Document C99 and C++11 requirement in README.md
[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 - x86 (i386, i486, i586, i686)
41 - amd64 / x86_64
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 - m68k
53 - RISC-V
54
55 Tested on:
56
57 - Linux all architectures
58 - FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64
59 - Solaris 10/11 i386
60 - Cygwin i386/amd64
61 - MacOS amd64/arm64
62
63 Should also work on:
64
65 - Android
66 - NetBSD 5
67 - OpenBSD
68 - Darwin
69
70 (more testing needed before claiming support for these OS).
71
72 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
73 better.
74
75 The C compiler used needs to support at least C99. The C++ compiler used
76 needs to support at least C++11.
77
78 The GCC compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
79 supported, with the following exceptions:
80
81 - GCC 3.3 and 3.4 have a bug that prevents them from generating volatile
82 accesses to offsets in a TLS structure on 32-bit x86. These versions are
83 therefore not compatible with `liburcu` on x86 32-bit
84 (i386, i486, i586, i686).
85 The problem has been reported to the GCC community:
86 http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
87 - GCC 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
88 See http://kerneltrap.org/node/7507
89 - Alpha, ia64 and ARM architectures depend on GCC 4.x with atomic builtins
90 support. For ARM this was introduced with GCC 4.4:
91 http://gcc.gnu.org/gcc-4.4/changes.html.
92 - Linux aarch64 depends on GCC 5.1 or better because prior versions
93 perform unsafe access to deallocated stack.
94
95 Clang version 3.0 (based on LLVM 3.0) is supported.
96
97 Glibc >= 2.4 should work but the older version we test against is
98 currently 2.17.
99
100 For developers using the Git tree:
101
102 This source tree is based on the autotools suite from GNU to simplify
103 portability. Here are some things you should have on your system in order to
104 compile the git repository tree :
105
106 - GNU autotools (automake >=1.12, autoconf >=2.69)
107 (make sure your system wide `automake` points to a recent version!)
108 - GNU Libtool >=2.2
109 (for more information, go to http://www.gnu.org/software/autoconf/)
110
111 If you get the tree from the repository, you will need to use the `bootstrap`
112 script in the root of the tree. It calls all the GNU tools needed to prepare
113 the tree configuration.
114
115 Test scripts provided in the `tests/` directory of the source tree depend
116 on `bash` and the `seq` program.
117
118
119 API
120 ---
121
122 See the relevant API documentation files in `doc/`. The APIs provided by
123 Userspace RCU are, by prefix:
124
125 - `rcu_`: Read-Copy Update (see [`doc/rcu-api.md`](doc/rcu-api.md))
126 - `cmm_`: Concurrent Memory Model
127 - `caa_`: Concurrent Architecture Abstraction
128 - `cds_`: Concurrent Data Structures
129 (see [`doc/cds-api.md`](doc/cds-api.md))
130 - `uatomic_`: Userspace Atomic
131 (see [`doc/uatomic-api.md`](doc/uatomic-api.md))
132
133
134 Quick start guide
135 -----------------
136
137 ### Usage of all urcu libraries:
138
139 - Define `_LGPL_SOURCE` (only) if your code is LGPL or GPL compatible
140 before including the `urcu.h` or `urcu-qsbr.h` header. If your application
141 is distributed under another license, function calls will be generated
142 instead of inlines, so your application can link with the library.
143 - Linking with one of the libraries below is always necessary even for
144 LGPL and GPL applications.
145 - Define `URCU_INLINE_SMALL_FUNCTIONS` before including Userspace RCU
146 headers if you want Userspace RCU to inline small functions (10
147 lines or less) into the application. It can be used by applications
148 distributed under any kind of license, and does *not* make the
149 application a derived work of Userspace RCU.
150
151 Those small inlined functions are guaranteed to match the library
152 content as long as the library major version is unchanged.
153 Therefore, the application *must* be compiled with headers matching
154 the library major version number. Applications using
155 `URCU_INLINE_SMALL_FUNCTIONS` may be unable to use debugging
156 features of Userspace RCU without being recompiled.
157
158 There are multiple flavors of liburcu available:
159
160 - `memb`,
161 - `qsbr`,
162 - `mb`,
163 - `signal`,
164 - `bp`.
165
166 The API members start with the prefix "urcu_<flavor>_", where
167 <flavor> is the chosen flavor name.
168
169
170 ### Usage of `liburcu-memb`
171
172 1. `#include <urcu/urcu-memb.h>`
173 2. Link the application with `-lurcu-memb`
174
175 This is the preferred version of the library, in terms of
176 grace-period detection speed, read-side speed and flexibility.
177 Dynamically detects kernel support for `sys_membarrier()`. Falls back
178 on `urcu-mb` scheme if support is not present, which has slower
179 read-side. Use the --disable-sys-membarrier-fallback configure option
180 to disable the fall back, thus requiring `sys_membarrier()` to be
181 available. This gives a small speedup when `sys_membarrier()` is
182 supported by the kernel, and aborts in the library constructor if not
183 supported.
184
185
186 ### Usage of `liburcu-qsbr`
187
188 1. `#include <urcu/urcu-qsbr.h>`
189 2. Link with `-lurcu-qsbr`
190
191 The QSBR flavor of RCU needs to have each reader thread executing
192 `rcu_quiescent_state()` periodically to progress. `rcu_thread_online()`
193 and `rcu_thread_offline()` can be used to mark long periods for which
194 the threads are not active. It provides the fastest read-side at the
195 expense of more intrusiveness in the application code.
196
197
198 ### Usage of `liburcu-mb`
199
200 1. `#include <urcu/urcu-mb.h>`
201 2. Link with `-lurcu-mb`
202
203 This version of the urcu library uses memory barriers on the writer
204 and reader sides. This results in faster grace-period detection, but
205 results in slower reads.
206
207
208 ### Usage of `liburcu-signal`
209
210 1. `#include <urcu/urcu-signal.h>`
211 2. Link the application with `-lurcu-signal`
212
213 Version of the library that requires a signal, typically `SIGUSR1`. Can
214 be overridden with `-DSIGRCU` by modifying `Makefile.build.inc`.
215
216
217 ### Usage of `liburcu-bp`
218
219 1. `#include <urcu/urcu-bp.h>`
220 2. Link with `-lurcu-bp`
221
222 The BP library flavor stands for "bulletproof". It is specifically
223 designed to help tracing library to hook on applications without
224 requiring to modify these applications. `urcu_bp_init()`, and
225 `urcu_bp_unregister_thread()` all become nops, whereas calling
226 `urcu_bp_register_thread()` becomes optional. The state is dealt with by
227 the library internally at the expense of read-side and write-side
228 performance.
229
230
231 ### Initialization
232
233 Each thread that has reader critical sections (that uses
234 `urcu_<flavor>_read_lock()`/`urcu_<flavor>_read_unlock()` must first
235 register to the URCU library. This is done by calling
236 `urcu_<flavor>_register_thread()`. Unregistration must be performed
237 before exiting the thread by using `urcu_<flavor>_unregister_thread()`.
238
239
240 ### Reading
241
242 Reader critical sections must be protected by locating them between
243 calls to `urcu_<flavor>_read_lock()` and `urcu_<flavor>_read_unlock()`.
244 Inside that lock, `rcu_dereference()` may be called to read an RCU
245 protected pointer.
246
247
248 ### Writing
249
250 `rcu_assign_pointer()` and `rcu_xchg_pointer()` may be called anywhere.
251 After, `urcu_<flavor>_synchronize_rcu()` must be called. When it
252 returns, the old values are not in usage anymore.
253
254
255 ### Usage of `liburcu-defer`
256
257 - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`,
258 `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
259 The `liburcu-defer` functionality is pulled into each of
260 those library modules.
261 - Provides `urcu_<flavor>_defer_rcu()` primitive to enqueue delayed
262 callbacks. Queued callbacks are executed in batch periodically after
263 a grace period. Do _not_ use `urcu_<flavor>_defer_rcu()` within a
264 read-side critical section, because it may call
265 `urcu_<flavor>_synchronize_rcu()` if the thread queue is full. This
266 can lead to deadlock or worse.
267 - Requires that `urcu_<flavor>_defer_barrier()` must be called in
268 library destructor if a library queues callbacks and is expected to
269 be unloaded with `dlclose()`.
270
271 Its API is currently experimental. It may change in future library releases.
272
273
274 ### Usage of `urcu-call-rcu`
275
276 - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`,
277 `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
278 The `urcu-call-rcu` functionality is pulled into each of
279 those library modules.
280 - Provides the `urcu_<flavor>_call_rcu()` primitive to enqueue delayed
281 callbacks in a manner similar to `urcu_<flavor>_defer_rcu()`, but
282 without ever delaying for a grace period. On the other hand,
283 `urcu_<flavor>_call_rcu()`'s best-case overhead is not quite as good
284 as that of `urcu_<flavor>_defer_rcu()`.
285 - Provides `urcu_<flavor>_call_rcu()` to allow asynchronous handling
286 of RCU grace periods. A number of additional functions are provided
287 to manage the helper threads used by `urcu_<flavor>_call_rcu()`, but
288 reasonable defaults are used if these additional functions are not
289 invoked. See [`doc/rcu-api.md`](doc/rcu-api.md) in userspace-rcu
290 documentation for more details.
291
292
293 ### Being careful with signals
294
295 The `liburcu-signal` library uses signals internally. The signal handler is
296 registered with the `SA_RESTART` flag. However, these signals may cause
297 some non-restartable system calls to fail with `errno = EINTR`. Care
298 should be taken to restart system calls manually if they fail with this
299 error. A list of non-restartable system calls may be found in
300 `signal(7)`.
301
302 Read-side critical sections are allowed in a signal handler,
303 except those setup with `sigaltstack(2)`, with `liburcu-memb` and
304 `liburcu-mb`. Be careful, however, to disable these signals
305 between thread creation and calls to `urcu_<flavor>_register_thread()`,
306 because a signal handler nesting on an unregistered thread would not be
307 allowed to call `urcu_<flavor>_read_lock()`.
308
309 Read-side critical sections are _not_ allowed in a signal handler with
310 `liburcu-qsbr`, unless signals are disabled explicitly around each
311 `urcu_qsbr_quiescent_state()` calls, when threads are put offline and around
312 calls to `urcu_qsbr_synchronize_rcu()`. Even then, we do not recommend it.
313
314
315 ### Interaction with mutexes
316
317 One must be careful to do not cause deadlocks due to interaction of
318 `urcu_<flavor>_synchronize_rcu()` and RCU read-side with mutexes. If
319 `urcu_<flavor>_synchronize_rcu()` is called with a mutex held, this
320 mutex (or any mutex which has this mutex in its dependency chain) should
321 not be acquired from within a RCU read-side critical section.
322
323 This is especially important to understand in the context of the
324 QSBR flavor: a registered reader thread being "online" by
325 default should be considered as within a RCU read-side critical
326 section unless explicitly put "offline". Therefore, if
327 `urcu_qsbr_synchronize_rcu()` is called with a mutex held, this mutex,
328 as well as any mutex which has this mutex in its dependency chain should
329 only be taken when the RCU reader thread is "offline" (this can be
330 performed by calling `urcu_qsbr_thread_offline()`).
331
332
333 ### Interaction with `fork()`
334
335 Special care must be taken for applications performing `fork()` without
336 any following `exec()`. This is caused by the fact that Linux only clones
337 the thread calling `fork()`, and thus never replicates any of the other
338 parent thread into the child process. Most `liburcu` implementations
339 require that all registrations (as reader, `defer_rcu` and `call_rcu`
340 threads) should be released before a `fork()` is performed, except for the
341 rather common scenario where `fork()` is immediately followed by `exec()` in
342 the child process. The only implementation not subject to that rule is
343 `liburcu-bp`, which is designed to handle `fork()` by calling
344 `urcu_bp_before_fork`, `urcu_bp_after_fork_parent` and
345 `urcu_bp_after_fork_child`.
346
347 Applications that use `urcu_<flavor>_call_rcu()` and that `fork()`
348 without doing an immediate `exec()` must take special action. The
349 parent must invoke `urcu_<flavor>_call_rcu_before_fork()` before the
350 `fork()` and `urcu_<flavor>_call_rcu_after_fork_parent()` after the
351 `fork()`. The child process must invoke
352 `urcu_<flavor>_call_rcu_after_fork_child()`. Even though these three
353 APIs are suitable for passing to `pthread_atfork()`, use of
354 `pthread_atfork()` is **STRONGLY DISCOURAGED** for programs calling the
355 glibc memory allocator (`malloc()`, `calloc()`, `free()`, ...) within
356 `urcu_<flavor>_call_rcu` callbacks. This is due to limitations in the
357 way glibc memory allocator handles calls to the memory allocator from
358 concurrent threads while the `pthread_atfork()` handlers are executing.
359
360 Combining e.g.:
361
362 - call to `free()` from callbacks executed within
363 `urcu_<flavor>_call_rcu` worker threads,
364 - executing `urcu_<flavor>_call_rcu` atfork handlers within the glibc
365 pthread atfork mechanism,
366
367 will sometimes trigger interesting process hangs. This usually
368 hangs on a memory allocator lock within glibc.
369
370
371 ### Thread Local Storage (TLS)
372
373 Userspace RCU can fall back on `pthread_getspecific()` to emulate
374 TLS variables on systems where it is not available. This behavior
375 can be forced by specifying `--disable-compiler-tls` as configure
376 argument.
377
378
379 ### Usage of `DEBUG_RCU` & `--enable-rcu-debug`
380
381 By default the library is configured with internal debugging
382 self-checks disabled.
383
384 For always-on debugging self-checks:
385 ./configure --enable-rcu-debug
386
387 For fine grained enabling of debugging self-checks, build
388 userspace-rcu with DEBUG_RCU defined and compile dependent
389 applications with DEBUG_RCU defined when necessary.
390
391 Warning: Enabling this feature result in a performance penalty.
392
393
394 ### Usage of `DEBUG_YIELD`
395
396 `DEBUG_YIELD` is used to add random delays in the code for testing
397 purposes.
398
399
400 ### SMP support
401
402 By default the library is configured to use synchronization primitives
403 adequate for SMP systems. On uniprocessor systems, support for SMP
404 systems can be disabled with:
405
406 ./configure --disable-smp-support
407
408 theoretically yielding slightly better performance.
409
410
411 ### Usage of `--enable-cds-lfht-iter-debug`
412
413 By default the library is configured with extra debugging checks for
414 lock-free hash table iterator traversal disabled.
415
416 Building liburcu with --enable-cds-lfht-iter-debug and rebuilding
417 application to match the ABI change allows finding cases where the hash
418 table iterator is re-purposed to be used on a different hash table while
419 still being used to iterate on a hash table.
420
421 This option alters the rculfhash ABI. Make sure to compile both library
422 and application with matching configuration.
423
424
425 Make targets
426 ------------
427
428 In addition to the usual `make check` target, Userspace RCU features
429 `make regtest` and `make bench` targets:
430
431 - `make check`: short tests, meant to be run when rebuilding or
432 porting Userspace RCU.
433 - `make regtest`: long (many hours) test, meant to be run when
434 modifying Userspace RCU or porting it to a new architecture or
435 operating system.
436 - `make bench`: long (many hours) benchmarks.
437
438
439 Known issues
440 ------------
441
442 There is an application vs library compatibility issue between
443 applications built using Userspace RCU 0.10 headers linked against
444 Userspace RCU 0.11 or 0.12 shared objects. The problem occurs as
445 follows:
446
447 - An application executable is built with _LGPL_SOURCE defined, includes
448 any of the Userspace RCU 0.10 urcu flavor headers, and is built
449 without the -fpic compiler option.
450
451 - The Userspace RCU 0.10 library shared objects are updated to 0.11
452 or 0.12 without rebuilding the application.
453
454 - The application will hang, typically when RCU grace period
455 (synchronize_rcu) is invoked.
456
457 Some possible work-arounds for this are:
458
459 - Rebuild the application against Userspace RCU 0.11+.
460
461 - Rebuild the application with -fpic.
462
463 - Upgrade Userspace RCU to 0.13+ without installing 0.11 nor 0.12.
464
465
466 Contacts
467 --------
468
469 You can contact the maintainers on the following mailing list:
470 `lttng-dev@lists.lttng.org`.
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