[urcu.git] / README

Userspace RCU Implementation
by Mathieu Desnoyers and Paul E. McKenney

BUILDING
--------

	./bootstrap (skip if using tarball)
	./configure
	make
	make install

	Note:	Forcing 32-bit build:
		* CFLAGS=-m32 ./configure

		Forcing 64-bit build:
		* CFLAGS=-m64 ./configure

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
48d848c7 PMF	7	./bootstrap (skip if using tarball)
48d848c7 PMF	8	./configure
c97ae6eb PMF	9	make
c97ae6eb PMF	10	make install
9ca52251 MD	11
	12	Note: Forcing 32-bit build:
	13	* CFLAGS=-m32 ./configure
	14
	15	Forcing 64-bit build:
	16	* CFLAGS=-m64 ./configure
aa8c36e0	17
c97ae6eb PMF	18	QUICK START GUIDE
c97ae6eb PMF	19	-----------------
aa8c36e0	20
0a1d290b MD	21	Usage of all urcu libraries
	22
	23	* Define _LGPL_SOURCE (only) if your code is LGPL or GPL compatible
	24	before including the urcu.h or urcu-qsbr.h header. If your application
	25	is distributed under another license, function calls will be generated
	26	instead of inlines, so your application can link with the library.
	27	* Linking with one of the libraries below is always necessary even for
	28	LGPL and GPL applications.
	29
	30	Usage of liburcu
	31
	32	* #include <urcu.h>
	33	* Link the application with "-lurcu".
	34	* This is the preferred version of the library, both in terms of speed
	35	and flexibility. Requires a signal, typically SIGUSR1. Can be
	36	overridden with -DSIGURCU by modifying Makefile.build.inc.
	37
	38	Usage of liburcu-mb
	39
	40	* #include <urcu.h>
256dd999	41	* Compile any _LGPL_SOURCE code using this library with "-DURCU_MB".
0a1d290b MD	42	* Link with "-lurcu-mb".
	43	* This version of the urcu library does not need to
	44	reserve a signal number. URCU_MB uses full memory barriers for
	45	readers. This eliminates the need for signals but results in slower
	46	reads.
	47
	48	Usage of liburcu-qsbr
	49
	50	* #include <urcu-qsbr.h>
	51	* Link with "-lurcu-qsbr".
	52	* The QSBR flavor of RCU needs to have each reader thread executing
	53	rcu_quiescent_state() periodically to progress. rcu_thread_online()
	54	and rcu_thread_offline() can be used to mark long periods for which
	55	the threads are not active. It provides the fastest read-side at the
	56	expense of more intrusiveness in the application code.
	57
fdee2e6d MD	58	Usage of liburcu-bp
	59
	60	* #include <urcu-bp.h>
	61	* Link with "-lurcu-bp".
	62	* The BP library flavor stands for "bulletproof". It is specifically
	63	designed to help tracing library to hook on applications without
	64	requiring to modify these applications. urcu_init(),
	65	rcu_register_thread() and rcu_unregister_thread() all become nops.
	66	The state is dealt with by the library internally at the expense of
	67	read-side and write-side performance.
	68
c97ae6eb PMF	69	Initialization
	70
	71	Each thread that has reader critical sections (that uses
	72	rcu_read_lock()/rcu_read_unlock() must first register to the URCU
4c1471de MD	73	library. This is done by calling rcu_register_thread(). Unregistration
	74	must be performed before exiting the thread by using
	75	rcu_unregister_thread().
c97ae6eb PMF	76
	77	Reading
	78
	79	Reader critical sections must be protected by locating them between
	80	calls to rcu_read_lock() and rcu_read_unlock(). Inside that lock,
	81	rcu_dereference() may be called to read an RCU protected pointer.
	82
	83	Writing
	84
	85	rcu_assign_pointer() and rcu_xchg_pointer() may be called anywhere.
9fb223da MD	86	After, synchronize_rcu() must be called. When it returns, the old
9fb223da MD	87	values are not in usage anymore.
c97ae6eb	88
ec4e58a3 MD	89	Usage of liburcu-defer
	90
	91	* #include <urcu-defer.h>
	92	* Link with "-lurcu-defer"
	93	* Provides call_rcu() primitive to enqueue delayed callbacks. Queued
	94	callbacks are executed in batch periodically after a grace period.
	95	Do _not_ use call_rcu() within a read-side critical section, because
	96	it may call synchronize_rcu() if the thread queue is full.
	97
dd052bd3 PMF	98	Being careful with signals
dd052bd3 PMF	99
0a1d290b	100	The liburcu library uses signals internally. The signal handler is
dd052bd3 PMF	101	registered with the SA_RESTART flag. However, these signals may cause
	102	some non-restartable system calls to fail with errno = EINTR. Care
	103	should be taken to restart system calls manually if they fail with this
	104	error. A list of non-restartable system calls may be found in
0a1d290b MD	105	signal(7). The liburcu-mb and liburcu-qsbr versions of the Userspace RCU
0a1d290b MD	106	library do not require any signal.
c97ae6eb	107
0a1d290b	108	Read-side critical sections are allowed in a signal handler with
7ac06cef MD	109	liburcu and liburcu-mb. Be careful, however, to disable these signals
	110	between thread creation and calls to rcu_register_thread(), because a
	111	signal handler nesting on an unregistered thread would not be allowed to
	112	call rcu_read_lock().
cee02f0a	113
0a1d290b MD	114	Read-side critical sections are _not_ allowed in a signal handler with
	115	liburcu-qsbr, unless signals are disabled explicitly around each
	116	rcu_quiescent_state() calls, when threads are put offline and around
	117	calls to synchronize_rcu(). Even then, we do not recommend it.
c97ae6eb	118
cee02f0a MD	119	Usage of DEBUG_RCU
	120
	121	DEBUG_RCU is used to add internal debugging self-checks to the
0a1d290b	122	RCU library. This define adds a performance penalty when enabled.
fb6e510b MD	123	Can be enabled by uncommenting the corresponding line in
fb6e510b MD	124	Makefile.build.inc.
c97ae6eb PMF	125
	126	Usage of DEBUG_YIELD
	127
	128	DEBUG_YIELD is used to add random delays in the code for testing
	129	purposes.