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, the following architectures are supported:

• x86 (i386, i486, i586, i686)
• amd64 / x86_64
• PowerPC 32/64
• S390, S390x
• ARM 32/64
• MIPS
• NIOS2
• Alpha
• ia64
• Sparcv9 32/64
• Tilera
• hppa/PA-RISC
• m68k
• RISC-V
• LoongArch

Tested on:

• Linux all architectures
• FreeBSD 13 i386/amd64
• Cygwin i386/amd64
• MacOS amd64/arm64

Should also work on:

• Android
• NetBSD 5
• OpenBSD
• Solaris

(more testing needed before claiming support for these OS).

Toolchain support

The C compiler used needs to support at least C99. The C++ compiler used needs to support at least C++11. The oldest GCC version officialy supported and tested is 4.8.

Older GCC versions might still work 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.
• 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.

Build system

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 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.

API

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

• rcu_: Read-Copy Update (see doc/rcu-api.md)
• cmm_: Concurrent Memory Model
• caa_: Concurrent Architecture Abstraction
• cds_: Concurrent Data Structures (see doc/cds-api.md)
• uatomic_: Userspace Atomic (see doc/uatomic-api.md)

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.

There are multiple flavors of liburcu available:

• memb,
• qsbr,
• mb,
• bp.

The API members start with the prefix urcu_<flavor>_, where <flavor> is the chosen flavor name.

Usage of liburcu-memb

1. #include <urcu/urcu-memb.h>
2. Link the application with -lurcu-memb

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. 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 sys_membarrier() is supported by the kernel, and aborts in the library constructor if not supported.

Usage of liburcu-qsbr

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

1. #include <urcu/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. urcu_bp_init(), and 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 performance.

Initialization

Each thread that has reader critical sections (that uses urcu_<flavor>_read_lock()/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 urcu_<flavor>_unregister_thread().

Reading

Reader critical sections must be protected by locating them between calls to urcu_<flavor>_read_lock() and urcu_<flavor>_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, urcu_<flavor>_synchronize_rcu() must be called. When it returns, the old values are not in usage anymore.

As an alternative to urcu_<flavor>_synchronize_rcu(), it is also possible to use the urcu polling mechanism to wait for a grace period to elapse. This can be done by using urcu_<flavor>_start_poll_synchronize_rcu() to start the grace period polling, and then invoke urcu_<flavor>_poll_state_synchronize_rcu(), which returns true if the grace period has completed, false otherwise.

Usage of liburcu-defer

• Follow instructions for either liburcu-memb, liburcu-qsbr, liburcu-mb, or liburcu-bp above. The liburcu-defer functionality is pulled into each of those library modules.
• Provides 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 dlclose().

Its API is currently experimental. It may change in future library releases.

Usage of urcu-call-rcu

• Follow instructions for either liburcu-memb, liburcu-qsbr, liburcu-mb, or liburcu-bp above. The urcu-call-rcu functionality 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>_defer_rcu().
• Provides 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.md in userspace-rcu documentation for more details.

Being careful with signals

Read-side critical sections are allowed in a signal handler, except those setup with sigaltstack(2), with liburcu-memb and liburcu-mb. Be careful, however, to disable these signals between thread creation and calls to urcu_<flavor>_register_thread(), because a signal handler nesting on an unregistered thread would not be allowed to call urcu_<flavor>_read_lock().

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 calls to 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 urcu_qsbr_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 urcu_bp_before_fork, urcu_bp_after_fork_parent and urcu_bp_after_fork_child.

Applications that use urcu_<flavor>_call_rcu() and that fork() without doing an immediate exec() must take special action. The parent must invoke urcu_<flavor>_call_rcu_before_fork() before the fork() and 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 (malloc(), calloc(), 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.

Combining e.g.:

• call to free() from callbacks executed within urcu_<flavor>_call_rcu worker threads,
• executing urcu_<flavor>_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 & --enable-rcu-debug

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.

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.

Usage of --enable-cds-lfht-iter-debug

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.

Usage of --enable-compiler-atomic-builtins

Building liburcu with --enable-compiler-atomic-builtins implements the uatomic API with the compiler atomic builtins if supported.

Make targets

In addition to the usual make check target, Userspace RCU features make regtest, make short_bench and make long_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 short_bench: short benchmarks, 3 seconds per test.
• make long_bench: long (many hours) benchmarks, 30 seconds per test.

Known issues

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.

Contacts

You can contact the maintainers on the following mailing list: lttng-dev@lists.lttng.org.

IRC channel: #lttng on the OFTC network

Bug tracker: Userspace RCU bug tracker

Code review: userspace-rcu project on LTTng Review

Continuous integration: Userspace RCU on LTTng's CI

GitHub mirror: urcu/userspace-rcu

Patches are principally submitted and reviewed on LTTng Review, but may also be submitted to the mailing list with the subject prefix PATCH urcu or by pull request on the GitHub mirror.