Dr. Dobb's Journal - November 2007 - (Page 59)

d11sutt_p6ma 9/10/07 11:29 AM Page 59 one thread before the synchronization, and another thread after the synchronization), and there isn’t enough synchronization here to do that. Let’s see why. The problem is that this anti-idiom is trying to abusively invert the usual meanings of a locked section. Thread 2 is abusively trying to use Thread 1’s entering a critical section as a release event, which it isn’t. Remember, entering a critical section is an acquire event, and leaving a critical section is a release event. On this, the whole world depends, as we saw last month [1]. In particular, compiler and processor optimizations are guaranteed to respect normal critical section boundaries, and therefore not change the semantics of correctly synchronized code; specifically, they respect the rule that code can be reordered to move into, but not out of, a critical section. In the context of Thread 1, that means an optimizer or processor is free to actually execute line a after line b… and therefore, Thread 2 could well see a value of 0 for x, despite its attempts to abuse the global visibility of locking. In reality, the code may happen to work all the time on a given system that doesn’t happen to reorder lines a and b. The original programmer might be long gone by the time some other poor sod tries to port it to a new compiler or platform that does manifest the race, and gets to experience the joy of debugging the intermittent problem over a holiday weekend. (Note that, besides the issues we’ve discussed, this code has other potential problems; for example, Thread 2 can wait forever if Thread 1’s lock is taken too early.) Treat a critical section as you would treat another person: Turning either inside out is at least cruel, and usually illegal. For all the sensational, lurid, and unsavory details, see [3]. ordered atomic variable; and exit a critical section by releasing a lock or writing to an ordered atomic variable. Never pervert these meanings; in particular, don’t abuse trylock to (try to) make a lock acquire in another thread act like the end of a critical section. Notes [1] H. Sutter. “Use Critical Sections (Preferably Locks) to Eliminate Races,” Dr. Dobb’s Journal, October 2007. [2] H. Sutter. “The Trouble With Locks,” C/C++ Users Journal, March 2005. Available at http://gotw.ca/publications/mill36.htm. [3] H. Boehm. “Reordering Constraints for Pthread-Style Locks,” Proceedings of the 12th ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming (PPoPP’07), March 2007. DDJ Herb is a software architect at Microsoft and chair of the ISO C++ Standards committee. He can be contacted at www.gotw.ca. Summary Apply critical sections consistently to protect shared objects: Enter a critical section by taking a lock or reading from an Make Networked and Multithreaded Apps a Breeze with ACE! • Wrap up projects sooner –– plug your app-specific code into ACE's C++ frameworks and go • Stress-free porting across a wide array of platforms • Open Source with unlimited commercial use –– no strings attached • Riverace offers world-class expertise, training, and support to help you make the most of ACE's power and flexibility • Come on over and download the latest ACE source kit free www.riverace.com · 888 384 8154 Adding Service to Open Source Software November 2007 l www.ddj.com l Dr. Dobb’s Journal 59 http://gotw.ca/publications/mill36.htm http://www.gotw.ca http://www.riverace.com http://www.riverace.com http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - November 2007

Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook Smart Compilers - But Smart Enough? Conversations Grid-Enabling Resource-Intensive Applications Distributed Computing: Windows and Linux Adobe AIR: Desktop/Web Convergence Transparency on Demand Reusable Associations Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - November 2007

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