| 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 | Initialization |
| 15 | |
| 16 | Each thread that has reader critical sections (that uses |
| 17 | rcu_read_lock()/rcu_read_unlock() must first register to the URCU |
| 18 | library. This is done by calling rcu_register_thread(). |
| 19 | |
| 20 | Reading |
| 21 | |
| 22 | Reader critical sections must be protected by locating them between |
| 23 | calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock, |
| 24 | rcu_dereference() may be called to read an RCU protected pointer. |
| 25 | |
| 26 | Writing |
| 27 | |
| 28 | rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere. |
| 29 | After, synchronize_rcu() must be called. When it returns, the old |
| 30 | values are not in usage anymore. |
| 31 | |
| 32 | Being careful with signals |
| 33 | |
| 34 | The urcu library uses signals internally. The signal handler is |
| 35 | registered with the SA_RESTART flag. However, these signals may cause |
| 36 | some non-restartable system calls to fail with errno = EINTR. Care |
| 37 | should be taken to restart system calls manually if they fail with this |
| 38 | error. A list of non-restartable system calls may be found in |
| 39 | signal(7). To ensure the Userspace RCU library does not use signals, |
| 40 | see "Usage of liburcu-mb" below. |
| 41 | |
| 42 | Read-side critical sections can are allowed in a signal handler with |
| 43 | liburcu and liburcu-mb. Be careful, however, to disable these signals |
| 44 | between thread creation and calls to rcu_register_thread(), because a |
| 45 | signal handler nesting on an unregistered thread would not be allowed to |
| 46 | call rcu_read_lock(). |
| 47 | |
| 48 | Usage of liburcu |
| 49 | |
| 50 | This is the preferred version of the library, both in terms of speed and |
| 51 | flexibility. Define _LGPL_SOURCE if your code is LGPL or GPL (otherwise |
| 52 | function calls will be generated instead of inlines). Use the urcu.h |
| 53 | header. Link the application with "-lurcu". |
| 54 | |
| 55 | Usage of liburcu-mb |
| 56 | |
| 57 | Compile code with "-DCONFIG_URCU_AVOID_SIGNALS" and link with |
| 58 | "-lurcu-mb" to use a version of the urcu library which does not need to |
| 59 | reserve a signal number. CONFIG_URCU_AVOID_SIGNALS uses full SMP |
| 60 | barriers for readers. This eliminates the need for signals but results |
| 61 | in slower reads. |
| 62 | |
| 63 | Usage of liburcu-qsbr |
| 64 | |
| 65 | The QSBR flavor of RCU needs to have each reader thread executing |
| 66 | rcu_quiescent_state() periodically to progress. rcu_thread_online() and |
| 67 | rcu_thread_offline() can be used to mark long periods for which the |
| 68 | threads are not active. Link with "-lurcu-qsbr" and use urcu-qsbr.h. |
| 69 | |
| 70 | Usage of DEBUG_RCU |
| 71 | |
| 72 | DEBUG_RCU is used to add internal debugging self-checks to the |
| 73 | RCU library. This define adds a performance penality when enabled. |
| 74 | Can be enabled by uncommenting the corresponding line in |
| 75 | Makefile.build.inc. |
| 76 | |
| 77 | Usage of DEBUG_YIELD |
| 78 | |
| 79 | DEBUG_YIELD is used to add random delays in the code for testing |
| 80 | purposes. |
| 81 | |
| 82 | |