| 1 | Userspace RCU Implementation |
| 2 | by Mathieu Desnoyers and Paul E. McKenney |
| 3 | |
| 4 | BUILDING |
| 5 | -------- |
| 6 | |
| 7 | make |
| 8 | #force 32-bit build with: make -f Makefile32 |
| 9 | #force 64-bit build with: make -f Makefile64 |
| 10 | make install |
| 11 | |
| 12 | |
| 13 | QUICK START GUIDE |
| 14 | ----------------- |
| 15 | |
| 16 | Usage of all urcu libraries |
| 17 | |
| 18 | * Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible |
| 19 | before including the urcu.h or urcu-qsbr.h header. If your application |
| 20 | is distributed under another license, function calls will be generated |
| 21 | instead of inlines, so your application can link with the library. |
| 22 | * Linking with one of the libraries below is always necessary even for |
| 23 | LGPL and GPL applications. |
| 24 | |
| 25 | Usage of liburcu |
| 26 | |
| 27 | * #include <urcu.h> |
| 28 | * Link the application with "-lurcu". |
| 29 | * This is the preferred version of the library, both in terms of speed |
| 30 | and flexibility. Requires a signal, typically SIGUSR1. Can be |
| 31 | overridden with -DSIGURCU by modifying Makefile.build.inc. |
| 32 | |
| 33 | Usage of liburcu-mb |
| 34 | |
| 35 | * #include <urcu.h> |
| 36 | * Compile any _LGPL_SOURCE code using this library with "-DURCU_MB". |
| 37 | * Link with "-lurcu-mb". |
| 38 | * This version of the urcu library does not need to |
| 39 | reserve a signal number. URCU_MB uses full memory barriers for |
| 40 | readers. This eliminates the need for signals but results in slower |
| 41 | reads. |
| 42 | |
| 43 | Usage of liburcu-qsbr |
| 44 | |
| 45 | * #include <urcu-qsbr.h> |
| 46 | * Link with "-lurcu-qsbr". |
| 47 | * The QSBR flavor of RCU needs to have each reader thread executing |
| 48 | rcu_quiescent_state() periodically to progress. rcu_thread_online() |
| 49 | and rcu_thread_offline() can be used to mark long periods for which |
| 50 | the threads are not active. It provides the fastest read-side at the |
| 51 | expense of more intrusiveness in the application code. |
| 52 | |
| 53 | Initialization |
| 54 | |
| 55 | Each thread that has reader critical sections (that uses |
| 56 | rcu_read_lock()/rcu_read_unlock() must first register to the URCU |
| 57 | library. This is done by calling rcu_register_thread(). Unregistration |
| 58 | must be performed before exiting the thread by using |
| 59 | rcu_unregister_thread(). |
| 60 | |
| 61 | Reading |
| 62 | |
| 63 | Reader critical sections must be protected by locating them between |
| 64 | calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock, |
| 65 | rcu_dereference() may be called to read an RCU protected pointer. |
| 66 | |
| 67 | Writing |
| 68 | |
| 69 | rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere. |
| 70 | After, synchronize_rcu() must be called. When it returns, the old |
| 71 | values are not in usage anymore. |
| 72 | |
| 73 | Being careful with signals |
| 74 | |
| 75 | The liburcu library uses signals internally. The signal handler is |
| 76 | registered with the SA_RESTART flag. However, these signals may cause |
| 77 | some non-restartable system calls to fail with errno = EINTR. Care |
| 78 | should be taken to restart system calls manually if they fail with this |
| 79 | error. A list of non-restartable system calls may be found in |
| 80 | signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU |
| 81 | library do not require any signal. |
| 82 | |
| 83 | Read-side critical sections are allowed in a signal handler with |
| 84 | liburcu and liburcu-mb. Be careful, however, to disable these signals |
| 85 | between thread creation and calls to rcu_register_thread(), because a |
| 86 | signal handler nesting on an unregistered thread would not be allowed to |
| 87 | call rcu_read_lock(). |
| 88 | |
| 89 | Read-side critical sections are _not_ allowed in a signal handler with |
| 90 | liburcu-qsbr, unless signals are disabled explicitly around each |
| 91 | rcu_quiescent_state() calls, when threads are put offline and around |
| 92 | calls to synchronize_rcu(). Even then, we do not recommend it. |
| 93 | |
| 94 | Usage of DEBUG_RCU |
| 95 | |
| 96 | DEBUG_RCU is used to add internal debugging self-checks to the |
| 97 | RCU library. This define adds a performance penalty when enabled. |
| 98 | Can be enabled by uncommenting the corresponding line in |
| 99 | Makefile.build.inc. |
| 100 | |
| 101 | Usage of DEBUG_YIELD |
| 102 | |
| 103 | DEBUG_YIELD is used to add random delays in the code for testing |
| 104 | purposes. |