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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 Tested on: - Linux all architectures - FreeBSD 8.2/8.3/9.0/9.1/10.0 i386/amd64 - Solaris 10/11 i386 - Cygwin i386/amd64 - MacOSX amd64 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 (see [`doc/rcu-api.md`](doc/rcu-api.md)) - `cmm_`: Concurrent Memory Model - `caa_`: Concurrent Architecture Abstraction - `cds_`: Concurrent Data Structures (see [`doc/cds-api.md`](doc/cds-api.md)) - `uatomic_`: Userspace Atomic (see [`doc/uatomic-api.md`](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`, - `signal`, - `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-signal` 1. `#include <urcu/urcu-signal.h>` 2. 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` 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. ### Usage of `liburcu-defer` - Follow instructions for either `liburcu-memb`, `liburcu-qsbr`, `liburcu-mb`, `liburcu-signal`, 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`, `liburcu-signal`, 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`](doc/rcu-api.md) in userspace-rcu documentation for more details. ### Being careful with signals The `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 `signal(7)`. 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 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. ### 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. 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`.