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   - NIOS2
  47   - Alpha
  48   - ia64
  49   - Sparcv9 32/64
  50   - Tilera
  51   - hppa/PA-RISC
  52
  53 Tested on Linux, FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64, and Cygwin.
  54 Should also work on:
  55
  56   - Android
  57   - NetBSD 5
  58   - OpenBSD
  59   - Darwin
  60
  61 (more testing needed before claiming support for these OS).
  62
  63 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
  64 better.
  65
  66 The GCC compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
  67 supported, with the following exceptions:
  68
  69   - GCC 3.3 and 3.4 have a bug that prevents them from generating volatile
  70     accesses to offsets in a TLS structure on 32-bit x86. These versions are
  71     therefore not compatible with `liburcu` on x86 32-bit
  72     (i386, i486, i586, i686).
  73     The problem has been reported to the GCC community:
  74     http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
  75   - GCC 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
  76     See http://kerneltrap.org/node/7507
  77   - Alpha, ia64 and ARM architectures depend on GCC 4.x with atomic builtins
  78     support. For ARM this was introduced with GCC 4.4:
  79     http://gcc.gnu.org/gcc-4.4/changes.html.
  80
  81 Clang version 3.0 (based on LLVM 3.0) is supported.
  82
  83 Building on MacOS X (Darwin) requires a work-around for processor
  84 detection:
  85
  86   - 32-bit:
  87
  88         ./configure --build=i686-apple-darwin11
  89
  90   - 64-bit:
  91
  92         ./configure --build=x86_64-apple-darwin11
  93
  94 For developers using the Git tree:
  95
  96 This source tree is based on the autotools suite from GNU to simplify
  97 portability. Here are some things you should have on your system in order to
  98 compile the git repository tree :
  99
 100   - GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
 101     (make sure your system wide `automake` points to a recent version!)
 102   - GNU Libtool >=2.2
 103     (for more information, go to http://www.gnu.org/software/autoconf/)
 104
 105 If you get the tree from the repository, you will need to use the `bootstrap`
 106 script in the root of the tree. It calls all the GNU tools needed to prepare
 107 the tree configuration.
 108
 109 Test scripts provided in the `tests/` directory of the source tree depend
 110 on `bash` and the `seq` program.
 111
 112
 113 API
 114 ---
 115
 116 See the relevant API documentation files in `doc/`. The APIs provided by
 117 Userspace RCU are, by prefix:
 118
 119   - `rcu_`: Read-Copy Update (see [`doc/rcu-api.md`](doc/rcu-api.md))
 120   - `cmm_`: Concurrent Memory Model
 121   - `caa_`: Concurrent Architecture Abstraction
 122   - `cds_`: Concurrent Data Structures
 123     (see [`doc/cds-api.md`](doc/cds-api.md))
 124   - `uatomic_`: Userspace Atomic
 125     (see [`doc/uatomic-api.md`](doc/uatomic-api.md))
 126
 127
 128 Quick start guide
 129 -----------------
 130
 131 ### Usage of all urcu libraries:
 132
 133   - Define `_LGPL_SOURCE` (only) if your code is LGPL or GPL compatible
 134     before including the `urcu.h` or `urcu-qsbr.h` header. If your application
 135     is distributed under another license, function calls will be generated
 136     instead of inlines, so your application can link with the library.
 137   - Linking with one of the libraries below is always necessary even for
 138     LGPL and GPL applications.
 139   - Define `URCU_INLINE_SMALL_FUNCTIONS` before including Userspace RCU
 140     headers if you want Userspace RCU to inline small functions (10
 141     lines or less) into the application. It can be used by applications
 142     distributed under any kind of license, and does *not* make the
 143     application a derived work of Userspace RCU.
 144
 145 Those small inlined functions are guaranteed to match the library
 146 content as long as the library major version is unchanged.
 147 Therefore, the application *must* be compiled with headers matching
 148 the library major version number. Applications using
 149 `URCU_INLINE_SMALL_FUNCTIONS` may be unable to use debugging
 150 features of Userspace RCU without being recompiled.
 151
 152
 153 ### Usage of `liburcu`
 154
 155   1. `#include <urcu.h>`
 156   2. Link the application with `-lurcu`
 157
 158 This is the preferred version of the library, in terms of
 159 grace-period detection speed, read-side speed and flexibility.
 160 Dynamically detects kernel support for `sys_membarrier()`. Falls back
 161 on `urcu-mb` scheme if support is not present, which has slower
 162 read-side.
 163
 164
 165 ### Usage of `liburcu-qsbr`
 166
 167   1. `#include <urcu-qsbr.h>`
 168   2. Link with `-lurcu-qsbr`
 169
 170 The QSBR flavor of RCU needs to have each reader thread executing
 171 `rcu_quiescent_state()` periodically to progress. `rcu_thread_online()`
 172 and `rcu_thread_offline()` can be used to mark long periods for which
 173 the threads are not active. It provides the fastest read-side at the
 174 expense of more intrusiveness in the application code.
 175
 176
 177 ### Usage of `liburcu-mb`
 178
 179   1. `#include <urcu.h>`
 180   2. Compile any `_LGPL_SOURCE` code using this library with `-DRCU_MB`
 181   3. Link with `-lurcu-mb`
 182
 183 This version of the urcu library uses memory barriers on the writer
 184 and reader sides. This results in faster grace-period detection, but
 185 results in slower reads.
 186
 187
 188 ### Usage of `liburcu-signal`
 189
 190   1. `#include <urcu.h>`
 191   2. Compile any `_LGPL_SOURCE` code using this library with `-DRCU_SIGNAL`
 192   3. Link the application with `-lurcu-signal`
 193
 194 Version of the library that requires a signal, typically `SIGUSR1`. Can
 195 be overridden with `-DSIGRCU` by modifying `Makefile.build.inc`.
 196
 197
 198 ### Usage of `liburcu-bp`
 199
 200   1. `#include <urcu-bp.h>`
 201   2. Link with `-lurcu-bp`
 202
 203 The BP library flavor stands for "bulletproof". It is specifically
 204 designed to help tracing library to hook on applications without
 205 requiring to modify these applications. `rcu_init()`,
 206 `rcu_register_thread()` and `rcu_unregister_thread()` all become nops.
 207 The state is dealt with by the library internally at the expense of
 208 read-side and write-side performance.
 209
 210
 211 ### Initialization
 212
 213 Each thread that has reader critical sections (that uses
 214 `rcu_read_lock()`/`rcu_read_unlock()` must first register to the URCU
 215 library. This is done by calling `rcu_register_thread()`. Unregistration
 216 must be performed before exiting the thread by using
 217 `rcu_unregister_thread()`.
 218
 219
 220 ### Reading
 221
 222 Reader critical sections must be protected by locating them between
 223 calls to `rcu_read_lock()` and `rcu_read_unlock()`. Inside that lock,
 224 `rcu_dereference()` may be called to read an RCU protected pointer.
 225
 226
 227 ### Writing
 228
 229 `rcu_assign_pointer()` and `rcu_xchg_pointer()` may be called anywhere.
 230 After, `synchronize_rcu()` must be called. When it returns, the old
 231 values are not in usage anymore.
 232
 233
 234 ### Usage of `liburcu-defer`
 235
 236   - Follow instructions for either `liburcu`, `liburcu-qsbr`,
 237     `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
 238     The `liburcu-defer` functionality is pulled into each of
 239     those library modules.
 240   - Provides `defer_rcu()` primitive to enqueue delayed callbacks. Queued
 241     callbacks are executed in batch periodically after a grace period.
 242     Do _not_ use `defer_rcu()` within a read-side critical section, because
 243     it may call `synchronize_rcu()` if the thread queue is full.
 244     This can lead to deadlock or worse.
 245   - Requires that `rcu_defer_barrier()` must be called in library destructor
 246     if a library queues callbacks and is expected to be unloaded with
 247     `dlclose()`.
 248
 249 Its API is currently experimental. It may change in future library releases.
 250
 251
 252 ### Usage of `urcu-call-rcu`
 253
 254   - Follow instructions for either `liburcu`, `liburcu-qsbr`,
 255     `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
 256     The `urcu-call-rcu` functionality is pulled into each of
 257     those library modules.
 258   - Provides the `call_rcu()` primitive to enqueue delayed callbacks
 259     in a manner similar to `defer_rcu()`, but without ever delaying
 260     for a grace period.  On the other hand, `call_rcu()`'s best-case
 261     overhead is not quite as good as that of `defer_rcu()`.
 262   - Provides `call_rcu()` to allow asynchronous handling of RCU
 263     grace periods.  A number of additional functions are provided
 264     to manage the helper threads used by `call_rcu()`, but reasonable
 265     defaults are used if these additional functions are not invoked.
 266     See [`doc/rcu-api.md`](doc/rcu-api.md) in userspace-rcu documentation
 267     for more details.
 268
 269
 270 ### Being careful with signals
 271
 272 The `liburcu` library uses signals internally. The signal handler is
 273 registered with the `SA_RESTART` flag. However, these signals may cause
 274 some non-restartable system calls to fail with `errno = EINTR`. Care
 275 should be taken to restart system calls manually if they fail with this
 276 error. A list of non-restartable system calls may be found in
 277 `signal(7)`. The `liburcu-mb` and `liburcu-qsbr` versions of the Userspace RCU
 278 library do not require any signal.
 279
 280 Read-side critical sections are allowed in a signal handler,
 281 except those setup with `sigaltstack(2)`, with `liburcu` and
 282 `liburcu-mb`. Be careful, however, to disable these signals
 283 between thread creation and calls to `rcu_register_thread()`, because a
 284 signal handler nesting on an unregistered thread would not be
 285 allowed to call `rcu_read_lock()`.
 286
 287 Read-side critical sections are _not_ allowed in a signal handler with
 288 `liburcu-qsbr`, unless signals are disabled explicitly around each
 289 `rcu_quiescent_state()` calls, when threads are put offline and around
 290 calls to `synchronize_rcu()`. Even then, we do not recommend it.
 291
 292
 293 ### Interaction with mutexes
 294
 295 One must be careful to do not cause deadlocks due to interaction of
 296 `synchronize_rcu()` and RCU read-side with mutexes. If `synchronize_rcu()`
 297 is called with a mutex held, this mutex (or any mutex which has this
 298 mutex in its dependency chain) should not be acquired from within a RCU
 299 read-side critical section.
 300
 301 This is especially important to understand in the context of the
 302 QSBR flavor: a registered reader thread being "online" by
 303 default should be considered as within a RCU read-side critical
 304 section unless explicitly put "offline". Therefore, if
 305 `synchronize_rcu()` is called with a mutex held, this mutex, as
 306 well as any mutex which has this mutex in its dependency chain
 307 should only be taken when the RCU reader thread is "offline"
 308 (this can be performed by calling `rcu_thread_offline()`).
 309
 310
 311 ### Interaction with `fork()`
 312
 313 Special care must be taken for applications performing `fork()` without
 314 any following `exec()`. This is caused by the fact that Linux only clones
 315 the thread calling `fork()`, and thus never replicates any of the other
 316 parent thread into the child process. Most `liburcu` implementations
 317 require that all registrations (as reader, `defer_rcu` and `call_rcu`
 318 threads) should be released before a `fork()` is performed, except for the
 319 rather common scenario where `fork()` is immediately followed by `exec()` in
 320 the child process. The only implementation not subject to that rule is
 321 `liburcu-bp`, which is designed to handle `fork()` by calling
 322 `rcu_bp_before_fork`, `rcu_bp_after_fork_parent` and
 323 `rcu_bp_after_fork_child`.
 324
 325 Applications that use `call_rcu()` and that `fork()` without
 326 doing an immediate `exec()` must take special action.  The parent
 327 must invoke `call_rcu_before_fork()` before the `fork()` and
 328 `call_rcu_after_fork_parent()` after the `fork()`. The child
 329 process must invoke `call_rcu_after_fork_child()`.
 330 Even though these three APIs are suitable for passing to
 331 `pthread_atfork()`, use of `pthread_atfork()` is **STRONGLY
 332 DISCOURAGED** for programs calling the glibc memory allocator
 333 (`malloc()`, `calloc()`, `free()`, ...) within `call_rcu` callbacks.
 334 This is due to limitations in the way glibc memory allocator
 335 handles calls to the memory allocator from concurrent threads
 336 while the `pthread_atfork()` handlers are executing.
 337
 338 Combining e.g.:
 339
 340   - call to `free()` from callbacks executed within `call_rcu` worker
 341     threads,
 342   - executing `call_rcu` atfork handlers within the glibc pthread
 343     atfork mechanism,
 344
 345 will sometimes trigger interesting process hangs. This usually
 346 hangs on a memory allocator lock within glibc.
 347
 348
 349 ### Thread Local Storage (TLS)
 350
 351 Userspace RCU can fall back on `pthread_getspecific()` to emulate
 352 TLS variables on systems where it is not available. This behavior
 353 can be forced by specifying `--disable-compiler-tls` as configure
 354 argument.
 355
 356
 357 ### Usage of `DEBUG_RCU`
 358
 359 `DEBUG_RCU` is used to add internal debugging self-checks to the
 360 RCU library. This define adds a performance penalty when enabled.
 361 Can be enabled by uncommenting the corresponding line in
 362 `Makefile.build.inc`.
 363
 364
 365 ### Usage of `DEBUG_YIELD`
 366
 367 `DEBUG_YIELD` is used to add random delays in the code for testing
 368 purposes.
 369
 370
 371 ### SMP support
 372
 373 By default the library is configured to use synchronization primitives
 374 adequate for SMP systems. On uniprocessor systems, support for SMP
 375 systems can be disabled with:
 376
 377     ./configure --disable-smp-support
 378
 379 theoretically yielding slightly better performance.
 380
 381
 382 Make targets
 383 ------------
 384
 385 In addition to the usual `make check` target, Userspace RCU features
 386 `make regtest` and `make bench` targets:
 387
 388   - `make check`: short tests, meant to be run when rebuilding or
 389     porting Userspace RCU.
 390   - `make regtest`: long (many hours) test, meant to be run when
 391     modifying Userspace RCU or porting it to a new architecture or
 392     operating system.
 393   - `make bench`: long (many hours) benchmarks.
 394
 395
 396 Contacts
 397 --------
 398
 399 You can contact the maintainers on the following mailing list:
 400 `lttng-dev@lists.lttng.org`.