README

   1 Userspace RCU Implementation
   2 by Mathieu Desnoyers and Paul E. McKenney
   3
   4 BUILDING
   5 --------
   6
   7         ./bootstrap (skip if using tarball)
   8         ./configure
   9         make
  10         make install
  11
  12         Hints:  Forcing 32-bit build:
  13                 * CFLAGS="-m32 -g -O2" ./configure
  14
  15                 Forcing 64-bit build:
  16                 * CFLAGS="-m64 -g -O2" ./configure
  17
  18                 Forcing a 32-bit build with 386 backward compatibility:
  19                 * CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu
  20
  21                 Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
  22                 * CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure
  23
  24 ARCHITECTURES SUPPORTED
  25 -----------------------
  26
  27 Currently, x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64, S390, S390x,
  28 ARM, Alpha, ia64 and Sparcv9 32/64 are supported. Only tested on Linux so
  29 far, but should theoretically work on other operating systems.
  30
  31 ARM depends on running a Linux kernel 2.6.15 or better.
  32
  33 The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
  34 supported, with the following exceptions:
  35
  36 - gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
  37   accesses to offsets in a TLS structure on 32-bit x86. These versions are
  38   therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
  39   The problem has been reported to the gcc community:
  40     http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
  41 - Alpha, ia64 and ARM architectures depend on 4.x gcc with atomic builtins
  42   support.
  43
  44
  45 QUICK START GUIDE
  46 -----------------
  47
  48 Usage of all urcu libraries
  49
  50         * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
  51           before including the urcu.h or urcu-qsbr.h header. If your application
  52           is distributed under another license, function calls will be generated
  53           instead of inlines, so your application can link with the library.
  54         * Linking with one of the libraries below is always necessary even for
  55           LGPL and GPL applications.
  56
  57 Usage of liburcu
  58
  59         * #include <urcu.h>
  60         * Link the application with "-lurcu".
  61         * This is the preferred version of the library, in terms of
  62           grace-period detection speed, read-side speed and flexibility.
  63           Dynamically detects kernel support for sys_membarrier(). Falls back
  64           on urcu-mb scheme if support is not present, which has slower
  65           read-side.
  66
  67 Usage of liburcu-qsbr
  68
  69         * #include <urcu-qsbr.h>
  70         * Link with "-lurcu-qsbr".
  71         * The QSBR flavor of RCU needs to have each reader thread executing
  72           rcu_quiescent_state() periodically to progress. rcu_thread_online()
  73           and rcu_thread_offline() can be used to mark long periods for which
  74           the threads are not active. It provides the fastest read-side at the
  75           expense of more intrusiveness in the application code.
  76
  77 Usage of liburcu-mb
  78
  79         * #include <urcu.h>
  80         * Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
  81         * Link with "-lurcu-mb".
  82         * This version of the urcu library uses memory barriers on the writer
  83           and reader sides. This results in faster grace-period detection, but
  84           results in slower reads.
  85
  86 Usage of liburcu-signal
  87
  88         * #include <urcu.h>
  89         * Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
  90         * Link the application with "-lurcu-signal".
  91         * Version of the library that requires a signal, typically SIGUSR1. Can
  92           be overridden with -DSIGRCU by modifying Makefile.build.inc.
  93
  94 Usage of liburcu-bp
  95
  96         * #include <urcu-bp.h>
  97         * Link with "-lurcu-bp".
  98         * The BP library flavor stands for "bulletproof". It is specifically
  99           designed to help tracing library to hook on applications without
 100           requiring to modify these applications. rcu_init(),
 101           rcu_register_thread() and rcu_unregister_thread() all become nops.
 102           The state is dealt with by the library internally at the expense of
 103           read-side and write-side performance.
 104
 105 Initialization
 106
 107         Each thread that has reader critical sections (that uses
 108         rcu_read_lock()/rcu_read_unlock() must first register to the URCU
 109         library. This is done by calling rcu_register_thread(). Unregistration
 110         must be performed before exiting the thread by using
 111         rcu_unregister_thread().
 112
 113 Reading
 114
 115         Reader critical sections must be protected by locating them between
 116         calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
 117         rcu_dereference() may be called to read an RCU protected pointer.
 118
 119 Writing
 120
 121         rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
 122         After, synchronize_rcu() must be called. When it returns, the old
 123         values are not in usage anymore.
 124
 125 Usage of liburcu-defer
 126
 127         * Follow instructions for either liburcu, liburcu-qsbr,
 128           liburcu-mb, liburcu-signal, or liburcu-bp above.
 129           The liburcu-defer functionality is pulled into each of
 130           those library modules.
 131         * Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
 132           callbacks are executed in batch periodically after a grace period.
 133           Do _not_ use defer_rcu() within a read-side critical section, because
 134           it may call synchronize_rcu() if the thread queue is full.
 135           This can lead to deadlock or worse.
 136         * Requires that rcu_defer_barrier() must be called in library destructor
 137           if a library queues callbacks and is expected to be unloaded with
 138           dlclose().
 139         * Its API is currently experimental. It may change in future library
 140           releases.
 141
 142 Usage of urcu-call-rcu
 143
 144         * Follow instructions for either liburcu, liburcu-qsbr,
 145           liburcu-mb, liburcu-signal, or liburcu-bp above.
 146           The urcu-call-rcu functionality is provided for each of
 147           these library modules.
 148         * Provides the call_rcu() primitive to enqueue delayed callbacks
 149           in a manner similar to defer_rcu(), but without ever delaying
 150           for a grace period.  On the other hand, call_rcu()'s best-case
 151           overhead is not quite as good as that of defer_rcu().
 152         * Provides call_rcu() to allow asynchronous handling of RCU
 153           grace periods.  A number of additional functions are provided
 154           to manage the helper threads used by call_rcu(), but reasonable
 155           defaults are used if these additional functions are not invoked.
 156           See API.txt for more details.
 157
 158 Being careful with signals
 159
 160         The liburcu library uses signals internally. The signal handler is
 161         registered with the SA_RESTART flag. However, these signals may cause
 162         some non-restartable system calls to fail with errno = EINTR. Care
 163         should be taken to restart system calls manually if they fail with this
 164         error. A list of non-restartable system calls may be found in
 165         signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
 166         library do not require any signal.
 167
 168         Read-side critical sections are allowed in a signal handler with
 169         liburcu and liburcu-mb. Be careful, however, to disable these signals
 170         between thread creation and calls to rcu_register_thread(), because a
 171         signal handler nesting on an unregistered thread would not be allowed to
 172         call rcu_read_lock().
 173
 174         Read-side critical sections are _not_ allowed in a signal handler with
 175         liburcu-qsbr, unless signals are disabled explicitly around each
 176         rcu_quiescent_state() calls, when threads are put offline and around
 177         calls to synchronize_rcu(). Even then, we do not recommend it.
 178
 179 Interaction with mutexes
 180
 181         One must be careful to do not cause deadlocks due to interaction of
 182         synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
 183         is called with a mutex held, this mutex (or any mutex which has this
 184         mutex in its dependency chain) should not be acquired from within a RCU
 185         read-side critical section.
 186
 187 Usage of DEBUG_RCU
 188
 189         DEBUG_RCU is used to add internal debugging self-checks to the
 190         RCU library. This define adds a performance penalty when enabled.
 191         Can be enabled by uncommenting the corresponding line in
 192         Makefile.build.inc.
 193
 194 Usage of DEBUG_YIELD
 195
 196         DEBUG_YIELD is used to add random delays in the code for testing
 197         purposes.
 198
 199 SMP support
 200
 201         By default the library is configured to use synchronization primitives
 202         adequate for SMP systems. On uniprocessor systems, support for SMP
 203         systems can be disabled with:
 204
 205                 ./configure --disable-smp-support
 206
 207         theoretically yielding slightly better performance.
 208
 209 Interaction with fork()
 210
 211         Special care must be taken for applications performing fork() without
 212         any following exec(). This is caused by the fact that Linux only clones
 213         the thread calling fork(), and thus never replicates any of the other
 214         parent thread into the child process. Most liburcu implementations
 215         require that all registrations (as reader, defer_rcu and call_rcu
 216         threads) should be released before a fork() is performed, except for the
 217         rather common scenario where fork() is immediately followed by exec() in
 218         the child process. The only implementation not subject to that rule is
 219         liburcu-bp, which is designed to handle fork() by calling
 220         rcu_bp_before_fork, rcu_bp_after_fork_parent and
 221         rcu_bp_after_fork_child.
 222
 223         Applications that use call_rcu() and that fork() without
 224         doing an immediate exec() must take special action.  The parent
 225         must invoke call_rcu_before_fork() before the fork() and
 226         call_rcu_after_fork_parent() after the fork().  The child
 227         process must invoke call_rcu_after_fork_child().
 228         These three APIs are suitable for passing to pthread_atfork().