before the `fork()` and
332 `call_rcu_after_fork_parent()` after the `fork()`. The child
333 process must invoke `call_rcu_after_fork_child()`.
334 Even though these three APIs are suitable for passing to
335 `pthread_atfork()`, use of `pthread_atfork()` is **STRONGLY
336 DISCOURAGED** for programs calling the glibc memory allocator
337 (`malloc()`, `calloc()`, `free()`, ...) within `call_rcu` callbacks.
338 This is due to limitations in the way glibc memory allocator
339 handles calls to the memory allocator from concurrent threads
340 while the `pthread_atfork()` handlers are executing.
341
342 Combining e.g.:
343
344 - call to `free()` from callbacks executed within `call_rcu` worker
345 threads,
346 - executing `call_rcu` atfork handlers within the glibc pthread
347 atfork mechanism,
348
349 will sometimes trigger interesting process hangs. This usually
350 hangs on a memory allocator lock within glibc.
351
352
353 ### Thread Local Storage (TLS)
354
355 Userspace RCU can fall back on `pthread_getspecific()` to emulate
356 TLS variables on systems where it is not available. This behavior
357 can be forced by specifying `--disable-compiler-tls` as configure
358 argument.
359
360
361 ### Usage of `DEBUG_RCU` & `--enable-rcu-debug`
362
363 By default the library is configured with internal debugging
364 self-checks disabled.
365
366 For always-on debugging self-checks:
367 ./configure --enable-rcu-debug
368
369 For fine grained enabling of debugging self-checks, build
370 urserspace-rcu with DEBUG_RCU defined and compile dependent
371 applications with DEBUG_RCU defined when necessary.
372
373 Warning: Enabling this feature result in a performance penalty.
374
375
376 ### Usage of `DEBUG_YIELD`
377
378 `DEBUG_YIELD` is used to add random delays in the code for testing
379 purposes.
380
381
382 ### SMP support
383
384 By default the library is configured to use synchronization primitives
385 adequate for SMP systems. On uniprocessor systems, support for SMP
386 systems can be disabled with:
387
388 ./configure --disable-smp-support
389
390 theoretically yielding slightly better performance.
391
392
393 Make targets
394 ------------
395
396 In addition to the usual `make check` target, Userspace RCU features
397 `make regtest` and `make bench` targets:
398
399 - `make check`: short tests, meant to be run when rebuilding or
400 porting Userspace RCU.
401 - `make regtest`: long (many hours) test, meant to be run when
402 modifying Userspace RCU or porting it to a new architecture or
403 operating system.
404 - `make bench`: long (many hours) benchmarks.
405
406
407 Contacts
408 --------
409
410 You can contact the maintainers on the following mailing list:
411 `lttng-dev@lists.lttng.org`.
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