[urcu.git] / README

Userspace RCU Implementation
by Mathieu Desnoyers and Paul E. McKenney

BUILDING
--------

	make
	#force 32-bit build with: make -f Makefile32
	#force 64-bit build with: make -f Makefile64
	make install
	

QUICK START GUIDE
-----------------

Usage of all urcu libraries

	* Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
	  before including the urcu.h or urcu-qsbr.h header. If your application
	  is distributed under another license, function calls will be generated
	  instead of inlines, so your application can link with the library.
	* Linking with one of the libraries below is always necessary even for
	  LGPL and GPL applications.

Usage of liburcu

	* #include <urcu.h>
	* Link the application with "-lurcu".
	* This is the preferred version of the library, both in terms of speed
	  and flexibility. Requires a signal, typically SIGUSR1. Can be
	  overridden with -DSIGURCU by modifying Makefile.build.inc.

Usage of liburcu-mb

	* #include <urcu.h>
	* Compile any _LGPL_SOURCE code using this library with "-DURCU_MB".
	* Link with "-lurcu-mb".
	* This version of the urcu library does not need to
	  reserve a signal number. URCU_MB uses full memory barriers for
	  readers. This eliminates the need for signals but results in slower
	  reads.

Usage of liburcu-qsbr

	* #include <urcu-qsbr.h>
	* Link with "-lurcu-qsbr".
	* The QSBR flavor of RCU needs to have each reader thread executing
	  rcu_quiescent_state() periodically to progress. rcu_thread_online()
	  and rcu_thread_offline() can be used to mark long periods for which
	  the threads are not active. It provides the fastest read-side at the
	  expense of more intrusiveness in the application code.

Usage of liburcu-bp

	* #include <urcu-bp.h>
	* Link with "-lurcu-bp".
	* The BP library flavor stands for "bulletproof". It is specifically
	  designed to help tracing library to hook on applications without
	  requiring to modify these applications. urcu_init(),
	  rcu_register_thread() and rcu_unregister_thread() all become nops.
	  The state is dealt with by the library internally at the expense of
	  read-side and write-side performance.

Initialization

	Each thread that has reader critical sections (that uses
	rcu_read_lock()/rcu_read_unlock() must first register to the URCU
	library. This is done by calling rcu_register_thread(). Unregistration
	must be performed before exiting the thread by using
	rcu_unregister_thread().

Reading

	Reader critical sections must be protected by locating them between
	calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
	rcu_dereference() may be called to read an RCU protected pointer.

Writing

	rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
	After, synchronize_rcu() must be called. When it returns, the old
	values are not in usage anymore.

Usage of liburcu-defer

	* #include <urcu-defer.h>
	* Link with "-lurcu-defer"
	* Provides call_rcu() primitive to enqueue delayed callbacks. Queued
	  callbacks are executed in batch periodically after a grace period.
	  Do _not_ use call_rcu() within a read-side critical section, because
	  it may call synchronize_rcu() if the thread queue is full.

Being careful with signals

	The liburcu library uses signals internally. The signal handler is
	registered with the SA_RESTART flag. However, these signals may cause
	some non-restartable system calls to fail with errno = EINTR. Care
	should be taken to restart system calls manually if they fail with this
	error. A list of non-restartable system calls may be found in
	signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
	library do not require any signal.

	Read-side critical sections are allowed in a signal handler with
	liburcu and liburcu-mb. Be careful, however, to disable these signals
	between thread creation and calls to rcu_register_thread(), because a
	signal handler nesting on an unregistered thread would not be allowed to
	call rcu_read_lock().

	Read-side critical sections are _not_ allowed in a signal handler with
	liburcu-qsbr, unless signals are disabled explicitly around each
	rcu_quiescent_state() calls, when threads are put offline and around
	calls to synchronize_rcu(). Even then, we do not recommend it.

Usage of DEBUG_RCU

	DEBUG_RCU is used to add internal debugging self-checks to the
	RCU library. This define adds a performance penalty when enabled.
	Can be enabled by uncommenting the corresponding line in
	Makefile.build.inc.

Usage of DEBUG_YIELD

	DEBUG_YIELD is used to add random delays in the code for testing
	purposes.
Commit	Line	Data
7ac06cef	1	Userspace RCU Implementation
c97ae6eb	2	by Mathieu Desnoyers and Paul E. McKenney
6991f61a	3
c97ae6eb PMF	4	BUILDING
c97ae6eb PMF	5	--------
6991f61a	6
c97ae6eb	7	make
a669219f MD	8	#force 32-bit build with: make -f Makefile32
a669219f MD	9	#force 64-bit build with: make -f Makefile64
c97ae6eb PMF	10	make install
c97ae6eb PMF	11
aa8c36e0	12
c97ae6eb PMF	13	QUICK START GUIDE
c97ae6eb PMF	14	-----------------
aa8c36e0	15
0a1d290b MD	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>
256dd999	36	* Compile any _LGPL_SOURCE code using this library with "-DURCU_MB".
0a1d290b MD	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
fdee2e6d MD	53	Usage of liburcu-bp
	54
	55	* #include <urcu-bp.h>
	56	* Link with "-lurcu-bp".
	57	* The BP library flavor stands for "bulletproof". It is specifically
	58	designed to help tracing library to hook on applications without
	59	requiring to modify these applications. urcu_init(),
	60	rcu_register_thread() and rcu_unregister_thread() all become nops.
	61	The state is dealt with by the library internally at the expense of
	62	read-side and write-side performance.
	63
c97ae6eb PMF	64	Initialization
	65
	66	Each thread that has reader critical sections (that uses
	67	rcu_read_lock()/rcu_read_unlock() must first register to the URCU
4c1471de MD	68	library. This is done by calling rcu_register_thread(). Unregistration
	69	must be performed before exiting the thread by using
	70	rcu_unregister_thread().
c97ae6eb PMF	71
	72	Reading
	73
	74	Reader critical sections must be protected by locating them between
	75	calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
	76	rcu_dereference() may be called to read an RCU protected pointer.
	77
	78	Writing
	79
	80	rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
9fb223da MD	81	After, synchronize_rcu() must be called. When it returns, the old
9fb223da MD	82	values are not in usage anymore.
c97ae6eb	83
ec4e58a3 MD	84	Usage of liburcu-defer
	85
	86	* #include <urcu-defer.h>
	87	* Link with "-lurcu-defer"
	88	* Provides call_rcu() primitive to enqueue delayed callbacks. Queued
	89	callbacks are executed in batch periodically after a grace period.
	90	Do _not_ use call_rcu() within a read-side critical section, because
	91	it may call synchronize_rcu() if the thread queue is full.
	92
dd052bd3 PMF	93	Being careful with signals
dd052bd3 PMF	94
0a1d290b	95	The liburcu library uses signals internally. The signal handler is
dd052bd3 PMF	96	registered with the SA_RESTART flag. However, these signals may cause
	97	some non-restartable system calls to fail with errno = EINTR. Care
	98	should be taken to restart system calls manually if they fail with this
	99	error. A list of non-restartable system calls may be found in
0a1d290b MD	100	signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
0a1d290b MD	101	library do not require any signal.
c97ae6eb	102
0a1d290b	103	Read-side critical sections are allowed in a signal handler with
7ac06cef MD	104	liburcu and liburcu-mb. Be careful, however, to disable these signals
	105	between thread creation and calls to rcu_register_thread(), because a
	106	signal handler nesting on an unregistered thread would not be allowed to
	107	call rcu_read_lock().
cee02f0a	108
0a1d290b MD	109	Read-side critical sections are _not_ allowed in a signal handler with
	110	liburcu-qsbr, unless signals are disabled explicitly around each
	111	rcu_quiescent_state() calls, when threads are put offline and around
	112	calls to synchronize_rcu(). Even then, we do not recommend it.
c97ae6eb	113
cee02f0a MD	114	Usage of DEBUG_RCU
	115
	116	DEBUG_RCU is used to add internal debugging self-checks to the
0a1d290b	117	RCU library. This define adds a performance penalty when enabled.
fb6e510b MD	118	Can be enabled by uncommenting the corresponding line in
fb6e510b MD	119	Makefile.build.inc.
c97ae6eb PMF	120
	121	Usage of DEBUG_YIELD
	122
	123	DEBUG_YIELD is used to add random delays in the code for testing
	124	purposes.