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

d12sutt_p4ma 10/11/07 9:48 AM Page 59 The second approach is to pass copies of data, which solves the correctness problem at the expense of space and performance. Variants of this approach include passing a subset of the data, and passing the copies via messages to run the callee asynchronously. To improve Example 1, for instance, it might be appropriate to change the RenderElements method to hold the lock only long enough to take copies of the necessary shared information in a local container, then doing processing outside the lock, passing the copied elements. (This could be inappropriate if the data is very expensive to copy, or the callee needs to work on the real data.) Alternatively, perhaps the callee doesn’t really need all the information it gets from being given direct access to the protected object, and it would be both sufficient and efficient to pass copies of just those parts of the data the callee does need. The third option is to reduce the power or granularity of the critical section, which implicitly trades off ease-of-use because making your synchronization finer-tuned and/or finer-grained also makes it harder to code correctly. One example of reducing the power of the critical section is to replace a mutex with a reader-writer mutex so that multiple concurrent readers are allowed; if the only deadlocks could arise among threads that are only performing reads of the protected resources, then this can be a valid solution by enabling the use of a read-only lock instead of a read-write lock. And an example of making the critical section finergrained is to replace a single mutex protecting a large data structure with mutexes protecting parts of the structure; if the only deadlocks possible are among threads that use different parts of the structure, then this can be a valid solution (Example 1 is not such a case). The fourth option is to tell the callee not to block, which trades off enforceability. In particular, if you have the power to impose requirements on the callee (as you do with plug-ins to your software, but not with simple calls into existing third-party libraries), then you can require them to not take locks or otherwise perform blocking actions. Alas, these requirements are typically going to be limited to documentation, and are typically not enforceable automatically. Tell the callee what (not) to do, and hope he follows the yellow brick road. Summary Be aware of the many opportunities modern languages give us to call “someone else’s code,” and eliminate external opportunities for deadlock by not calling unknown code from within a critical section. If you additionally eliminate internal opportunities for deadlock by applying a lock hierarchy discipline within the code you control, your use of locks will be highly likely to be correct…and we’ll consider lock hierarchies next month. Stay tuned.. DDJ Herb is a software architect at Microsoft and chair of the ISO C++ Standards committee. He can be contacted at www.gotw.ca. 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 December 2007 l www.ddj.com l Dr. Dobb’s Journal 59 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 - December 2007

Dr. Dobb's Journal - December 2007 Contents Hmmmm Alia Vox Developer Diaries Developer’s Notebook Computer Books: Reading Between the Lines Conversations Query Anything with SQLite XQuery Web Maps with the Google Map API OpenALM and Its Manifesto Transactional Programming Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - December 2007

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