02b903a2e18ea9d533cc4cd31e0b6288a32cc3b6
[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 GCC compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
76 supported, with the following exceptions:
77
78 - GCC 3.3 and 3.4 have a bug that prevents them from generating volatile
79 accesses to offsets in a TLS structure on 32-bit x86. These versions are
80 therefore not compatible with `liburcu` on x86 32-bit
81 (i386, i486, i586, i686).
82 The problem has been reported to the GCC community:
83 http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
84 - GCC 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
85 See http://kerneltrap.org/node/7507
86 - Alpha, ia64 and ARM architectures depend on GCC 4.x with atomic builtins
87 support. For ARM this was introduced with GCC 4.4:
88 http://gcc.gnu.org/gcc-4.4/changes.html.
89 - Linux aarch64 depends on GCC 5.1 or better because prior versions
90 perform unsafe access to deallocated stack.
91
92 Clang version 3.0 (based on LLVM 3.0) is supported.
93
94 Glibc >= 2.4 should work but the older version we test against is
95 currently 2.17.
96
97 For developers using the Git tree:
98
99 This source tree is based on the autotools suite from GNU to simplify
100 portability. Here are some things you should have on your system in order to
101 compile the git repository tree :
102
103 - GNU autotools (automake >=1.12, autoconf >=2.69)
104 (make sure your system wide `automake` points to a recent version!)
105 - GNU Libtool >=2.2
106 (for more information, go to http://www.gnu.org/software/autoconf/)
107
108 If you get the tree from the repository, you will need to use the `bootstrap`
109 script in the root of the tree. It calls all the GNU tools needed to prepare
110 the tree configuration.
111
112 Test scripts provided in the `tests/` directory of the source tree depend
113 on `bash` and the `seq` program.
114
115
116 API
117 ---
118
119 See the relevant API documentation files in `doc/`. The APIs provided by
120 Userspace RCU are, by prefix:
121
122 - `rcu_`: Read-Copy Update (see [`doc/rcu-api.md`](doc/rcu-api.md))
123 - `cmm_`: Concurrent Memory Model
124 - `caa_`: Concurrent Architecture Abstraction
125 - `cds_`: Concurrent Data Structures
126 (see [`doc/cds-api.md`](doc/cds-api.md))
127 - `uatomic_`: Userspace Atomic
128 (see [`doc/uatomic-api.md`](doc/uatomic-api.md))
129
130
131 Quick start guide
132 -----------------
133
134 ### Usage of all urcu libraries:
135
136 - Define `_LGPL_SOURCE` (only) if your code is LGPL or GPL compatible
137 before including the `urcu.h` or `urcu-qsbr.h` header. If your application
138 is distributed under another license, function calls will be generated
139 instead of inlines, so your application can link with the library.
140 - Linking with one of the libraries below is always necessary even for
141 LGPL and GPL applications.
142 - Define `URCU_INLINE_SMALL_FUNCTIONS` before including Userspace RCU
143 headers if you want Userspace RCU to inline small functions (10
144 lines or less) into the application. It can be used by applications
145 distributed under any kind of license, and does *not* make the
146 application a derived work of Userspace RCU.
147
148 Those small inlined functions are guaranteed to match the library
149 content as long as the library major version is unchanged.
150 Therefore, the application *must* be compiled with headers matching
151 the library major version number. Applications using
152 `URCU_INLINE_SMALL_FUNCTIONS` may be unable to use debugging
153 features of Userspace RCU without being recompiled.
154
155 There are multiple flavors of liburcu available:
156
157 - `memb`,
158 - `qsbr`,
159 - `mb`,
160 - `signal`,
161 - `bp`.
162
163 The API members start with the prefix "urcu_<flavor>_", where
164 <flavor> is the chosen flavor name.
165
166
167 ### Usage of `liburcu-memb`
168
169 1. `#include <urcu/urcu-memb.h>`
170 2. Link the application with `-lurcu-memb`
171
172 This is the preferred version of the library, in terms of
173 grace-period detection speed, read-side speed and flexibility.
174 Dynamically detects kernel support for `sys_membarrier()`. Falls back
175 on `urcu-mb` scheme if support is not present, which has slower
176 read-side. Use the --disable-sys-membarrier-fallback configure option
177 to disable the fall back, thus requiring `sys_membarrier()` to be
178 available. This gives a small speedup when `sys_membarrier()` is
179 supported by the kernel, and aborts in the library constructor if not
180 supported.
181
182
183 ### Usage of `liburcu-qsbr`
184
185 1. `#include <urcu/urcu-qsbr.h>`
186 2. Link with `-lurcu-qsbr`
187
188 The QSBR flavor of RCU needs to have each reader thread executing
189 `rcu_quiescent_state()` periodically to progress. `rcu_thread_online()`
190 and `rcu_thread_offline()` can be used to mark long periods for which
191 the threads are not active. It provides the fastest read-side at the
192 expense of more intrusiveness in the application code.
193
194
195 ### Usage of `liburcu-mb`
196
197 1. `#include <urcu/urcu-mb.h>`
198 2. Link with `-lurcu-mb`
199
200 This version of the urcu library uses memory barriers on the writer
201 and reader sides. This results in faster grace-period detection, but
202 results in slower reads.
203
204
205 ### Usage of `liburcu-signal`
206
207 1. `#include <urcu/urcu-signal.h>`
208 2. Link the application with `-lurcu-signal`
209
210 Version of the library that requires a signal, typically `SIGUSR1`. Can
211 be overridden with `-DSIGRCU` by modifying `Makefile.build.inc`.
212
213
214 ### Usage of `liburcu-bp`
215
216 1. `#include <urcu/urcu-bp.h>`
217 2. Link with `-lurcu-bp`
218
219 The BP library flavor stands for "bulletproof". It is specifically
220 designed to help tracing library to hook on applications without
221 requiring to modify these applications. `urcu_bp_init()`, and
222 `urcu_bp_unregister_thread()` all become nops, whereas calling
223 `urcu_bp_register_thread()` becomes optional. The state is dealt with by
224 the library internally at the expense of read-side and write-side
225 performance.
226
227
228 ### Initialization
229
230 Each thread that has reader critical sections (that uses
231 `urcu_<flavor>_read_lock()`/`urcu_<flavor>_read_unlock()` must first
232 register to the URCU library. This is done by calling
233 `urcu_<flavor>_register_thread()`. Unregistration must be performed
234 before exiting the thread by using `urcu_<flavor>_unregister_thread()`.
235
236
237 ### Reading
238
239 Reader critical sections must be protected by locating them between
240 calls to `urcu_<flavor>_read_lock()` and `urcu_<flavor>_read_unlock()`.
241 Inside that lock, `rcu_dereference()` may be called to read an RCU
242 protected pointer.
243
244
245 ### Writing
246
247 `rcu_assign_pointer()` and `rcu_xchg_pointer()` may be called anywhere.
248 After, `urcu_<flavor>_synchronize_rcu()` must be called. When it
249 returns, the old values are not in usage anymore.
250
251
252 ### Usage of `liburcu-defer`
253
254 - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`,
255 `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
256 The `liburcu-defer` functionality is pulled into each of
257 those library modules.
258 - Provides `urcu_<flavor>_defer_rcu()` primitive to enqueue delayed
259 callbacks. Queued callbacks are executed in batch periodically after
260 a grace period. Do _not_ use `urcu_<flavor>_defer_rcu()` within a
261 read-side critical section, because it may call
262 `urcu_<flavor>_synchronize_rcu()` if the thread queue is full. This
263 can lead to deadlock or worse.
264 - Requires that `urcu_<flavor>_defer_barrier()` must be called in
265 library destructor if a library queues callbacks and is expected to
266 be unloaded with `dlclose()`.
267
268 Its API is currently experimental. It may change in future library releases.
269
270
271 ### Usage of `urcu-call-rcu`
272
273 - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`,
274 `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
275 The `urcu-call-rcu` functionality is pulled into each of
276 those library modules.
277 - Provides the `urcu_<flavor>_call_rcu()` primitive to enqueue delayed
278 callbacks in a manner similar to `urcu_<flavor>_defer_rcu()`, but
279 without ever delaying for a grace period. On the other hand,
280 `urcu_<flavor>_call_rcu()`'s best-case overhead is not quite as good
281 as that of `urcu_<flavor>_defer_rcu()`.
282 - Provides `urcu_<flavor>_call_rcu()` to allow asynchronous handling
283 of RCU grace periods. A number of additional functions are provided
284 to manage the helper threads used by `urcu_<flavor>_call_rcu()`, but
285 reasonable defaults are used if these additional functions are not
286 invoked. See [`doc/rcu-api.md`](doc/rcu-api.md) in userspace-rcu
287 documentation for more details.
288
289
290 ### Being careful with signals
291
292 The `liburcu-signal` library uses signals internally. The signal handler is
293 registered with the `SA_RESTART` flag. However, these signals may cause
294 some non-restartable system calls to fail with `errno = EINTR`. Care
295 should be taken to restart system calls manually if they fail with this
296 error. A list of non-restartable system calls may be found in
297 `signal(7)`.
298
299 Read-side critical sections are allowed in a signal handler,
300 except those setup with `sigaltstack(2)`, with `liburcu-memb` and
301 `liburcu-mb`. Be careful, however, to disable these signals
302 between thread creation and calls to `urcu_<flavor>_register_thread()`,
303 because a signal handler nesting on an unregistered thread would not be
304 allowed to call `urcu_<flavor>_read_lock()`.
305
306 Read-side critical sections are _not_ allowed in a signal handler with
307 `liburcu-qsbr`, unless signals are disabled explicitly around each
308 `urcu_qsbr_quiescent_state()` calls, when threads are put offline and around
309 calls to `urcu_qsbr_synchronize_rcu()`. Even then, we do not recommend it.
310
311
312 ### Interaction with mutexes
313
314 One must be careful to do not cause deadlocks due to interaction of
315 `urcu_<flavor>_synchronize_rcu()` and RCU read-side with mutexes. If
316 `urcu_<flavor>_synchronize_rcu()` is called with a mutex held, this
317 mutex (or any mutex which has this mutex in its dependency chain) should
318 not be acquired from within a RCU read-side critical section.
319
320 This is especially important to understand in the context of the
321 QSBR flavor: a registered reader thread being "online" by
322 default should be considered as within a RCU read-side critical
323 section unless explicitly put "offline". Therefore, if
324 `urcu_qsbr_synchronize_rcu()` is called with a mutex held, this mutex,
325 as well as any mutex which has this mutex in its dependency chain should
326 only be taken when the RCU reader thread is "offline" (this can be
327 performed by calling `urcu_qsbr_thread_offline()`).
328
329
330 ### Interaction with `fork()`
331
332 Special care must be taken for applications performing `fork()` without
333 any following `exec()`. This is caused by the fact that Linux only clones
334 the thread calling `fork()`, and thus never replicates any of the other
335 parent thread into the child process. Most `liburcu` implementations
336 require that all registrations (as reader, `defer_rcu` and `call_rcu`
337 threads) should be released before a `fork()` is performed, except for the
338 rather common scenario where `fork()` is immediately followed by `exec()` in
339 the child process. The only implementation not subject to that rule is
340 `liburcu-bp`, which is designed to handle `fork()` by calling
341 `urcu_bp_before_fork`, `urcu_bp_after_fork_parent` and
342 `urcu_bp_after_fork_child`.
343
344 Applications that use `urcu_<flavor>_call_rcu()` and that `fork()`
345 without doing an immediate `exec()` must take special action. The
346 parent must invoke `urcu_<flavor>_call_rcu_before_fork()` before the
347 `fork()` and `urcu_<flavor>_call_rcu_after_fork_parent()` after the
348 `fork()`. The child process must invoke
349 `urcu_<flavor>_call_rcu_after_fork_child()`. Even though these three
350 APIs are suitable for passing to `pthread_atfork()`, use of
351 `pthread_atfork()` is **STRONGLY DISCOURAGED** for programs calling the
352 glibc memory allocator (`malloc()`, `calloc()`, `free()`, ...) within
353 `urcu_<flavor>_call_rcu` callbacks. This is due to limitations in the
354 way glibc memory allocator handles calls to the memory allocator from
355 concurrent threads while the `pthread_atfork()` handlers are executing.
356
357 Combining e.g.:
358
359 - call to `free()` from callbacks executed within
360 `urcu_<flavor>_call_rcu` worker threads,
361 - executing `urcu_<flavor>_call_rcu` atfork handlers within the glibc
362 pthread atfork mechanism,
363
364 will sometimes trigger interesting process hangs. This usually
365 hangs on a memory allocator lock within glibc.
366
367
368 ### Thread Local Storage (TLS)
369
370 Userspace RCU can fall back on `pthread_getspecific()` to emulate
371 TLS variables on systems where it is not available. This behavior
372 can be forced by specifying `--disable-compiler-tls` as configure
373 argument.
374
375
376 ### Usage of `DEBUG_RCU` & `--enable-rcu-debug`
377
378 By default the library is configured with internal debugging
379 self-checks disabled.
380
381 For always-on debugging self-checks:
382 ./configure --enable-rcu-debug
383
384 For fine grained enabling of debugging self-checks, build
385 userspace-rcu with DEBUG_RCU defined and compile dependent
386 applications with DEBUG_RCU defined when necessary.
387
388 Warning: Enabling this feature result in a performance penalty.
389
390
391 ### Usage of `DEBUG_YIELD`
392
393 `DEBUG_YIELD` is used to add random delays in the code for testing
394 purposes.
395
396
397 ### SMP support
398
399 By default the library is configured to use synchronization primitives
400 adequate for SMP systems. On uniprocessor systems, support for SMP
401 systems can be disabled with:
402
403 ./configure --disable-smp-support
404
405 theoretically yielding slightly better performance.
406
407
408 ### Usage of `--enable-cds-lfht-iter-debug`
409
410 By default the library is configured with extra debugging checks for
411 lock-free hash table iterator traversal disabled.
412
413 Building liburcu with --enable-cds-lfht-iter-debug and rebuilding
414 application to match the ABI change allows finding cases where the hash
415 table iterator is re-purposed to be used on a different hash table while
416 still being used to iterate on a hash table.
417
418 This option alters the rculfhash ABI. Make sure to compile both library
419 and application with matching configuration.
420
421
422 Make targets
423 ------------
424
425 In addition to the usual `make check` target, Userspace RCU features
426 `make regtest` and `make bench` targets:
427
428 - `make check`: short tests, meant to be run when rebuilding or
429 porting Userspace RCU.
430 - `make regtest`: long (many hours) test, meant to be run when
431 modifying Userspace RCU or porting it to a new architecture or
432 operating system.
433 - `make bench`: long (many hours) benchmarks.
434
435
436 Known issues
437 ------------
438
439 There is an application vs library compatibility issue between
440 applications built using Userspace RCU 0.10 headers linked against
441 Userspace RCU 0.11 or 0.12 shared objects. The problem occurs as
442 follows:
443
444 - An application executable is built with _LGPL_SOURCE defined, includes
445 any of the Userspace RCU 0.10 urcu flavor headers, and is built
446 without the -fpic compiler option.
447
448 - The Userspace RCU 0.10 library shared objects are updated to 0.11
449 or 0.12 without rebuilding the application.
450
451 - The application will hang, typically when RCU grace period
452 (synchronize_rcu) is invoked.
453
454 Some possible work-arounds for this are:
455
456 - Rebuild the application against Userspace RCU 0.11+.
457
458 - Rebuild the application with -fpic.
459
460 - Upgrade Userspace RCU to 0.13+ without installing 0.11 nor 0.12.
461
462
463 Contacts
464 --------
465
466 You can contact the maintainers on the following mailing list:
467 `lttng-dev@lists.lttng.org`.
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