--- /dev/null
+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
+ - 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 (see [`doc/rcu-api.txt`](doc/rcu-api.txt))
+ - `cmm_`: Concurrent Memory Model
+ - `caa_`: Concurrent Architecture Abstraction
+ - `cds_`: Concurrent Data Structures
+ (see [`doc/cds-api.txt`](doc/cds-api.txt))
+ - `uatomic_`: Userspace Atomic
+ (see [`doc/uatomic-api.txt`](doc/uatomic-api.txt))
+
+
+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 `Makefile.build.inc`.
+
+
+### 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.
+
+
+### 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.
+
+
+### 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/rcu-api.txt`](doc/rcu-api.txt) 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
+`Makefile.build.inc`.
+
+
+### 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.
+
+
+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`.
\ No newline at end of file
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