Userspace RCU Implementation
by Mathieu Desnoyers and Paul E. McKenney

BUILDING
--------

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

	Hints:	Forcing 32-bit build:
		* CFLAGS="-m32 -g -O2" ./configure

		Forcing 64-bit build:
		* CFLAGS="-m64 -g -O2" ./configure

		Forcing a 32-bit build with 386 backward compatibility:
		* CFLAGS="-m32 -g -O2" ./configure --host=i386-pc-linux-gnu

		Forcing a 32-bit build for Sparcv9 (typical for Sparc v9)
		* CFLAGS="-m32 -Wa,-Av9a -g -O2" ./configure

ARCHITECTURES SUPPORTED
-----------------------

Currently, Linux x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64,
S390, S390x, ARM, Alpha, ia64 and Sparcv9 32/64 are supported. Tested on
Linux, FreeBSD 8.2/9.0, and Cygwin. Should also work on: Android, NetBSD 5,
OpenBSD, Darwin (more testing needed before claiming support for these OS).

Linux ARM depends on running a Linux kernel 2.6.15 or better, GCC 4.4 or
better.

The gcc compiler versions 3.3, 3.4, 4.0, 4.1, 4.2, 4.3, 4.4 and 4.5 are
supported, with the following exceptions:

- gcc 3.3 and 3.4 have a bug that prevents them from generating volatile
  accesses to offsets in a TLS structure on 32-bit x86. These versions are
  therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
  The problem has been reported to the gcc community:
    http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
- gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
  See: http://kerneltrap.org/node/7507
- Alpha, ia64 and ARM architectures depend on gcc 4.x with atomic builtins
  support. For ARM this was introduced with gcc 4.4:
    http://gcc.gnu.org/gcc-4.4/changes.html

Clang version 3.0 (based on LLVM 3.0) is supported.

For developers using the git tree:

This source tree is based on the autotools suite from GNU to simplify
portability. Here are some things you should have on your system in order to
compile the git repository tree :

- GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
  (make sure your system wide "automake" points to a recent version!)
- GNU Libtool >=2.2
  (for more information, go to http://www.gnu.org/software/autoconf/)

If you get the tree from the repository, you will need to use the "bootstrap"
script in the root of the tree. It calls all the GNU tools needed to prepare the
tree configuration.

Test scripts provided in the tests/ directory of the source tree depend
on "bash" and the "seq" program.


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, in terms of
	  grace-period detection speed, read-side speed and flexibility.
	  Dynamically detects kernel support for sys_membarrier(). Falls back
	  on urcu-mb scheme if support is not present, which has slower
	  read-side.

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-mb

	* #include <urcu.h>
	* Compile any _LGPL_SOURCE code using this library with "-DRCU_MB".
	* Link with "-lurcu-mb".
	* This version of the urcu library uses memory barriers on the writer
	  and reader sides. This results in faster grace-period detection, but
	  results in slower reads.

Usage of liburcu-signal

	* #include <urcu.h>
	* Compile any _LGPL_SOURCE code using this library with "-DRCU_SIGNAL".
	* Link the application with "-lurcu-signal".
	* Version of the library that requires a signal, typically SIGUSR1. Can
	  be overridden with -DSIGRCU by modifying Makefile.build.inc.

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. rcu_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

	* Follow instructions for either liburcu, liburcu-qsbr,
	  liburcu-mb, liburcu-signal, or liburcu-bp above.
	  The liburcu-defer functionality is pulled into each of
	  those library modules.
	* Provides defer_rcu() primitive to enqueue delayed callbacks. Queued
	  callbacks are executed in batch periodically after a grace period.
	  Do _not_ use defer_rcu() within a read-side critical section, because
	  it may call synchronize_rcu() if the thread queue is full.
	  This can lead to deadlock or worse.
	* Requires that rcu_defer_barrier() must be called in library destructor
	  if a library queues callbacks and is expected to be unloaded with
	  dlclose().
	* Its API is currently experimental. It may change in future library
	  releases.

Usage of urcu-call-rcu

	* Follow instructions for either liburcu, liburcu-qsbr,
	  liburcu-mb, liburcu-signal, or liburcu-bp above.
	  The urcu-call-rcu functionality is provided for each of
	  these library modules.
	* Provides the call_rcu() primitive to enqueue delayed callbacks
	  in a manner similar to defer_rcu(), but without ever delaying
	  for a grace period.  On the other hand, call_rcu()'s best-case
	  overhead is not quite as good as that of defer_rcu().
	* Provides call_rcu() to allow asynchronous handling of RCU
	  grace periods.  A number of additional functions are provided
	  to manage the helper threads used by call_rcu(), but reasonable
	  defaults are used if these additional functions are not invoked.
	  See rcu-api.txt in userspace-rcu documentation for more details.

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,
	except those setup with sigaltstack(2), 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.

Interaction with mutexes

	One must be careful to do not cause deadlocks due to interaction of
	synchronize_rcu() and RCU read-side with mutexes. If synchronize_rcu()
	is called with a mutex held, this mutex (or any mutex which has this
	mutex in its dependency chain) should not be acquired from within a RCU
	read-side critical section.

	This is especially important to understand in the context of the
	QSBR flavor: a registered reader thread being "online" by
	default should be considered as within a RCU read-side critical
	section unless explicitly put "offline". Therefore, if
	synchronize_rcu() is called with a mutex held, this mutex, as
	well as any mutex which has this mutex in its dependency chain
	should only be taken when the RCU reader thread is "offline"
	(this can be performed by calling rcu_thread_offline()).

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.

SMP support

	By default the library is configured to use synchronization primitives
	adequate for SMP systems. On uniprocessor systems, support for SMP
	systems can be disabled with:

		./configure --disable-smp-support

	theoretically yielding slightly better performance.

Interaction with fork()

	Special care must be taken for applications performing fork() without
	any following exec(). This is caused by the fact that Linux only clones
	the thread calling fork(), and thus never replicates any of the other
	parent thread into the child process. Most liburcu implementations
	require that all registrations (as reader, defer_rcu and call_rcu
	threads) should be released before a fork() is performed, except for the
	rather common scenario where fork() is immediately followed by exec() in
	the child process. The only implementation not subject to that rule is
	liburcu-bp, which is designed to handle fork() by calling
	rcu_bp_before_fork, rcu_bp_after_fork_parent and
	rcu_bp_after_fork_child.

	Applications that use call_rcu() and that fork() without
	doing an immediate exec() must take special action.  The parent
	must invoke call_rcu_before_fork() before the fork() and
	call_rcu_after_fork_parent() after the fork().  The child
	process must invoke call_rcu_after_fork_child().
	Even though these three APIs are suitable for passing to
	pthread_atfork(), use of pthread_atfork() is *STRONGLY
	DISCOURAGED* for programs calling the glibc memory allocator
	(malloc(), calloc(), free(), ...) within call_rcu callbacks.
	This is due to limitations in the way glibc memory allocator
	handles calls to the memory allocator from concurrent threads
	while the pthread_atfork() handlers are executing.
	Combining e.g.:
	* call to free() from callbacks executed within call_rcu worker
	  threads,
	* executing call_rcu atfork handlers within the glibc pthread
	  atfork mechanism,
	will sometimes trigger interesting process hangs. This usually
	hangs on a memory allocator lock within glibc.

Thread Local Storage (TLS)

	Userspace RCU can fall back on pthread_getspecific() to emulate
	TLS variables on systems where it is not available. This behavior
	can be forced by specifying --disable-compiler-tls as configure
	argument.
