Userspace RCU Implementation
by Mathieu Desnoyers and Paul E. McKenney
./bootstrap # skip if using tarball ./configure make make install ldconfig
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
Currently, the following architectures are supported:
- x86 (i386, i486, i586, i686)
- amd64 / x86_64
- PowerPC 32/64
- S390, S390x
- ARM 32/64
- Sparcv9 32/64
- Linux all architectures
- FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64
- Solaris 10/11 i386
- Cygwin i386/amd64
- MacOS amd64/arm64
Should also work on:
- NetBSD 5
(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 C compiler used needs to support at least C99. The C++ compiler used needs to support at least C++11.
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
liburcuon x86 32-bit (i386, i486, i586, i686). The problem has been reported to the GCC community: http://firstname.lastname@example.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.
- Linux aarch64 depends on GCC 5.1 or better because prior versions perform unsafe access to deallocated stack.
Clang version 3.0 (based on LLVM 3.0) is supported.
Glibc >= 2.4 should work but the older version we test against is currently 2.17.
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 >=1.12, autoconf >=2.69)
(make sure your system wide
automakepoints 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
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
bash and the
See the relevant API documentation files in
doc/. The APIs provided by
Userspace RCU are, by prefix:
rcu_: Read-Copy Update (see
cmm_: Concurrent Memory Model
caa_: Concurrent Architecture Abstraction
cds_: Concurrent Data Structures (see
uatomic_: Userspace Atomic (see
Quick start guide
Usage of all urcu libraries:
_LGPL_SOURCE(only) if your code is LGPL or GPL compatible before including the
urcu-qsbr.hheader. 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.
URCU_INLINE_SMALL_FUNCTIONSbefore 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.
There are multiple flavors of liburcu available:
The API members start with the prefix "urcu__", where is the chosen flavor name.
- Link the application with
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
urcu-mb scheme if support is not present, which has slower
read-side. Use the --disable-sys-membarrier-fallback configure option
to disable the fall back, thus requiring
sys_membarrier() to be
available. This gives a small speedup when
supported by the kernel, and aborts in the library constructor if not
- Link with
The QSBR flavor of RCU needs to have each reader thread executing
rcu_quiescent_state() periodically to progress.
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.
- Link with
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.
- Link the application with
Version of the library that requires a signal, typically
be overridden with
-DSIGRCU by modifying
- Link with
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_bp_unregister_thread() all become nops, whereas calling
urcu_bp_register_thread() becomes optional. The state is dealt with by
the library internally at the expense of read-side and write-side
Each thread that has reader critical sections (that uses
urcu_<flavor>_read_unlock() must first
register to the URCU library. This is done by calling
urcu_<flavor>_register_thread(). Unregistration must be performed
before exiting the thread by using
Reader critical sections must be protected by locating them between
Inside that lock,
rcu_dereference() may be called to read an RCU
rcu_xchg_pointer() may be called anywhere.
urcu_<flavor>_synchronize_rcu() must be called. When it
returns, the old values are not in usage anymore.
- Follow instructions for either
liburcu-deferfunctionality is pulled into each of those library modules.
urcu_<flavor>_defer_rcu()primitive to enqueue delayed callbacks. Queued callbacks are executed in batch periodically after a grace period. Do not use
urcu_<flavor>_defer_rcu()within a read-side critical section, because it may call
urcu_<flavor>_synchronize_rcu()if the thread queue is full. This can lead to deadlock or worse.
- Requires that
urcu_<flavor>_defer_barrier()must be called in library destructor if a library queues callbacks and is expected to be unloaded with
Its API is currently experimental. It may change in future library releases.
- Follow instructions for either
urcu-call-rcufunctionality is pulled into each of those library modules.
- Provides the
urcu_<flavor>_call_rcu()primitive to enqueue delayed callbacks in a manner similar to
urcu_<flavor>_defer_rcu(), but without ever delaying for a grace period. On the other hand,
urcu_<flavor>_call_rcu()'s best-case overhead is not quite as good as that of
urcu_<flavor>_call_rcu()to allow asynchronous handling of RCU grace periods. A number of additional functions are provided to manage the helper threads used by
urcu_<flavor>_call_rcu(), but reasonable defaults are used if these additional functions are not invoked. See
doc/rcu-api.mdin userspace-rcu documentation for more details.
Being careful with signals
liburcu-signal 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
Read-side critical sections are allowed in a signal handler,
except those setup with
liburcu-mb. Be careful, however, to disable these signals
between thread creation and calls to
because a signal handler nesting on an unregistered thread would not be
allowed to call
Read-side critical sections are not allowed in a signal handler with
liburcu-qsbr, unless signals are disabled explicitly around each
urcu_qsbr_quiescent_state() calls, when threads are put offline and around
urcu_qsbr_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
urcu_<flavor>_synchronize_rcu() and RCU read-side with mutexes. If
urcu_<flavor>_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
urcu_qsbr_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
Special care must be taken for applications performing
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
require that all registrations (as reader,
threads) should be released before a
fork() is performed, except for the
rather common scenario where
fork() is immediately followed by
the child process. The only implementation not subject to that rule is
liburcu-bp, which is designed to handle
fork() by calling
Applications that use
urcu_<flavor>_call_rcu() and that
without doing an immediate
exec() must take special action. The
parent must invoke
urcu_<flavor>_call_rcu_before_fork() before the
urcu_<flavor>_call_rcu_after_fork_parent() after the
fork(). The child process must invoke
urcu_<flavor>_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 (
free(), ...) within
urcu_<flavor>_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.
- call to
free()from callbacks executed within
urcu_<flavor>_call_rcuatfork 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
By default the library is configured with internal debugging self-checks disabled.
For always-on debugging self-checks: ./configure --enable-rcu-debug
For fine grained enabling of debugging self-checks, build userspace-rcu with DEBUG_RCU defined and compile dependent applications with DEBUG_RCU defined when necessary.
Warning: Enabling this feature result in a performance penalty.
DEBUG_YIELD is used to add random delays in the code for testing
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:
theoretically yielding slightly better performance.
By default the library is configured with extra debugging checks for lock-free hash table iterator traversal disabled.
Building liburcu with --enable-cds-lfht-iter-debug and rebuilding application to match the ABI change allows finding cases where the hash table iterator is re-purposed to be used on a different hash table while still being used to iterate on a hash table.
This option alters the rculfhash ABI. Make sure to compile both library and application with matching configuration.
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.
There is an application vs library compatibility issue between applications built using Userspace RCU 0.10 headers linked against Userspace RCU 0.11 or 0.12 shared objects. The problem occurs as follows:
An application executable is built with _LGPL_SOURCE defined, includes any of the Userspace RCU 0.10 urcu flavor headers, and is built without the -fpic compiler option.
The Userspace RCU 0.10 library shared objects are updated to 0.11 or 0.12 without rebuilding the application.
The application will hang, typically when RCU grace period (synchronize_rcu) is invoked.
Some possible work-arounds for this are:
Rebuild the application against Userspace RCU 0.11+.
Rebuild the application with -fpic.
Upgrade Userspace RCU to 0.13+ without installing 0.11 nor 0.12.
You can contact the maintainers on the following mailing list: