Fix: update ax_pthread macro to handle newer clang
[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.
163
164
165 ### Usage of `liburcu-qsbr`
166
167 1. `#include <urcu-qsbr.h>`
168 2. Link with `-lurcu-qsbr`
169
170 The QSBR flavor of RCU needs to have each reader thread executing
171 `rcu_quiescent_state()` periodically to progress. `rcu_thread_online()`
172 and `rcu_thread_offline()` can be used to mark long periods for which
173 the threads are not active. It provides the fastest read-side at the
174 expense of more intrusiveness in the application code.
175
176
177 ### Usage of `liburcu-mb`
178
179 1. `#include <urcu.h>`
180 2. Compile any `_LGPL_SOURCE` code using this library with `-DRCU_MB`
181 3. Link with `-lurcu-mb`
182
183 This version of the urcu library uses memory barriers on the writer
184 and reader sides. This results in faster grace-period detection, but
185 results in slower reads.
186
187
188 ### Usage of `liburcu-signal`
189
190 1. `#include <urcu.h>`
191 2. Compile any `_LGPL_SOURCE` code using this library with `-DRCU_SIGNAL`
192 3. Link the application with `-lurcu-signal`
193
194 Version of the library that requires a signal, typically `SIGUSR1`. Can
195 be overridden with `-DSIGRCU` by modifying `Makefile.build.inc`.
196
197
198 ### Usage of `liburcu-bp`
199
200 1. `#include <urcu-bp.h>`
201 2. Link with `-lurcu-bp`
202
203 The BP library flavor stands for "bulletproof". It is specifically
204 designed to help tracing library to hook on applications without
205 requiring to modify these applications. `rcu_init()`,
206 `rcu_register_thread()` and `rcu_unregister_thread()` all become nops.
207 The state is dealt with by the library internally at the expense of
208 read-side and write-side performance.
209
210
211 ### Initialization
212
213 Each thread that has reader critical sections (that uses
214 `rcu_read_lock()`/`rcu_read_unlock()` must first register to the URCU
215 library. This is done by calling `rcu_register_thread()`. Unregistration
216 must be performed before exiting the thread by using
217 `rcu_unregister_thread()`.
218
219
220 ### Reading
221
222 Reader critical sections must be protected by locating them between
223 calls to `rcu_read_lock()` and `rcu_read_unlock()`. Inside that lock,
224 `rcu_dereference()` may be called to read an RCU protected pointer.
225
226
227 ### Writing
228
229 `rcu_assign_pointer()` and `rcu_xchg_pointer()` may be called anywhere.
230 After, `synchronize_rcu()` must be called. When it returns, the old
231 values are not in usage anymore.
232
233
234 ### Usage of `liburcu-defer`
235
236 - Follow instructions for either `liburcu`, `liburcu-qsbr`,
237 `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
238 The `liburcu-defer` functionality is pulled into each of
239 those library modules.
240 - Provides `defer_rcu()` primitive to enqueue delayed callbacks. Queued
241 callbacks are executed in batch periodically after a grace period.
242 Do _not_ use `defer_rcu()` within a read-side critical section, because
243 it may call `synchronize_rcu()` if the thread queue is full.
244 This can lead to deadlock or worse.
245 - Requires that `rcu_defer_barrier()` must be called in library destructor
246 if a library queues callbacks and is expected to be unloaded with
247 `dlclose()`.
248
249 Its API is currently experimental. It may change in future library releases.
250
251
252 ### Usage of `urcu-call-rcu`
253
254 - Follow instructions for either `liburcu`, `liburcu-qsbr`,
255 `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
256 The `urcu-call-rcu` functionality is pulled into each of
257 those library modules.
258 - Provides the `call_rcu()` primitive to enqueue delayed callbacks
259 in a manner similar to `defer_rcu()`, but without ever delaying
260 for a grace period. On the other hand, `call_rcu()`'s best-case
261 overhead is not quite as good as that of `defer_rcu()`.
262 - Provides `call_rcu()` to allow asynchronous handling of RCU
263 grace periods. A number of additional functions are provided
264 to manage the helper threads used by `call_rcu()`, but reasonable
265 defaults are used if these additional functions are not invoked.
266 See [`doc/rcu-api.md`](doc/rcu-api.md) in userspace-rcu documentation
267 for more details.
268
269
270 ### Being careful with signals
271
272 The `liburcu` library uses signals internally. The signal handler is
273 registered with the `SA_RESTART` flag. However, these signals may cause
274 some non-restartable system calls to fail with `errno = EINTR`. Care
275 should be taken to restart system calls manually if they fail with this
276 error. A list of non-restartable system calls may be found in
277 `signal(7)`. The `liburcu-mb` and `liburcu-qsbr` versions of the Userspace RCU
278 library do not require any signal.
279
280 Read-side critical sections are allowed in a signal handler,
281 except those setup with `sigaltstack(2)`, with `liburcu` and
282 `liburcu-mb`. Be careful, however, to disable these signals
283 between thread creation and calls to `rcu_register_thread()`, because a
284 signal handler nesting on an unregistered thread would not be
285 allowed to call `rcu_read_lock()`.
286
287 Read-side critical sections are _not_ allowed in a signal handler with
288 `liburcu-qsbr`, unless signals are disabled explicitly around each
289 `rcu_quiescent_state()` calls, when threads are put offline and around
290 calls to `synchronize_rcu()`. Even then, we do not recommend it.
291
292
293 ### Interaction with mutexes
294
295 One must be careful to do not cause deadlocks due to interaction of
296 `synchronize_rcu()` and RCU read-side with mutexes. If `synchronize_rcu()`
297 is called with a mutex held, this mutex (or any mutex which has this
298 mutex in its dependency chain) should not be acquired from within a RCU
299 read-side critical section.
300
301 This is especially important to understand in the context of the
302 QSBR flavor: a registered reader thread being "online" by
303 default should be considered as within a RCU read-side critical
304 section unless explicitly put "offline". Therefore, if
305 `synchronize_rcu()` is called with a mutex held, this mutex, as
306 well as any mutex which has this mutex in its dependency chain
307 should only be taken when the RCU reader thread is "offline"
308 (this can be performed by calling `rcu_thread_offline()`).
309
310
311 ### Interaction with `fork()`
312
313 Special care must be taken for applications performing `fork()` without
314 any following `exec()`. This is caused by the fact that Linux only clones
315 the thread calling `fork()`, and thus never replicates any of the other
316 parent thread into the child process. Most `liburcu` implementations
317 require that all registrations (as reader, `defer_rcu` and `call_rcu`
318 threads) should be released before a `fork()` is performed, except for the
319 rather common scenario where `fork()` is immediately followed by `exec()` in
320 the child process. The only implementation not subject to that rule is
321 `liburcu-bp`, which is designed to handle `fork()` by calling
322 `rcu_bp_before_fork`, `rcu_bp_after_fork_parent` and
323 `rcu_bp_after_fork_child`.
324
325 Applications that use `call_rcu()` and that `fork()` without
326 doing an immediate `exec()` must take special action. The parent
327 must invoke `call_rcu_before_fork()` before the `fork()` and
328 `call_rcu_after_fork_parent()` after the `fork()`. The child
329 process must invoke `call_rcu_after_fork_child()`.
330 Even though these three APIs are suitable for passing to
331 `pthread_atfork()`, use of `pthread_atfork()` is **STRONGLY
332 DISCOURAGED** for programs calling the glibc memory allocator
333 (`malloc()`, `calloc()`, `free()`, ...) within `call_rcu` callbacks.
334 This is due to limitations in the way glibc memory allocator
335 handles calls to the memory allocator from concurrent threads
336 while the `pthread_atfork()` handlers are executing.
337
338 Combining e.g.:
339
340 - call to `free()` from callbacks executed within `call_rcu` worker
341 threads,
342 - executing `call_rcu` atfork handlers within the glibc pthread
343 atfork mechanism,
344
345 will sometimes trigger interesting process hangs. This usually
346 hangs on a memory allocator lock within glibc.
347
348
349 ### Thread Local Storage (TLS)
350
351 Userspace RCU can fall back on `pthread_getspecific()` to emulate
352 TLS variables on systems where it is not available. This behavior
353 can be forced by specifying `--disable-compiler-tls` as configure
354 argument.
355
356
357 ### Usage of `DEBUG_RCU`
358
359 `DEBUG_RCU` is used to add internal debugging self-checks to the
360 RCU library. This define adds a performance penalty when enabled.
361 Can be enabled by uncommenting the corresponding line in
362 `Makefile.build.inc`.
363
364
365 ### Usage of `DEBUG_YIELD`
366
367 `DEBUG_YIELD` is used to add random delays in the code for testing
368 purposes.
369
370
371 ### SMP support
372
373 By default the library is configured to use synchronization primitives
374 adequate for SMP systems. On uniprocessor systems, support for SMP
375 systems can be disabled with:
376
377 ./configure --disable-smp-support
378
379 theoretically yielding slightly better performance.
380
381
382 Make targets
383 ------------
384
385 In addition to the usual `make check` target, Userspace RCU features
386 `make regtest` and `make bench` targets:
387
388 - `make check`: short tests, meant to be run when rebuilding or
389 porting Userspace RCU.
390 - `make regtest`: long (many hours) test, meant to be run when
391 modifying Userspace RCU or porting it to a new architecture or
392 operating system.
393 - `make bench`: long (many hours) benchmarks.
394
395
396 Contacts
397 --------
398
399 You can contact the maintainers on the following mailing list:
400 `lttng-dev@lists.lttng.org`.
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