Dr. Dobb's Journal - August 2008 - (Page 54)

Effective Concurrency Dealing With Deadlock The good news is that we have tools and techniques to detect and prevent many kinds of deadlock. In particular, consider ways to deal with two popular forms of blocking: Waiting to acquire a mutex, and waiting to receive a message. First, to prevent locking cycles within the code you control, use lock hierarchies and other ordering techniques to ensure that your program always acquires all mutexes in the same order [1]. Note: Not all locks need to participate directly in a lock hierarchy; for example, some groups of related locks already have a total ordering among themselves, such as a chain of locks always traversed in the same order (hand-over-hand locking along a singly linked list, for instance). Second, to prevent many kinds of message cycles, disciplines have been proposed to express contracts on communication channels, which helps to impose a known ordering on messages. One example is WS-CDL [3]. The general idea is to define rules for the orders in which messages must be sent and received, which can be enforced statically or dynamically to ensure that components communicating via messages won’t tie themselves into a knot. A message ordering contract is typically expressed as some form of state machine. For example, here’s how we might express a simple interface in pseudocode for a buyer requesting a purchase order: contract PORequest { // messages from requester to // provider (arbitrarily // choose “in” to be from the // provider’s point of view) in void Request( Info ); // messages from provider // to requester out void Ack(); out void Response( Details ); // now declare states and transitions state Start { Request -> RequestSent; } state RequestSent { Ack -> RequestReceived; } state RequestReceived { Response -> End; } } us a way to guarantee that a provider fulfills the contract, or at least to detect violations. General Deadlock Detection The bad news is that, as far as I know, there’s no tool on the planet that identifies all kinds of blocking cycles for you, especially ones that consist of more than one kind of blocking. Lock hierarchies only guarantee freedom from deadlock among locks in the code you control; message contracts on communication channels only guarantee freedom from deadlock among messages. Probably the best you can do today is to roll your own deadlock detection in code, by adopting a discipline like the following: • Identify every condition or resource that can be waited for (a mutex, a message, a value of an atomic variable, an exclusive use of a file) and give it a unique name by creating a dummy object to represent it. • Instrument every “start wait” and “end wait” point in your code by calling two helper functions: The first records that a condition or resource is about to be waited for (e.g,. StartWait(thing)) and should internally record the current thread’s ID and the thing being waited for; it can also check to see if there is now a waiting cycle among threads and resources, and report the deadlock if that occurs. The second records that a wait has ended (e.g., EndWait(thing)) and will remove the waiting edge. To illustrate how we can apply such a discipline, consider this deadlock between a mutex and a message that arises in the execution A->B->C: // Global data Mutex mut; MessageQueue queue; // Thread 1 mut.lock(); queue.receive(); mut.unlock(); // Thread 2 mut.lock(); queue.send( msg ); // Thread 1 StartWait( w_mut ); mut.lock(); EndWait( w_mut ); StartWait( w_queue ); queue.receive(); EndWait( w_queue ); mut.unlock(); // Thread 2 StartWait( w_mut ); mut.lock(); EndWait( w_mut ); queue.send( msg ); // A // B: blocks // C: blocks That’s a sketch of the idea. It’s only a coding discipline, but it’s an approach that can help you to instrument all waiting in a unified way, at least within the code you control. Summary Deadlock can arise whenever there is a blocking (or waiting) cycle among concurrent tasks, where each one is waiting for the next to produce some value or release some resource. Eliminate deadlocks as much as possible by applying ordering techniques like lock hierarchies and message contracts; these techniques are important, even though they are incomplete because each one deals with only a specific kind of waiting. Then consider adding your own deadlock detection by instrumenting the wait points in your code in a uniform way. Whimsically, we might say that a more correct name for deadlock could be “deadblock” …but the world has already adopted a common spelling that’s one letter shorter, and this isn’t the time to try to change that. When reasoning about deadlock, remember not to forget the important “b” even though , it’s silent in pronunciation and in the common spelling. Notes // A // B: blocks // C: blocks The only valid message order is Request-> Ack->Response. If a provider could mistakenly send a Response without first sending an Ack, a requester could hang indefinitely waiting for the missing Ack message. Expressing the message order contract gives 54 Dr. Dobb’s Journal l www.ddj.com l August 2008 We could apply a wait start/end instrumentation discipline as follows. Here the implementation of StartWait and EndWait is left for the reader, but should record which threads are waiting for which objects as described above: // Global data Mutex mut; WaitableObject w_mut; MessageQueue queue; WaitableObject w_queue; [1] H. Sutter. “Use Lock Hierarchies To Avoid Deadlock” (DDJ, January 2008). [2] H. Sutter. “Avoid Calling Unknown Code While Inside a Critical Section” (DDJ, December 2007). [3] Web Services Choreography Description Language (WS-CDL) (W3C, 2005) (www.w3 .org/TR/ws-cdl-10/). DDJ Herb is a software development consultant, a software architect at Microsoft, and chair of the ISO C++ Standards committee. He can be contacted at www.gotw.ca. http://www.w3.org/TR/ws-cdl-10/ http://www.w3.org/TR/ws-cdl-10/ http://www.gotw.ca http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - August 2008

Dr. Dobb's Journal - August 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer’s Notebook A Conversation with Christos Papadimitriou OpenGL and Mobile Devices: Round 2 Ellipse Specification Using Vectors Embed Custom GUIs in WPF Building RIAs on J2EE Foundations Disentangling Concepts in Object-Oriented Systems The Agile Edge Effective Concurrency Swaine’s Flames

Dr. Dobb's Journal - August 2008

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