From d589a916c59393f17a063db2752db89e91eb2296 Mon Sep 17 00:00:00 2001 From: Philippe Proulx Date: Mon, 5 May 2014 13:03:20 -0400 Subject: [PATCH] Modernize README using Markdown Signed-off-by: Philippe Proulx Signed-off-by: Mathieu Desnoyers --- README | 341 ---------------------------------------------- README.md | 399 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 399 insertions(+), 341 deletions(-) delete mode 100644 README create mode 100644 README.md diff --git a/README b/README deleted file mode 100644 index 00eaae9..0000000 --- a/README +++ /dev/null @@ -1,341 +0,0 @@ -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, Linux x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64, -S390, S390x, ARM 32/64, MIPS, Alpha, ia64, Sparcv9 32/64, Tilera, and -hppa/PA-RISC are supported. -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) -- cmm_ : Concurrent Memory Model -- caa_ : Concurrent Architecture Abstraction -- cds_ : Concurrent Data Structures (see doc/cds-api.txt) -- uatomic_: Userspace Atomic (see 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 - - * #include - * 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 - - * #include - * 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 - - * #include - * 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. - -Usage of liburcu-signal - - * #include - * 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. - -Usage of liburcu-bp - - * #include - * 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 provided for each of - these 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 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 diff --git a/README.md b/README.md new file mode 100644 index 0000000..b33095c --- /dev/null +++ b/README.md @@ -0,0 +1,399 @@ +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 ` + 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 ` + 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 ` + 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 ` + 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 ` + 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 -- 2.34.1