Subject: [Boost-commit] svn:boost r50696 - sandbox/thread_safe_signals/trunk/libs/signals2/doc
Date: 2009-01-20 22:43:07
Date: 2009-01-20 22:43:06 EST (Tue, 20 Jan 2009)
New Revision: 50696
Added beginnings of section on thread-safety.
sandbox/thread_safe_signals/trunk/libs/signals2/doc/thread_safety.xml (contents, props changed)
Text files modified:
sandbox/thread_safe_signals/trunk/libs/signals2/doc/Makefile | 1 +
sandbox/thread_safe_signals/trunk/libs/signals2/doc/signals.xml | 1 +
2 files changed, 2 insertions(+), 0 deletions(-)
--- sandbox/thread_safe_signals/trunk/libs/signals2/doc/Makefile (original)
+++ sandbox/thread_safe_signals/trunk/libs/signals2/doc/Makefile 2009-01-20 22:43:06 EST (Tue, 20 Jan 2009)
@@ -10,6 +10,7 @@
+ thread_safety.xml \
--- sandbox/thread_safe_signals/trunk/libs/signals2/doc/signals.xml (original)
+++ sandbox/thread_safe_signals/trunk/libs/signals2/doc/signals.xml 2009-01-20 22:43:06 EST (Tue, 20 Jan 2009)
@@ -46,6 +46,7 @@
+ <xi:include href="thread_safety.xml"/>
--- (empty file)
+++ sandbox/thread_safe_signals/trunk/libs/signals2/doc/thread_safety.xml 2009-01-20 22:43:06 EST (Tue, 20 Jan 2009)
@@ -0,0 +1,86 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE section PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
+<section last-revision="$Date: 2007-06-12 14:01:23 -0400 (Tue, 12 Jun 2007) $" id="signals2.thread-safety">
+ <using-namespace name="boost"/>
+ <using-namespace name="boost::signals2"/>
+ The primary motivation for Boost.Signals2 is to provide a version of
+ the original Boost.Signals library which can be used safely in a
+ multi-threaded environment.
+ This is achieved primarily through two changes from the original Boost.Signals
+ API. One is the introduction of a new automatic connection management scheme
+ relying on <classname>shared_ptr</classname> and <classname>weak_ptr</classname>,
+ as described in the <link linkend="signals2.tutorial.connection-management">tutorial</link>.
+ The second change was the introduction of a <code>Mutex</code> template type
+ parameter to the <classname>signal</classname> class. This section details how
+ <code>Mutex</code> objects are used by a signal to protect its internal
+ state from concurrent access and modification.
+ Each signal object default-constructs a <code>Mutex</code> object to protect
+ its internal state. Furthermore, a <code>Mutex</code> is created
+ for each slot which is connected to the signal, to protect the
+ associated signal-slot connection.
+ A signal's mutex is automatically locked whenever any of the
+ signal's methods are called. The mutex is usually held until the
+ method completes, however there is one major exception to this rule. When
+ a signal is invoked by calling
+ <methodname alt="signalN::operator()">signal::operator()</methodname>,
+ the invocation first acquires a lock on the signal's mutex. Then
+ it obtains a handle to the signal's slot list and combiner. Next
+ it releases the signal's mutex, before it invokes the combiner to
+ iterate through the slot list. Thus no mutexes are held by the
+ signal while a slot is executing. This design choice
+ makes it impossible for user code running in a slot
+ to deadlock against any of the
+ mutexes used internally by the Boost.Signals2 library.
+ It also prevents slots from accidentally attempting to
+ recursively lock any of the library's internal mutexes.
+ Therefore, if you invoke a signal concurrently from multiple threads,
+ it is possible for the signal's combiner to be invoked concurrently
+ and thus the slots to execute concurrently.
+ During a signal invocation, the following steps are performed in order to
+ find the next callable slot while iterating through the slot list.
+ <para>The <code>Mutex</code> associated with the connection to the
+ slot is locked.</para>
+ <para>All the tracked <classname>weak_ptr</classname> associated with the
+ slot are copied into temporary <classname>shared_ptr</classname> which
+ will be kept alive until we are done with the slot. If this fails due
+ to any of the
+ <classname>weak_ptr</classname> having expired, the connection is
+ automatically disconnected.
+ The connection to the slot is checked to see if it is blocked
+ or disconnected, and then the mutex is unlocked. If the connection
+ was either blocked or disconnected, we
+ start again from the beginning with the next slot in the slot list.
+ Otherwise, we will execute the slot when the combiner next
+ dereferences its slot call iterator (unless the combiner should increment
+ the iterator without ever dereferencing it).
+ Note that since we unlock the connection's mutex before executing
+ its associated slot, it is possible a slot will still be executing
+ after it has been disconnected by a
+ <code><methodname>connection::disconnect</methodname>()</code>, if
+ the disconnect was called concurrently with signal invocation.
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