Userspace RCU

liburcu is a LGPLv2.1 userspace RCU (read-copy-update) library. This data synchronization library provides read-side access which scales linearly with the number of cores.

liburcu-cds provides efficient data structures based on RCU and lock-free algorithms. Those structures include hash tables, queues, stacks, and doubly-linked lists.


Binary packages

liburcu is available on most major Linux distributions.


Packages are available here.


You can clone the source code from the official Userspace RCU:

git clone git://

Mailing list

For help and developer discussions, see the LTTng mailing list.

Bug tracker

Userspace RCU's bug tracker is the official system to report issues with the project and view their progress.

Initial motivation for creating the userspace RCU library: The userspace RCU library was initially needed for the LTTng userspace tracer port. The userspace tracer implementation is therefore the motivation and first user of liburcu.

Other projects use liburcu:

  • Knot DNS: Knot DNS is a high-performance authoritative-only DNS server.
  • Netsniff-ng: Netsniff-ng is a high-performance Linux networking toolkit.
  • Sheepdog: Sheepdog is a distributed storage system for QEMU/KVM.
  • GlusterFS: Gluster is a software scalable network filesystem.
  • gdnsd: gdnsd is an authoritative-only DNS server.
  • ISC BIND: BIND is a full-featured DNS system.
  • XFS User Tools (xfsprogs): xfsprogs are the userspace utilities that manage XFS filesystems.
View on GitHub

Userspace RCU Implementation

by Mathieu Desnoyers and Paul E. McKenney


./bootstrap # skip if using tarball
make install


  • 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, the following architectures are supported:

  • Linux x86 (i386, i486, i586, i686)
  • x86 64-bit
  • PowerPC 32/64
  • S390, S390x
  • ARM 32/64
  • MIPS
  • Alpha
  • ia64
  • Sparcv9 32/64
  • Tilera
  • hppa/PA-RISC

Tested on Linux, FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64, 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:

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

Building on MacOS X (Darwin) requires a work-around for processor detection:

  • 32-bit:

    ./configure --build=i686-apple-darwin11
  • 64-bit:

    ./configure --build=x86_64-apple-darwin11

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

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.


See the relevant API documentation files in doc/. The APIs provided by Userspace RCU are, by prefix:

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.
  • Define URCU_INLINE_SMALL_FUNCTIONS before including Userspace RCU headers if you want Userspace RCU to inline small functions (10 lines or less) into the application. It can be used by applications distributed under any kind of license, and does not make the application a derived work of Userspace RCU.

Those small inlined functions are guaranteed to match the library content as long as the library major version is unchanged. Therefore, the application must be compiled with headers matching the library major version number. Applications using URCU_INLINE_SMALL_FUNCTIONS may be unable to use debugging features of Userspace RCU without being recompiled.

Usage of liburcu

  1. #include <urcu.h>
  2. 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

  1. #include <urcu-qsbr.h>
  2. 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

  1. #include <urcu.h>
  2. Compile any _LGPL_SOURCE code using this library with -DRCU_MB
  3. 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

  1. #include <urcu.h>
  2. Compile any _LGPL_SOURCE code using this library with -DRCU_SIGNAL
  3. Link the application with -lurcu-signal

Version of the library that requires a signal, typically SIGUSR1. Can be overridden with -DSIGRCU by modifying

Usage of liburcu-bp

  1. #include <urcu-bp.h>
  2. 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.


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().


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.


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 pulled into each of those 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 doc/ 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()).

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.

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


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.

Make targets

In addition to the usual make check target, Userspace RCU features make regtest and make bench targets:

  • make check: short tests, meant to be run when rebuilding or porting Userspace RCU.
  • make regtest: long (many hours) test, meant to be run when modifying Userspace RCU or porting it to a new architecture or operating system.
  • make bench: long (many hours) benchmarks.


You can contact the maintainers on the following mailing list: