1 Userspace RCU Implementation
2 ============================
4 by Mathieu Desnoyers and Paul E. McKenney
10 ./bootstrap # skip if using tarball
18 - Forcing 32-bit build:
20 CFLAGS="-m32 -g -O2" ./configure
22 - Forcing 64-bit build:
24 CFLAGS="-m64 -g -O2" ./configure
26 - Forcing a 32-bit build with 386 backward compatibility:
28 CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
30 - Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
32 CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
35 Architectures supported
36 -----------------------
38 Currently, the following architectures are supported:
40 - x86 (i386, i486, i586, i686)
57 - Linux all architectures
58 - FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64
70 (more testing needed before claiming support for these OS).
72 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
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:
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.
92 Clang version 3.0 (based on LLVM 3.0) is supported.
94 Glibc >= 2.4 should work but the older version we test against is
97 For developers using the Git tree:
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 :
103 - GNU autotools (automake >=1.12, autoconf >=2.69)
104 (make sure your system wide `automake` points to a recent version!)
106 (for more information, go to http://www.gnu.org/software/autoconf/)
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.
112 Test scripts provided in the `tests/` directory of the source tree depend
113 on `bash` and the `seq` program.
119 See the relevant API documentation files in `doc/`. The APIs provided by
120 Userspace RCU are, by prefix:
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))
134 ### Usage of all urcu libraries:
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.
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.
155 There are multiple flavors of liburcu available:
163 The API members start with the prefix "urcu_<flavor>_", where
164 <flavor> is the chosen flavor name.
167 ### Usage of `liburcu-memb`
169 1. `#include <urcu/urcu-memb.h>`
170 2. Link the application with `-lurcu-memb`
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
183 ### Usage of `liburcu-qsbr`
185 1. `#include <urcu/urcu-qsbr.h>`
186 2. Link with `-lurcu-qsbr`
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.
195 ### Usage of `liburcu-mb`
197 1. `#include <urcu/urcu-mb.h>`
198 2. Link with `-lurcu-mb`
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.
205 ### Usage of `liburcu-signal`
207 1. `#include <urcu/urcu-signal.h>`
208 2. Link the application with `-lurcu-signal`
210 Version of the library that requires a signal, typically `SIGUSR1`. Can
211 be overridden with `-DSIGRCU` by modifying `Makefile.build.inc`.
214 ### Usage of `liburcu-bp`
216 1. `#include <urcu/urcu-bp.h>`
217 2. Link with `-lurcu-bp`
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
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()`.
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
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.
252 ### Usage of `liburcu-defer`
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()`.
268 Its API is currently experimental. It may change in future library releases.
271 ### Usage of `urcu-call-rcu`
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.
290 ### Being careful with signals
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
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()`.
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.
312 ### Interaction with mutexes
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.
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()`).
330 ### Interaction with `fork()`
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`.
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.
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,
364 will sometimes trigger interesting process hangs. This usually
365 hangs on a memory allocator lock within glibc.
368 ### Thread Local Storage (TLS)
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
376 ### Usage of `DEBUG_RCU` & `--enable-rcu-debug`
378 By default the library is configured with internal debugging
379 self-checks disabled.
381 For always-on debugging self-checks:
382 ./configure --enable-rcu-debug
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.
388 Warning: Enabling this feature result in a performance penalty.
391 ### Usage of `DEBUG_YIELD`
393 `DEBUG_YIELD` is used to add random delays in the code for testing
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:
403 ./configure --disable-smp-support
405 theoretically yielding slightly better performance.
408 ### Usage of `--enable-cds-lfht-iter-debug`
410 By default the library is configured with extra debugging checks for
411 lock-free hash table iterator traversal disabled.
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.
418 This option alters the rculfhash ABI. Make sure to compile both library
419 and application with matching configuration.
425 In addition to the usual `make check` target, Userspace RCU features
426 `make regtest` and `make bench` targets:
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
433 - `make bench`: long (many hours) benchmarks.
439 You can contact the maintainers on the following mailing list:
440 `lttng-dev@lists.lttng.org`.