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