README

   1 Userspace RCU Implementation
   2 by Mathieu Desnoyers and Paul E. McKenney
   3
   4 BUILDING
   5 --------
   6
   7         make
   8         make install
   9
  10
  11 QUICK START GUIDE
  12 -----------------
  13
  14 Usage of all urcu libraries
  15
  16         * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
  17           before including the urcu.h or urcu-qsbr.h header. If your application
  18           is distributed under another license, function calls will be generated
  19           instead of inlines, so your application can link with the library.
  20         * Linking with one of the libraries below is always necessary even for
  21           LGPL and GPL applications.
  22
  23 Usage of liburcu
  24
  25         * #include <urcu.h>
  26         * Link the application with "-lurcu".
  27         * This is the preferred version of the library, both in terms of speed
  28           and flexibility. Requires a signal, typically SIGUSR1. Can be
  29           overridden with -DSIGURCU by modifying Makefile.build.inc.
  30
  31 Usage of liburcu-mb
  32
  33         * #include <urcu.h>
  34         * Compile any _LGPL_SOURCE code using this library with "-DURCU_MB".
  35         * Link with "-lurcu-mb".
  36         * This version of the urcu library does not need to
  37           reserve a signal number. URCU_MB uses full memory barriers for
  38           readers. This eliminates the need for signals but results in slower
  39           reads.
  40
  41 Usage of liburcu-qsbr
  42
  43         * #include <urcu-qsbr.h>
  44         * Link with "-lurcu-qsbr".
  45         * The QSBR flavor of RCU needs to have each reader thread executing
  46           rcu_quiescent_state() periodically to progress. rcu_thread_online()
  47           and rcu_thread_offline() can be used to mark long periods for which
  48           the threads are not active. It provides the fastest read-side at the
  49           expense of more intrusiveness in the application code.
  50
  51 Initialization
  52
  53         Each thread that has reader critical sections (that uses
  54         rcu_read_lock()/rcu_read_unlock() must first register to the URCU
  55         library. This is done by calling rcu_register_thread(). Unregistration
  56         must be performed before exiting the thread by using
  57         rcu_unregister_thread().
  58
  59 Reading
  60
  61         Reader critical sections must be protected by locating them between
  62         calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
  63         rcu_dereference() may be called to read an RCU protected pointer.
  64
  65 Writing
  66
  67         rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
  68         After, synchronize_rcu() must be called. When it returns, the old
  69         values are not in usage anymore.
  70
  71 Being careful with signals
  72
  73         The liburcu library uses signals internally. The signal handler is
  74         registered with the SA_RESTART flag. However, these signals may cause
  75         some non-restartable system calls to fail with errno = EINTR. Care
  76         should be taken to restart system calls manually if they fail with this
  77         error. A list of non-restartable system calls may be found in
  78         signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
  79         library do not require any signal.
  80
  81         Read-side critical sections are allowed in a signal handler with
  82         liburcu and liburcu-mb. Be careful, however, to disable these signals
  83         between thread creation and calls to rcu_register_thread(), because a
  84         signal handler nesting on an unregistered thread would not be allowed to
  85         call rcu_read_lock().
  86
  87         Read-side critical sections are _not_ allowed in a signal handler with
  88         liburcu-qsbr, unless signals are disabled explicitly around each
  89         rcu_quiescent_state() calls, when threads are put offline and around
  90         calls to synchronize_rcu(). Even then, we do not recommend it.
  91
  92 Usage of DEBUG_RCU
  93
  94         DEBUG_RCU is used to add internal debugging self-checks to the
  95         RCU library. This define adds a performance penalty when enabled.
  96         Can be enabled by uncommenting the corresponding line in
  97         Makefile.build.inc.
  98
  99 Usage of DEBUG_YIELD
 100
 101         DEBUG_YIELD is used to add random delays in the code for testing
 102         purposes.