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:
- Linux x86 (i386, i486, i586, i686)
- x86 64-bit
- PowerPC 32/64
- S390, S390x
- ARM 32/64
- MIPS
- NIOS2
- 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:
- 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.
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 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 (seedoc/rcu-api.md
)cmm_
: Concurrent Memory Modelcaa_
: Concurrent Architecture Abstractioncds_
: Concurrent Data Structures (seedoc/cds-api.md
)uatomic_
: Userspace Atomic (seedoc/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 theurcu.h
orurcu-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.
liburcu
Usage of #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. 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.
liburcu-qsbr
Usage of #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.
liburcu-mb
Usage of #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.
liburcu-signal
Usage of #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
.
liburcu-bp
Usage of #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.
liburcu-defer
Usage of - Follow instructions for either
liburcu
,liburcu-qsbr
,liburcu-mb
,liburcu-signal
, orliburcu-bp
above. Theliburcu-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 usedefer_rcu()
within a read-side critical section, because it may callsynchronize_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 withdlclose()
.
Its API is currently experimental. It may change in future library releases.
urcu-call-rcu
Usage of - Follow instructions for either
liburcu
,liburcu-qsbr
,liburcu-mb
,liburcu-signal
, orliburcu-bp
above. Theurcu-call-rcu
functionality is pulled into each of those library modules. - Provides the
call_rcu()
primitive to enqueue delayed callbacks in a manner similar todefer_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 ofdefer_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 bycall_rcu()
, but reasonable defaults are used if these additional functions are not invoked. Seedoc/rcu-api.md
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()
).
fork()
Interaction with 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 withincall_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.
DEBUG_RCU
& --enable-rcu-debug
Usage of 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 urserspace-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
Usage of 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.
Contacts
You can contact the maintainers on the following mailing list:
lttng-dev@lists.lttng.org
.