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Userspace RCU Implementation
by Mathieu Desnoyers and Paul E. McKenney
BUILDING
--------
./bootstrap (skip if using tarball)
./configure
make
make install
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, x86 (i386, i486, i586, i686), x86 64-bit, PowerPC 32/64, S390, S390x
and Sparcv9 32/64 are supported. Only tested on Linux so far, but should
theoretically work on other operating systems.
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.
Usage of liburcu
* #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.
Usage of liburcu-qsbr
* #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.
Usage of liburcu-mb
* #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.
Usage of liburcu-signal
* #include <urcu-signal.h>
* 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 <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.
Usage of liburcu-defer
* #include <urcu-defer.h>
* Link with "-lurcu-defer", and also with one of the urcu library
(either urcu, urcu-bp, urcu-mb or urcu-qsbr).
* 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.
* Provides defer_rcu_ratelimit() primitive, which acts just like
defer_rcu(), but takes an additional rate limiter callback forcing
synchronized callback execution of the limiter returns non-zero.
* 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.
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 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.
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.
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