Documentation: rculfhash: cds_lfht_resize not within read-side C.S.
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1Userspace RCU Implementation
2by Mathieu Desnoyers and Paul E. McKenney
3
4BUILDING
5--------
6
7 ./bootstrap (skip if using tarball)
8 ./configure
9 make
10 make install
11 ldconfig
12
13 Hints: Forcing 32-bit build:
14 * CFLAGS="-m32 -g -O2" ./configure
15
16 Forcing 64-bit build:
17 * CFLAGS="-m64 -g -O2" ./configure
18
19 Forcing a 32-bit build with 386 backward compatibility:
20 * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
21
22 Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
23 * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
24
25
26ARCHITECTURES SUPPORTED
27-----------------------
28
29Currently, Linux x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64,
30S390, S390x, ARM, Alpha, ia64 and Sparcv9 32/64 are supported. Tested on
31Linux, FreeBSD 8.2/9.0, and Cygwin. Should also work on: Android, NetBSD 5,
32OpenBSD, Darwin (more testing needed before claiming support for these OS).
33
34Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
35better.
36
37The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
38supported, with the following exceptions:
39
40- gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
41 accesses to offsets in a TLS structure on 32-bit x86. These versions are
42 therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
43 The problem has been reported to the gcc community:
44 http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
45- gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
46 See: http://kerneltrap.org/node/7507
47- Alpha, ia64 and ARM architectures depend on gcc 4.x with atomic builtins
48 support. For ARM this was introduced with gcc 4.4:
49 http://gcc.gnu.org/gcc-4.4/changes.html
50
51Clang version 3.0 (based on LLVM 3.0) is supported.
52
53For developers using the git tree:
54
55This source tree is based on the autotools suite from GNU to simplify
56portability. Here are some things you should have on your system in order to
57compile the git repository tree :
58
59- GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
60 (make sure your system wide "automake" points to a recent version!)
61- GNU Libtool >=2.2
62 (for more information, go to http://www.gnu.org/software/autoconf/)
63
64If you get the tree from the repository, you will need to use the "bootstrap"
65script in the root of the tree. It calls all the GNU tools needed to prepare the
66tree configuration.
67
68Test scripts provided in the tests/ directory of the source tree depend
69on "bash" and the "seq" program.
70
71
72API
73---
74
75See the relevant API documentation files in doc/. The APIs provided by
76Userspace RCU are, by prefix:
77
78- rcu_ : Read-Copy Update (see doc/rcu-api.txt)
79- cmm_ : Concurrent Memory Model
80- caa_ : Concurrent Architecture Abstraction
81- cds_ : Concurrent Data Structures (see doc/cds-api.txt)
82- uatomic_: Userspace Atomic (see doc/uatomic-api.txt)
83
84
85QUICK START GUIDE
86-----------------
87
88Usage of all urcu libraries
89
90 * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
91 before including the urcu.h or urcu-qsbr.h header. If your application
92 is distributed under another license, function calls will be generated
93 instead of inlines, so your application can link with the library.
94 * Linking with one of the libraries below is always necessary even for
95 LGPL and GPL applications.
96
97Usage of liburcu
98
99 * #include <urcu.h>
100 * Link the application with "-lurcu".
101 * This is the preferred version of the library, in terms of
102 grace-period detection speed, read-side speed and flexibility.
103 Dynamically detects kernel support for sys_membarrier(). Falls back
104 on urcu-mb scheme if support is not present, which has slower
105 read-side.
106
107Usage of liburcu-qsbr
108
109 * #include <urcu-qsbr.h>
110 * Link with "-lurcu-qsbr".
111 * The QSBR flavor of RCU needs to have each reader thread executing
112 rcu_quiescent_state() periodically to progress. rcu_thread_online()
113 and rcu_thread_offline() can be used to mark long periods for which
114 the threads are not active. It provides the fastest read-side at the
115 expense of more intrusiveness in the application code.
116
117Usage of liburcu-mb
118
119 * #include <urcu.h>
120 * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
121 * Link with "-lurcu-mb".
122 * This version of the urcu library uses memory barriers on the writer
123 and reader sides. This results in faster grace-period detection, but
124 results in slower reads.
125
126Usage of liburcu-signal
127
128 * #include <urcu.h>
129 * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
130 * Link the application with "-lurcu-signal".
131 * Version of the library that requires a signal, typically SIGUSR1. Can
132 be overridden with -DSIGRCU by modifying Makefile.build.inc.
133
134Usage of liburcu-bp
135
136 * #include <urcu-bp.h>
137 * Link with "-lurcu-bp".
138 * The BP library flavor stands for "bulletproof". It is specifically
139 designed to help tracing library to hook on applications without
140 requiring to modify these applications. rcu_init(),
141 rcu_register_thread() and rcu_unregister_thread() all become nops.
142 The state is dealt with by the library internally at the expense of
143 read-side and write-side performance.
144
145Initialization
146
147 Each thread that has reader critical sections (that uses
148 rcu_read_lock()/rcu_read_unlock() must first register to the URCU
149 library. This is done by calling rcu_register_thread(). Unregistration
150 must be performed before exiting the thread by using
151 rcu_unregister_thread().
152
153Reading
154
155 Reader critical sections must be protected by locating them between
156 calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
157 rcu_dereference() may be called to read an RCU protected pointer.
158
159Writing
160
161 rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
162 After, synchronize_rcu() must be called. When it returns, the old
163 values are not in usage anymore.
164
165Usage of liburcu-defer
166
167 * Follow instructions for either liburcu, liburcu-qsbr,
168 liburcu-mb, liburcu-signal, or liburcu-bp above.
169 The liburcu-defer functionality is pulled into each of
170 those library modules.
171 * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
172 callbacks are executed in batch periodically after a grace period.
173 Do _not_ use defer_rcu() within a read-side critical section, because
174 it may call synchronize_rcu() if the thread queue is full.
175 This can lead to deadlock or worse.
176 * Requires that rcu_defer_barrier() must be called in library destructor
177 if a library queues callbacks and is expected to be unloaded with
178 dlclose().
179 * Its API is currently experimental. It may change in future library
180 releases.
181
182Usage of urcu-call-rcu
183
184 * Follow instructions for either liburcu, liburcu-qsbr,
185 liburcu-mb, liburcu-signal, or liburcu-bp above.
186 The urcu-call-rcu functionality is provided for each of
187 these library modules.
188 * Provides the call_rcu() primitive to enqueue delayed callbacks
189 in a manner similar to defer_rcu(), but without ever delaying
190 for a grace period. On the other hand, call_rcu()'s best-case
191 overhead is not quite as good as that of defer_rcu().
192 * Provides call_rcu() to allow asynchronous handling of RCU
193 grace periods. A number of additional functions are provided
194 to manage the helper threads used by call_rcu(), but reasonable
195 defaults are used if these additional functions are not invoked.
196 See rcu-api.txt in userspace-rcu documentation for more details.
197
198Being careful with signals
199
200 The liburcu library uses signals internally. The signal handler is
201 registered with the SA_RESTART flag. However, these signals may cause
202 some non-restartable system calls to fail with errno = EINTR. Care
203 should be taken to restart system calls manually if they fail with this
204 error. A list of non-restartable system calls may be found in
205 signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
206 library do not require any signal.
207
208 Read-side critical sections are allowed in a signal handler,
209 except those setup with sigaltstack(2), with liburcu and
210 liburcu-mb. Be careful, however, to disable these signals
211 between thread creation and calls to rcu_register_thread(), because a
212 signal handler nesting on an unregistered thread would not be
213 allowed to call rcu_read_lock().
214
215 Read-side critical sections are _not_ allowed in a signal handler with
216 liburcu-qsbr, unless signals are disabled explicitly around each
217 rcu_quiescent_state() calls, when threads are put offline and around
218 calls to synchronize_rcu(). Even then, we do not recommend it.
219
220Interaction with mutexes
221
222 One must be careful to do not cause deadlocks due to interaction of
223 synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
224 is called with a mutex held, this mutex (or any mutex which has this
225 mutex in its dependency chain) should not be acquired from within a RCU
226 read-side critical section.
227
228 This is especially important to understand in the context of the
229 QSBR flavor: a registered reader thread being "online" by
230 default should be considered as within a RCU read-side critical
231 section unless explicitly put "offline". Therefore, if
232 synchronize_rcu() is called with a mutex held, this mutex, as
233 well as any mutex which has this mutex in its dependency chain
234 should only be taken when the RCU reader thread is "offline"
235 (this can be performed by calling rcu_thread_offline()).
236
237Interaction with fork()
238
239 Special care must be taken for applications performing fork() without
240 any following exec(). This is caused by the fact that Linux only clones
241 the thread calling fork(), and thus never replicates any of the other
242 parent thread into the child process. Most liburcu implementations
243 require that all registrations (as reader, defer_rcu and call_rcu
244 threads) should be released before a fork() is performed, except for the
245 rather common scenario where fork() is immediately followed by exec() in
246 the child process. The only implementation not subject to that rule is
247 liburcu-bp, which is designed to handle fork() by calling
248 rcu_bp_before_fork, rcu_bp_after_fork_parent and
249 rcu_bp_after_fork_child.
250
251 Applications that use call_rcu() and that fork() without
252 doing an immediate exec() must take special action. The parent
253 must invoke call_rcu_before_fork() before the fork() and
254 call_rcu_after_fork_parent() after the fork(). The child
255 process must invoke call_rcu_after_fork_child().
256 Even though these three APIs are suitable for passing to
257 pthread_atfork(), use of pthread_atfork() is *STRONGLY
258 DISCOURAGED* for programs calling the glibc memory allocator
259 (malloc(), calloc(), free(), ...) within call_rcu callbacks.
260 This is due to limitations in the way glibc memory allocator
261 handles calls to the memory allocator from concurrent threads
262 while the pthread_atfork() handlers are executing.
263 Combining e.g.:
264 * call to free() from callbacks executed within call_rcu worker
265 threads,
266 * executing call_rcu atfork handlers within the glibc pthread
267 atfork mechanism,
268 will sometimes trigger interesting process hangs. This usually
269 hangs on a memory allocator lock within glibc.
270
271Thread Local Storage (TLS)
272
273 Userspace RCU can fall back on pthread_getspecific() to emulate
274 TLS variables on systems where it is not available. This behavior
275 can be forced by specifying --disable-compiler-tls as configure
276 argument.
277
278Usage of DEBUG_RCU
279
280 DEBUG_RCU is used to add internal debugging self-checks to the
281 RCU library. This define adds a performance penalty when enabled.
282 Can be enabled by uncommenting the corresponding line in
283 Makefile.build.inc.
284
285Usage of DEBUG_YIELD
286
287 DEBUG_YIELD is used to add random delays in the code for testing
288 purposes.
289
290SMP support
291
292 By default the library is configured to use synchronization primitives
293 adequate for SMP systems. On uniprocessor systems, support for SMP
294 systems can be disabled with:
295
296 ./configure --disable-smp-support
297
298 theoretically yielding slightly better performance.
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