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   - 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
  52 Tested on Linux, FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64, and Cygwin.
  53 Should 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
  62 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
  63 better.
  64
  65 The GCC compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
  66 supported, 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
  80 Clang version 3.0 (based on LLVM 3.0) is supported.
  81
  82 Building on MacOS X (Darwin) requires a work-around for processor
  83 detection:
  84
  85   - 32-bit:
  86
  87         ./configure --build=i686-apple-darwin11
  88
  89   - 64-bit:
  90
  91         ./configure --build=x86_64-apple-darwin11
  92
  93 For developers using the Git tree:
  94
  95 This source tree is based on the autotools suite from GNU to simplify
  96 portability. Here are some things you should have on your system in order to
  97 compile 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
 104 If you get the tree from the repository, you will need to use the `bootstrap`
 105 script in the root of the tree. It calls all the GNU tools needed to prepare
 106 the tree configuration.
 107
 108 Test scripts provided in the `tests/` directory of the source tree depend
 109 on `bash` and the `seq` program.
 110
 111
 112 API
 113 ---
 114
 115 See the relevant API documentation files in `doc/`. The APIs provided by
 116 Userspace RCU are, by prefix:
 117
 118   - `rcu_`: Read-Copy Update (see [`doc/rcu-api.md`](doc/rcu-api.md))
 119   - `cmm_`: Concurrent Memory Model
 120   - `caa_`: Concurrent Architecture Abstraction
 121   - `cds_`: Concurrent Data Structures
 122     (see [`doc/cds-api.md`](doc/cds-api.md))
 123   - `uatomic_`: Userspace Atomic
 124     (see [`doc/uatomic-api.md`](doc/uatomic-api.md))
 125
 126
 127 Quick 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
 144 Those small inlined functions are guaranteed to match the library
 145 content as long as the library major version is unchanged.
 146 Therefore, the application *must* be compiled with headers matching
 147 the library major version number. Applications using
 148 `URCU_INLINE_SMALL_FUNCTIONS` may be unable to use debugging
 149 features 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
 157 This is the preferred version of the library, in terms of
 158 grace-period detection speed, read-side speed and flexibility.
 159 Dynamically detects kernel support for `sys_membarrier()`. Falls back
 160 on `urcu-mb` scheme if support is not present, which has slower
 161 read-side.
 162
 163
 164 ### Usage of `liburcu-qsbr`
 165
 166   1. `#include <urcu-qsbr.h>`
 167   2. Link with `-lurcu-qsbr`
 168
 169 The QSBR flavor of RCU needs to have each reader thread executing
 170 `rcu_quiescent_state()` periodically to progress. `rcu_thread_online()`
 171 and `rcu_thread_offline()` can be used to mark long periods for which
 172 the threads are not active. It provides the fastest read-side at the
 173 expense 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
 182 This version of the urcu library uses memory barriers on the writer
 183 and reader sides. This results in faster grace-period detection, but
 184 results 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
 193 Version of the library that requires a signal, typically `SIGUSR1`. Can
 194 be 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
 202 The BP library flavor stands for "bulletproof". It is specifically
 203 designed to help tracing library to hook on applications without
 204 requiring to modify these applications. `rcu_init()`,
 205 `rcu_register_thread()` and `rcu_unregister_thread()` all become nops.
 206 The state is dealt with by the library internally at the expense of
 207 read-side and write-side performance.
 208
 209
 210 ### Initialization
 211
 212 Each thread that has reader critical sections (that uses
 213 `rcu_read_lock()`/`rcu_read_unlock()` must first register to the URCU
 214 library. This is done by calling `rcu_register_thread()`. Unregistration
 215 must be performed before exiting the thread by using
 216 `rcu_unregister_thread()`.
 217
 218
 219 ### Reading
 220
 221 Reader critical sections must be protected by locating them between
 222 calls 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.
 229 After, `synchronize_rcu()` must be called. When it returns, the old
 230 values 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
 248 Its 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.md`](doc/rcu-api.md) in userspace-rcu documentation
 266     for more details.
 267
 268
 269 ### Being careful with signals
 270
 271 The `liburcu` library uses signals internally. The signal handler is
 272 registered with the `SA_RESTART` flag. However, these signals may cause
 273 some non-restartable system calls to fail with `errno = EINTR`. Care
 274 should be taken to restart system calls manually if they fail with this
 275 error. 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
 277 library do not require any signal.
 278
 279 Read-side critical sections are allowed in a signal handler,
 280 except those setup with `sigaltstack(2)`, with `liburcu` and
 281 `liburcu-mb`. Be careful, however, to disable these signals
 282 between thread creation and calls to `rcu_register_thread()`, because a
 283 signal handler nesting on an unregistered thread would not be
 284 allowed to call `rcu_read_lock()`.
 285
 286 Read-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
 289 calls to `synchronize_rcu()`. Even then, we do not recommend it.
 290
 291
 292 ### Interaction with mutexes
 293
 294 One must be careful to do not cause deadlocks due to interaction of
 295 `synchronize_rcu()` and RCU read-side with mutexes. If `synchronize_rcu()`
 296 is called with a mutex held, this mutex (or any mutex which has this
 297 mutex in its dependency chain) should not be acquired from within a RCU
 298 read-side critical section.
 299
 300 This is especially important to understand in the context of the
 301 QSBR flavor: a registered reader thread being "online" by
 302 default should be considered as within a RCU read-side critical
 303 section unless explicitly put "offline". Therefore, if
 304 `synchronize_rcu()` is called with a mutex held, this mutex, as
 305 well as any mutex which has this mutex in its dependency chain
 306 should 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
 312 Special care must be taken for applications performing `fork()` without
 313 any following `exec()`. This is caused by the fact that Linux only clones
 314 the thread calling `fork()`, and thus never replicates any of the other
 315 parent thread into the child process. Most `liburcu` implementations
 316 require that all registrations (as reader, `defer_rcu` and `call_rcu`
 317 threads) should be released before a `fork()` is performed, except for the
 318 rather common scenario where `fork()` is immediately followed by `exec()` in
 319 the 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
 324 Applications that use `call_rcu()` and that `fork()` without
 325 doing an immediate `exec()` must take special action.  The parent
 326 must invoke `call_rcu_before_fork()` before the `fork()` and
 327 `call_rcu_after_fork_parent()` after the `fork()`. The child
 328 process must invoke `call_rcu_after_fork_child()`.
 329 Even though these three APIs are suitable for passing to
 330 `pthread_atfork()`, use of `pthread_atfork()` is **STRONGLY
 331 DISCOURAGED** for programs calling the glibc memory allocator
 332 (`malloc()`, `calloc()`, `free()`, ...) within `call_rcu` callbacks.
 333 This is due to limitations in the way glibc memory allocator
 334 handles calls to the memory allocator from concurrent threads
 335 while the `pthread_atfork()` handlers are executing.
 336
 337 Combining 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
 344 will sometimes trigger interesting process hangs. This usually
 345 hangs on a memory allocator lock within glibc.
 346
 347
 348 ### Thread Local Storage (TLS)
 349
 350 Userspace RCU can fall back on `pthread_getspecific()` to emulate
 351 TLS variables on systems where it is not available. This behavior
 352 can be forced by specifying `--disable-compiler-tls` as configure
 353 argument.
 354
 355
 356 ### Usage of `DEBUG_RCU`
 357
 358 `DEBUG_RCU` is used to add internal debugging self-checks to the
 359 RCU library. This define adds a performance penalty when enabled.
 360 Can 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
 367 purposes.
 368
 369
 370 ### SMP support
 371
 372 By default the library is configured to use synchronization primitives
 373 adequate for SMP systems. On uniprocessor systems, support for SMP
 374 systems can be disabled with:
 375
 376     ./configure --disable-smp-support
 377
 378 theoretically yielding slightly better performance.
 379
 380
 381 Make targets
 382 ------------
 383
 384 In 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
 395 Contacts
 396 --------
 397
 398 You can contact the maintainers on the following mailing list:
 399 `lttng-dev@lists.lttng.org`.