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
  55 Tested on:
  56
  57   - Linux all architectures
  58   - FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64
  59   - Solaris 10/11 i386
  60   - Cygwin i386/amd64
  61   - MacOSX amd64
  62
  63 Should also work on:
  64
  65   - Android
  66   - NetBSD 5
  67   - OpenBSD
  68   - Darwin
  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 GCC compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
  76 supported, with the following exceptions:
  77
  78   - GCC 3.3 and 3.4 have a bug that prevents them from generating volatile
  79     accesses to offsets in a TLS structure on 32-bit x86. These versions are
  80     therefore not compatible with `liburcu` on x86 32-bit
  81     (i386, i486, i586, i686).
  82     The problem has been reported to the GCC community:
  83     http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
  84   - GCC 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
  85     See http://kerneltrap.org/node/7507
  86   - Alpha, ia64 and ARM architectures depend on GCC 4.x with atomic builtins
  87     support. For ARM this was introduced with GCC 4.4:
  88     http://gcc.gnu.org/gcc-4.4/changes.html.
  89   - Linux aarch64 depends on GCC 5.1 or better because prior versions
  90     perform unsafe access to deallocated stack.
  91
  92 Clang version 3.0 (based on LLVM 3.0) is supported.
  93
  94 Building on MacOS X (Darwin) requires a work-around for processor
  95 detection:
  96
  97   - 32-bit:
  98
  99         ./configure --build=i686-apple-darwin11
 100
 101   - 64-bit:
 102
 103         ./configure --build=x86_64-apple-darwin11
 104
 105 For developers using the Git tree:
 106
 107 This source tree is based on the autotools suite from GNU to simplify
 108 portability. Here are some things you should have on your system in order to
 109 compile the git repository tree :
 110
 111   - GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
 112     (make sure your system wide `automake` points to a recent version!)
 113   - GNU Libtool >=2.2
 114     (for more information, go to http://www.gnu.org/software/autoconf/)
 115
 116 If you get the tree from the repository, you will need to use the `bootstrap`
 117 script in the root of the tree. It calls all the GNU tools needed to prepare
 118 the tree configuration.
 119
 120 Test scripts provided in the `tests/` directory of the source tree depend
 121 on `bash` and the `seq` program.
 122
 123
 124 API
 125 ---
 126
 127 See the relevant API documentation files in `doc/`. The APIs provided by
 128 Userspace RCU are, by prefix:
 129
 130   - `rcu_`: Read-Copy Update (see [`doc/rcu-api.md`](doc/rcu-api.md))
 131   - `cmm_`: Concurrent Memory Model
 132   - `caa_`: Concurrent Architecture Abstraction
 133   - `cds_`: Concurrent Data Structures
 134     (see [`doc/cds-api.md`](doc/cds-api.md))
 135   - `uatomic_`: Userspace Atomic
 136     (see [`doc/uatomic-api.md`](doc/uatomic-api.md))
 137
 138
 139 Quick start guide
 140 -----------------
 141
 142 ### Usage of all urcu libraries:
 143
 144   - Define `_LGPL_SOURCE` (only) if your code is LGPL or GPL compatible
 145     before including the `urcu.h` or `urcu-qsbr.h` header. If your application
 146     is distributed under another license, function calls will be generated
 147     instead of inlines, so your application can link with the library.
 148   - Linking with one of the libraries below is always necessary even for
 149     LGPL and GPL applications.
 150   - Define `URCU_INLINE_SMALL_FUNCTIONS` before including Userspace RCU
 151     headers if you want Userspace RCU to inline small functions (10
 152     lines or less) into the application. It can be used by applications
 153     distributed under any kind of license, and does *not* make the
 154     application a derived work of Userspace RCU.
 155
 156 Those small inlined functions are guaranteed to match the library
 157 content as long as the library major version is unchanged.
 158 Therefore, the application *must* be compiled with headers matching
 159 the library major version number. Applications using
 160 `URCU_INLINE_SMALL_FUNCTIONS` may be unable to use debugging
 161 features of Userspace RCU without being recompiled.
 162
 163 There are multiple flavors of liburcu available:
 164
 165   - `memb`,
 166   - `qsbr`,
 167   - `mb`,
 168   - `signal`,
 169   - `bp`.
 170
 171 The API members start with the prefix "urcu_<flavor>_", where
 172 <flavor> is the chosen flavor name.
 173
 174
 175 ### Usage of `liburcu-memb`
 176
 177   1. `#include <urcu/urcu-memb.h>`
 178   2. Link the application with `-lurcu-memb`
 179
 180 This is the preferred version of the library, in terms of
 181 grace-period detection speed, read-side speed and flexibility.
 182 Dynamically detects kernel support for `sys_membarrier()`. Falls back
 183 on `urcu-mb` scheme if support is not present, which has slower
 184 read-side. Use the --disable-sys-membarrier-fallback configure option
 185 to disable the fall back, thus requiring `sys_membarrier()` to be
 186 available. This gives a small speedup when `sys_membarrier()` is
 187 supported by the kernel, and aborts in the library constructor if not
 188 supported.
 189
 190
 191 ### Usage of `liburcu-qsbr`
 192
 193   1. `#include <urcu/urcu-qsbr.h>`
 194   2. Link with `-lurcu-qsbr`
 195
 196 The QSBR flavor of RCU needs to have each reader thread executing
 197 `rcu_quiescent_state()` periodically to progress. `rcu_thread_online()`
 198 and `rcu_thread_offline()` can be used to mark long periods for which
 199 the threads are not active. It provides the fastest read-side at the
 200 expense of more intrusiveness in the application code.
 201
 202
 203 ### Usage of `liburcu-mb`
 204
 205   1. `#include <urcu/urcu-mb.h>`
 206   2. Link with `-lurcu-mb`
 207
 208 This version of the urcu library uses memory barriers on the writer
 209 and reader sides. This results in faster grace-period detection, but
 210 results in slower reads.
 211
 212
 213 ### Usage of `liburcu-signal`
 214
 215   1. `#include <urcu/urcu-signal.h>`
 216   2. Link the application with `-lurcu-signal`
 217
 218 Version of the library that requires a signal, typically `SIGUSR1`. Can
 219 be overridden with `-DSIGRCU` by modifying `Makefile.build.inc`.
 220
 221
 222 ### Usage of `liburcu-bp`
 223
 224   1. `#include <urcu/urcu-bp.h>`
 225   2. Link with `-lurcu-bp`
 226
 227 The BP library flavor stands for "bulletproof". It is specifically
 228 designed to help tracing library to hook on applications without
 229 requiring to modify these applications. `urcu_bp_init()`,
 230 `urcu_bp_register_thread()` and `urcu_bp_unregister_thread()` all become
 231 nops. The state is dealt with by the library internally at the expense
 232 of read-side and write-side performance.
 233
 234
 235 ### Initialization
 236
 237 Each thread that has reader critical sections (that uses
 238 `urcu_<flavor>_read_lock()`/`urcu_<flavor>_read_unlock()` must first
 239 register to the URCU library. This is done by calling
 240 `urcu_<flavor>_register_thread()`. Unregistration must be performed
 241 before exiting the thread by using `urcu_<flavor>_unregister_thread()`.
 242
 243
 244 ### Reading
 245
 246 Reader critical sections must be protected by locating them between
 247 calls to `urcu_<flavor>_read_lock()` and `urcu_<flavor>_read_unlock()`.
 248 Inside that lock, `rcu_dereference()` may be called to read an RCU
 249 protected pointer.
 250
 251
 252 ### Writing
 253
 254 `rcu_assign_pointer()` and `rcu_xchg_pointer()` may be called anywhere.
 255 After, `urcu_<flavor>_synchronize_rcu()` must be called. When it
 256 returns, the old values are not in usage anymore.
 257
 258
 259 ### Usage of `liburcu-defer`
 260
 261   - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`,
 262     `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
 263     The `liburcu-defer` functionality is pulled into each of
 264     those library modules.
 265   - Provides `urcu_<flavor>_defer_rcu()` primitive to enqueue delayed
 266     callbacks. Queued callbacks are executed in batch periodically after
 267     a grace period.  Do _not_ use `urcu_<flavor>_defer_rcu()` within a
 268     read-side critical section, because it may call
 269     `urcu_<flavor>_synchronize_rcu()` if the thread queue is full.  This
 270     can lead to deadlock or worse.
 271   - Requires that `urcu_<flavor>_defer_barrier()` must be called in
 272     library destructor if a library queues callbacks and is expected to
 273     be unloaded with `dlclose()`.
 274
 275 Its API is currently experimental. It may change in future library releases.
 276
 277
 278 ### Usage of `urcu-call-rcu`
 279
 280   - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`,
 281     `liburcu-mb`, `liburcu-signal`, or `liburcu-bp` above.
 282     The `urcu-call-rcu` functionality is pulled into each of
 283     those library modules.
 284   - Provides the `urcu_<flavor>_call_rcu()` primitive to enqueue delayed
 285     callbacks in a manner similar to `urcu_<flavor>_defer_rcu()`, but
 286     without ever delaying for a grace period.  On the other hand,
 287     `urcu_<flavor>_call_rcu()`'s best-case overhead is not quite as good
 288     as that of `urcu_<flavor>_defer_rcu()`.
 289   - Provides `urcu_<flavor>_call_rcu()` to allow asynchronous handling
 290     of RCU grace periods.  A number of additional functions are provided
 291     to manage the helper threads used by `urcu_<flavor>_call_rcu()`, but
 292     reasonable defaults are used if these additional functions are not
 293     invoked. See [`doc/rcu-api.md`](doc/rcu-api.md) in userspace-rcu
 294     documentation for more details.
 295
 296
 297 ### Being careful with signals
 298
 299 The `liburcu-signal` library uses signals internally. The signal handler is
 300 registered with the `SA_RESTART` flag. However, these signals may cause
 301 some non-restartable system calls to fail with `errno = EINTR`. Care
 302 should be taken to restart system calls manually if they fail with this
 303 error. A list of non-restartable system calls may be found in
 304 `signal(7)`.
 305
 306 Read-side critical sections are allowed in a signal handler,
 307 except those setup with `sigaltstack(2)`, with `liburcu-memb` and
 308 `liburcu-mb`. Be careful, however, to disable these signals
 309 between thread creation and calls to `urcu_<flavor>_register_thread()`,
 310 because a signal handler nesting on an unregistered thread would not be
 311 allowed to call `urcu_<flavor>_read_lock()`.
 312
 313 Read-side critical sections are _not_ allowed in a signal handler with
 314 `liburcu-qsbr`, unless signals are disabled explicitly around each
 315 `urcu_qsbr_quiescent_state()` calls, when threads are put offline and around
 316 calls to `urcu_qsbr_synchronize_rcu()`. Even then, we do not recommend it.
 317
 318
 319 ### Interaction with mutexes
 320
 321 One must be careful to do not cause deadlocks due to interaction of
 322 `urcu_<flavor>_synchronize_rcu()` and RCU read-side with mutexes. If
 323 `urcu_<flavor>_synchronize_rcu()` is called with a mutex held, this
 324 mutex (or any mutex which has this mutex in its dependency chain) should
 325 not be acquired from within a RCU read-side critical section.
 326
 327 This is especially important to understand in the context of the
 328 QSBR flavor: a registered reader thread being "online" by
 329 default should be considered as within a RCU read-side critical
 330 section unless explicitly put "offline". Therefore, if
 331 `urcu_qsbr_synchronize_rcu()` is called with a mutex held, this mutex,
 332 as well as any mutex which has this mutex in its dependency chain should
 333 only be taken when the RCU reader thread is "offline" (this can be
 334 performed by calling `urcu_qsbr_thread_offline()`).
 335
 336
 337 ### Interaction with `fork()`
 338
 339 Special care must be taken for applications performing `fork()` without
 340 any following `exec()`. This is caused by the fact that Linux only clones
 341 the thread calling `fork()`, and thus never replicates any of the other
 342 parent thread into the child process. Most `liburcu` implementations
 343 require that all registrations (as reader, `defer_rcu` and `call_rcu`
 344 threads) should be released before a `fork()` is performed, except for the
 345 rather common scenario where `fork()` is immediately followed by `exec()` in
 346 the child process. The only implementation not subject to that rule is
 347 `liburcu-bp`, which is designed to handle `fork()` by calling
 348 `urcu_bp_before_fork`, `urcu_bp_after_fork_parent` and
 349 `urcu_bp_after_fork_child`.
 350
 351 Applications that use `urcu_<flavor>_call_rcu()` and that `fork()`
 352 without doing an immediate `exec()` must take special action.  The
 353 parent must invoke `urcu_<flavor>_call_rcu_before_fork()` before the
 354 `fork()` and `urcu_<flavor>_call_rcu_after_fork_parent()` after the
 355 `fork()`. The child process must invoke
 356 `urcu_<flavor>_call_rcu_after_fork_child()`.  Even though these three
 357 APIs are suitable for passing to `pthread_atfork()`, use of
 358 `pthread_atfork()` is **STRONGLY DISCOURAGED** for programs calling the
 359 glibc memory allocator (`malloc()`, `calloc()`, `free()`, ...) within
 360 `urcu_<flavor>_call_rcu` callbacks.  This is due to limitations in the
 361 way glibc memory allocator handles calls to the memory allocator from
 362 concurrent threads while the `pthread_atfork()` handlers are executing.
 363
 364 Combining e.g.:
 365
 366   - call to `free()` from callbacks executed within
 367     `urcu_<flavor>_call_rcu` worker threads,
 368   - executing `urcu_<flavor>_call_rcu` atfork handlers within the glibc
 369     pthread atfork mechanism,
 370
 371 will sometimes trigger interesting process hangs. This usually
 372 hangs on a memory allocator lock within glibc.
 373
 374
 375 ### Thread Local Storage (TLS)
 376
 377 Userspace RCU can fall back on `pthread_getspecific()` to emulate
 378 TLS variables on systems where it is not available. This behavior
 379 can be forced by specifying `--disable-compiler-tls` as configure
 380 argument.
 381
 382
 383 ### Usage of `DEBUG_RCU` & `--enable-rcu-debug`
 384
 385 By default the library is configured with internal debugging
 386 self-checks disabled.
 387
 388 For always-on debugging self-checks:
 389         ./configure --enable-rcu-debug
 390
 391 For fine grained enabling of debugging self-checks, build
 392 userspace-rcu with DEBUG_RCU defined and compile dependent
 393 applications with DEBUG_RCU defined when necessary.
 394
 395 Warning: Enabling this feature result in a performance penalty.
 396
 397
 398 ### Usage of `DEBUG_YIELD`
 399
 400 `DEBUG_YIELD` is used to add random delays in the code for testing
 401 purposes.
 402
 403
 404 ### SMP support
 405
 406 By default the library is configured to use synchronization primitives
 407 adequate for SMP systems. On uniprocessor systems, support for SMP
 408 systems can be disabled with:
 409
 410     ./configure --disable-smp-support
 411
 412 theoretically yielding slightly better performance.
 413
 414
 415 ### Usage of `--enable-cds-lfht-iter-debug`
 416
 417 By default the library is configured with extra debugging checks for
 418 lock-free hash table iterator traversal disabled.
 419
 420 Building liburcu with --enable-cds-lfht-iter-debug and rebuilding
 421 application to match the ABI change allows finding cases where the hash
 422 table iterator is re-purposed to be used on a different hash table while
 423 still being used to iterate on a hash table.
 424
 425 This option alters the rculfhash ABI. Make sure to compile both library
 426 and application with matching configuration.
 427
 428
 429 Make targets
 430 ------------
 431
 432 In addition to the usual `make check` target, Userspace RCU features
 433 `make regtest` and `make bench` targets:
 434
 435   - `make check`: short tests, meant to be run when rebuilding or
 436     porting Userspace RCU.
 437   - `make regtest`: long (many hours) test, meant to be run when
 438     modifying Userspace RCU or porting it to a new architecture or
 439     operating system.
 440   - `make bench`: long (many hours) benchmarks.
 441
 442
 443 Known issues
 444 ------------
 445
 446 There is an application vs library compatibility issue between
 447 applications built using Userspace RCU 0.10 headers linked against
 448 Userspace RCU 0.11 or 0.12 shared objects. The problem occurs as
 449 follows:
 450
 451   - An application executable is built with _LGPL_SOURCE defined, includes
 452     any of the Userspace RCU 0.10 urcu flavor headers, and is built
 453     without the -fpic compiler option.
 454
 455   - The Userspace RCU 0.10 library shared objects are updated to 0.11
 456     or 0.12 without rebuilding the application.
 457
 458   - The application will hang, typically when RCU grace period
 459     (synchronize_rcu) is invoked.
 460
 461 Some possible work-arounds for this are:
 462
 463   - Rebuild the application against Userspace RCU 0.11+.
 464
 465   - Rebuild the application with -fpic.
 466
 467   - Upgrade Userspace RCU to 0.13+ without installing 0.11 nor 0.12.
 468
 469
 470 Contacts
 471 --------
 472
 473 You can contact the maintainers on the following mailing list:
 474 `lttng-dev@lists.lttng.org`.