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   - x86 (i386, i486, i586, i686)
  41   - amd64 / x86\_64
  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   - m68k
  53   - RISC-V
  54   - LoongArch
  55
  56 Tested on:
  57
  58   - Linux all architectures
  59   - FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64
  60   - Solaris 10/11 i386
  61   - Cygwin i386/amd64
  62   - MacOS amd64/arm64
  63
  64 Should also work on:
  65
  66   - Android
  67   - NetBSD 5
  68   - OpenBSD
  69
  70 (more testing needed before claiming support for these OS).
  71
  72 Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
  73 better.
  74
  75 The C compiler used needs to support at least C99. The C++ compiler used
  76 needs to support at least C++11.
  77
  78 The GCC compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
  79 supported, with the following exceptions:
  80
  81   - GCC 3.3 and 3.4 have a bug that prevents them from generating volatile
  82     accesses to offsets in a TLS structure on 32-bit x86. These versions are
  83     therefore not compatible with `liburcu` on x86 32-bit
  84     (i386, i486, i586, i686).
  85     The problem has been reported to the GCC community:
  86     <http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html>
  87   - GCC 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
  88     See <http://kerneltrap.org/node/7507>
  89   - Alpha, ia64 and ARM architectures depend on GCC 4.x with atomic builtins
  90     support. For ARM this was introduced with GCC 4.4:
  91     <http://gcc.gnu.org/gcc-4.4/changes.html>.
  92   - Linux aarch64 depends on GCC 5.1 or better because prior versions
  93     perform unsafe access to deallocated stack.
  94
  95 Clang version 3.0 (based on LLVM 3.0) is supported.
  96
  97 Glibc >= 2.4 should work but the older version we test against is
  98 currently 2.17.
  99
 100 For developers using the Git tree:
 101
 102 This source tree is based on the autotools suite from GNU to simplify
 103 portability. Here are some things you should have on your system in order to
 104 compile the git repository tree :
 105
 106   - GNU autotools (automake >=1.12, autoconf >=2.69)
 107     (make sure your system wide `automake` points to a recent version!)
 108   - GNU Libtool >=2.2
 109     (for more information, go to <http://www.gnu.org/software/autoconf/>)
 110
 111 If you get the tree from the repository, you will need to use the `bootstrap`
 112 script in the root of the tree. It calls all the GNU tools needed to prepare
 113 the tree configuration.
 114
 115 Test scripts provided in the `tests/` directory of the source tree depend
 116 on `bash` and the `seq` program.
 117
 118
 119 API
 120 ---
 121
 122 See the relevant API documentation files in `doc/`. The APIs provided by
 123 Userspace RCU are, by prefix:
 124
 125   - `rcu_`: Read-Copy Update (see [`doc/rcu-api.md`](doc/rcu-api.md))
 126   - `cmm_`: Concurrent Memory Model
 127   - `caa_`: Concurrent Architecture Abstraction
 128   - `cds_`: Concurrent Data Structures
 129     (see [`doc/cds-api.md`](doc/cds-api.md))
 130   - `uatomic_`: Userspace Atomic
 131     (see [`doc/uatomic-api.md`](doc/uatomic-api.md))
 132
 133
 134 Quick start guide
 135 -----------------
 136
 137 ### Usage of all urcu libraries:
 138
 139   - Define `_LGPL_SOURCE` (only) if your code is LGPL or GPL compatible
 140     before including the `urcu.h` or `urcu-qsbr.h` header. If your application
 141     is distributed under another license, function calls will be generated
 142     instead of inlines, so your application can link with the library.
 143   - Linking with one of the libraries below is always necessary even for
 144     LGPL and GPL applications.
 145   - Define `URCU_INLINE_SMALL_FUNCTIONS` before including Userspace RCU
 146     headers if you want Userspace RCU to inline small functions (10
 147     lines or less) into the application. It can be used by applications
 148     distributed under any kind of license, and does *not* make the
 149     application a derived work of Userspace RCU.
 150
 151 Those small inlined functions are guaranteed to match the library
 152 content as long as the library major version is unchanged.
 153 Therefore, the application *must* be compiled with headers matching
 154 the library major version number. Applications using
 155 `URCU_INLINE_SMALL_FUNCTIONS` may be unable to use debugging
 156 features of Userspace RCU without being recompiled.
 157
 158 There are multiple flavors of liburcu available:
 159
 160   - `memb`,
 161   - `qsbr`,
 162   - `mb`,
 163   - `signal`,
 164   - `bp`.
 165
 166 The API members start with the prefix `urcu_<flavor>_`, where
 167 `<flavor>` is the chosen flavor name.
 168
 169
 170 ### Usage of `liburcu-memb`
 171
 172   1. `#include <urcu/urcu-memb.h>`
 173   2. Link the application with `-lurcu-memb`
 174
 175 This is the preferred version of the library, in terms of
 176 grace-period detection speed, read-side speed and flexibility.
 177 Dynamically detects kernel support for `sys_membarrier()`. Falls back
 178 on `urcu-mb` scheme if support is not present, which has slower
 179 read-side. Use the `--disable-sys-membarrier-fallback` configure option
 180 to disable the fall back, thus requiring `sys_membarrier()` to be
 181 available. This gives a small speedup when `sys_membarrier()` is
 182 supported by the kernel, and aborts in the library constructor if not
 183 supported.
 184
 185
 186 ### Usage of `liburcu-qsbr`
 187
 188   1. `#include <urcu/urcu-qsbr.h>`
 189   2. Link with `-lurcu-qsbr`
 190
 191 The QSBR flavor of RCU needs to have each reader thread executing
 192 `rcu_quiescent_state()` periodically to progress. `rcu_thread_online()`
 193 and `rcu_thread_offline()` can be used to mark long periods for which
 194 the threads are not active. It provides the fastest read-side at the
 195 expense of more intrusiveness in the application code.
 196
 197
 198 ### Usage of `liburcu-mb`
 199
 200   1. `#include <urcu/urcu-mb.h>`
 201   2. Link with `-lurcu-mb`
 202
 203 This version of the urcu library uses memory barriers on the writer
 204 and reader sides. This results in faster grace-period detection, but
 205 results in slower reads.
 206
 207
 208 ### Usage of `liburcu-signal`
 209
 210   1. `#include <urcu/urcu-signal.h>`
 211   2. Link the application with `-lurcu-signal`
 212
 213 Version of the library that requires a signal, typically `SIGUSR1`. Can
 214 be overridden with `-DSIGRCU` by modifying `Makefile.build.inc`.
 215
 216
 217 ### Usage of `liburcu-bp`
 218
 219   1. `#include <urcu/urcu-bp.h>`
 220   2. Link with `-lurcu-bp`
 221
 222 The BP library flavor stands for "bulletproof". It is specifically
 223 designed to help tracing library to hook on applications without
 224 requiring to modify these applications. `urcu_bp_init()`, and
 225 `urcu_bp_unregister_thread()` all become nops, whereas calling
 226 `urcu_bp_register_thread()` becomes optional. The state is dealt with by
 227 the library internally at the expense of read-side and write-side
 228 performance.
 229
 230
 231 ### Initialization
 232
 233 Each thread that has reader critical sections (that uses
 234 `urcu_<flavor>_read_lock()`/`urcu_<flavor>_read_unlock()` must first
 235 register to the URCU library. This is done by calling
 236 `urcu_<flavor>_register_thread()`. Unregistration must be performed
 237 before exiting the thread by using `urcu_<flavor>_unregister_thread()`.
 238
 239
 240 ### Reading
 241
 242 Reader critical sections must be protected by locating them between
 243 calls to `urcu_<flavor>_read_lock()` and `urcu_<flavor>_read_unlock()`.
 244 Inside that lock, `rcu_dereference()` may be called to read an RCU
 245 protected pointer.
 246
 247
 248 ### Writing
 249
 250 `rcu_assign_pointer()` and `rcu_xchg_pointer()` may be called anywhere.
 251 After, `urcu_<flavor>_synchronize_rcu()` must be called. When it
 252 returns, the old values are not in usage anymore.
 253
 254 As an alternative to `urcu_<flavor>_synchronize_rcu()`,
 255 it is also possible to use the urcu polling mechanism to wait for a
 256 grace period to elapse. This can be done by using
 257 `urcu_<flavor>_start_poll_synchronize_rcu()`
 258 to start the grace period polling, and then invoke
 259 `urcu_<flavor>_poll_state_synchronize_rcu()`, which returns true if
 260 the grace period has completed, false otherwise.
 261
 262
 263 ### Usage of `liburcu-defer`
 264
 265   - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`,
 266     `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
 267     The `liburcu-defer` functionality is pulled into each of
 268     those library modules.
 269   - Provides `urcu_<flavor>_defer_rcu()` primitive to enqueue delayed
 270     callbacks. Queued callbacks are executed in batch periodically after
 271     a grace period.  Do _not_ use `urcu_<flavor>_defer_rcu()` within a
 272     read-side critical section, because it may call
 273     `urcu_<flavor>_synchronize_rcu()` if the thread queue is full.  This
 274     can lead to deadlock or worse.
 275   - Requires that `urcu_<flavor>_defer_barrier()` must be called in
 276     library destructor if a library queues callbacks and is expected to
 277     be unloaded with `dlclose()`.
 278
 279 Its API is currently experimental. It may change in future library releases.
 280
 281
 282 ### Usage of `urcu-call-rcu`
 283
 284   - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`,
 285     `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
 286     The `urcu-call-rcu` functionality is pulled into each of
 287     those library modules.
 288   - Provides the `urcu_<flavor>_call_rcu()` primitive to enqueue delayed
 289     callbacks in a manner similar to `urcu_<flavor>_defer_rcu()`, but
 290     without ever delaying for a grace period.  On the other hand,
 291     `urcu_<flavor>_call_rcu()`'s best-case overhead is not quite as good
 292     as that of `urcu_<flavor>_defer_rcu()`.
 293   - Provides `urcu_<flavor>_call_rcu()` to allow asynchronous handling
 294     of RCU grace periods.  A number of additional functions are provided
 295     to manage the helper threads used by `urcu_<flavor>_call_rcu()`, but
 296     reasonable defaults are used if these additional functions are not
 297     invoked. See [`doc/rcu-api.md`](doc/rcu-api.md) in userspace-rcu
 298     documentation for more details.
 299
 300
 301 ### Being careful with signals
 302
 303 The `liburcu-signal` library uses signals internally. The signal handler is
 304 registered with the `SA_RESTART` flag. However, these signals may cause
 305 some non-restartable system calls to fail with `errno = EINTR`. Care
 306 should be taken to restart system calls manually if they fail with this
 307 error. A list of non-restartable system calls may be found in
 308 `signal(7)`.
 309
 310 Read-side critical sections are allowed in a signal handler,
 311 except those setup with `sigaltstack(2)`, with `liburcu-memb` and
 312 `liburcu-mb`. Be careful, however, to disable these signals
 313 between thread creation and calls to `urcu_<flavor>_register_thread()`,
 314 because a signal handler nesting on an unregistered thread would not be
 315 allowed to call `urcu_<flavor>_read_lock()`.
 316
 317 Read-side critical sections are _not_ allowed in a signal handler with
 318 `liburcu-qsbr`, unless signals are disabled explicitly around each
 319 `urcu_qsbr_quiescent_state()` calls, when threads are put offline and around
 320 calls to `urcu_qsbr_synchronize_rcu()`. Even then, we do not recommend it.
 321
 322
 323 ### Interaction with mutexes
 324
 325 One must be careful to do not cause deadlocks due to interaction of
 326 `urcu_<flavor>_synchronize_rcu()` and RCU read-side with mutexes. If
 327 `urcu_<flavor>_synchronize_rcu()` is called with a mutex held, this
 328 mutex (or any mutex which has this mutex in its dependency chain) should
 329 not be acquired from within a RCU read-side critical section.
 330
 331 This is especially important to understand in the context of the
 332 QSBR flavor: a registered reader thread being "online" by
 333 default should be considered as within a RCU read-side critical
 334 section unless explicitly put "offline". Therefore, if
 335 `urcu_qsbr_synchronize_rcu()` is called with a mutex held, this mutex,
 336 as well as any mutex which has this mutex in its dependency chain should
 337 only be taken when the RCU reader thread is "offline" (this can be
 338 performed by calling `urcu_qsbr_thread_offline()`).
 339
 340
 341 ### Interaction with `fork()`
 342
 343 Special care must be taken for applications performing `fork()` without
 344 any following `exec()`. This is caused by the fact that Linux only clones
 345 the thread calling `fork()`, and thus never replicates any of the other
 346 parent thread into the child process. Most `liburcu` implementations
 347 require that all registrations (as reader, `defer_rcu` and `call_rcu`
 348 threads) should be released before a `fork()` is performed, except for the
 349 rather common scenario where `fork()` is immediately followed by `exec()` in
 350 the child process. The only implementation not subject to that rule is
 351 `liburcu-bp`, which is designed to handle `fork()` by calling
 352 `urcu_bp_before_fork`, `urcu_bp_after_fork_parent` and
 353 `urcu_bp_after_fork_child`.
 354
 355 Applications that use `urcu_<flavor>_call_rcu()` and that `fork()`
 356 without doing an immediate `exec()` must take special action.  The
 357 parent must invoke `urcu_<flavor>_call_rcu_before_fork()` before the
 358 `fork()` and `urcu_<flavor>_call_rcu_after_fork_parent()` after the
 359 `fork()`. The child process must invoke
 360 `urcu_<flavor>_call_rcu_after_fork_child()`.  Even though these three
 361 APIs are suitable for passing to `pthread_atfork()`, use of
 362 `pthread_atfork()` is **STRONGLY DISCOURAGED** for programs calling the
 363 glibc memory allocator (`malloc()`, `calloc()`, `free()`, ...) within
 364 `urcu_<flavor>_call_rcu` callbacks.  This is due to limitations in the
 365 way glibc memory allocator handles calls to the memory allocator from
 366 concurrent threads while the `pthread_atfork()` handlers are executing.
 367
 368 Combining e.g.:
 369
 370   - call to `free()` from callbacks executed within
 371     `urcu_<flavor>_call_rcu` worker threads,
 372   - executing `urcu_<flavor>_call_rcu` atfork handlers within the glibc
 373     pthread atfork mechanism,
 374
 375 will sometimes trigger interesting process hangs. This usually
 376 hangs on a memory allocator lock within glibc.
 377
 378
 379 ### Thread Local Storage (TLS)
 380
 381 Userspace RCU can fall back on `pthread_getspecific()` to emulate
 382 TLS variables on systems where it is not available. This behavior
 383 can be forced by specifying `--disable-compiler-tls` as configure
 384 argument.
 385
 386
 387 ### Usage of `DEBUG_RCU` & `--enable-rcu-debug`
 388
 389 By default the library is configured with internal debugging
 390 self-checks disabled.
 391
 392 For always-on debugging self-checks:
 393
 394     ./configure --enable-rcu-debug
 395
 396 For fine grained enabling of debugging self-checks, build
 397 userspace-rcu with `DEBUG_RCU` defined and compile dependent
 398 applications with `DEBUG_RCU` defined when necessary.
 399
 400 Warning: Enabling this feature result in a performance penalty.
 401
 402
 403 ### Usage of `DEBUG_YIELD`
 404
 405 `DEBUG_YIELD` is used to add random delays in the code for testing
 406 purposes.
 407
 408
 409 ### SMP support
 410
 411 By default the library is configured to use synchronization primitives
 412 adequate for SMP systems. On uniprocessor systems, support for SMP
 413 systems can be disabled with:
 414
 415     ./configure --disable-smp-support
 416
 417 theoretically yielding slightly better performance.
 418
 419
 420 ### Usage of `--enable-cds-lfht-iter-debug`
 421
 422 By default the library is configured with extra debugging checks for
 423 lock-free hash table iterator traversal disabled.
 424
 425 Building liburcu with `--enable-cds-lfht-iter-debug` and rebuilding
 426 application to match the ABI change allows finding cases where the hash
 427 table iterator is re-purposed to be used on a different hash table while
 428 still being used to iterate on a hash table.
 429
 430 This option alters the rculfhash ABI. Make sure to compile both library
 431 and application with matching configuration.
 432
 433
 434 Make targets
 435 ------------
 436
 437 In addition to the usual `make check` target, Userspace RCU features
 438 `make regtest`, `make short_bench` and `make long_bench` targets:
 439
 440   - `make check`: short tests, meant to be run when rebuilding or
 441     porting Userspace RCU.
 442   - `make regtest`: long (many hours) test, meant to be run when
 443     modifying Userspace RCU or porting it to a new architecture or
 444     operating system.
 445   - `make short_bench`: short benchmarks, 3 seconds per test.
 446   - `make long_bench`: long (many hours) benchmarks, 30 seconds per test.
 447
 448
 449 Known issues
 450 ------------
 451
 452 There is an application vs library compatibility issue between
 453 applications built using Userspace RCU 0.10 headers linked against
 454 Userspace RCU 0.11 or 0.12 shared objects. The problem occurs as
 455 follows:
 456
 457   - An application executable is built with `_LGPL_SOURCE` defined, includes
 458     any of the Userspace RCU 0.10 urcu flavor headers, and is built
 459     without the `-fpic` compiler option.
 460
 461   - The Userspace RCU 0.10 library shared objects are updated to 0.11
 462     or 0.12 without rebuilding the application.
 463
 464   - The application will hang, typically when RCU grace period
 465     (synchronize_rcu) is invoked.
 466
 467 Some possible work-arounds for this are:
 468
 469   - Rebuild the application against Userspace RCU 0.11+.
 470
 471   - Rebuild the application with `-fpic`.
 472
 473   - Upgrade Userspace RCU to 0.13+ without installing 0.11 nor 0.12.
 474
 475
 476 Contacts
 477 --------
 478
 479 You can contact the maintainers on the following mailing list:
 480 `lttng-dev@lists.lttng.org`.