There are four general properties that must hold to produce a deadlock: But first, let's review the basics of deadlocks.Ī deadlock occurs when a set of concurrent workers are waiting on each other to make forward progress before any of them can make forward progress. The former of these disciplines is the primary topic of this article. You can combat deadlock using a combination of disciplined locking practices and aggressive stress testing on a range of hardware platforms. And even when you find the cause, you may have already painted yourself into a corner. You may find a small number of them by accident in your ordinary functional test suites, but it's more likely that many will lie dormant in your shipping product, waiting to spring onto an unsuspecting customer. Deadlocks, while more obvious than races (in the simplest case, the program just stops dead in its tracks), are still difficult to locate and eliminate. When the unexpected occurs, it can be extremely difficult to replay the steps leading up to the incident.
Today's shared memory programming models require that the programmer think about the possible interleaving of instructions throughout program execution. Assuming your system does nothing to mitigate deadlocks, a subtle misstep here is apt to gum up the complex machinery of your program. Disparate segments of code, each acquiring and releasing locks at varying granularities, tend to interact with each other in unforeseen ways. There's a trade-off between correctness and sequential performance (coarse granularity of locks) and parallel scalability (fine granularity of locks). Forgetting to lock memory in motion can lead to race conditions that lead to crashes at best and corrupt data at worst. Clearly the worst type of a hang is one that never comes back-in other words, a deadlock.Įngineering large concurrent applications using the standard Windows® shared-memory toolkit of threads and locks is significantly more involved than it might appear at first glance.
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Code that blocks a GUI thread's execution can prevent a window from processing incoming user input and system-generated messages, resulting in an app that is "Not Responding." Even non-GUI programs can suffer from responsiveness problems when using shared resources or when performing interthread, interprocess, or intermachine communication. They can result from slow I/O, a computationally expensive algorithm, or mutually exclusive access to resources, in any case reducing the overall responsiveness of an application. Hangs can range from temporary to permanent.
Users frequently shut down a frozen application before such information can be captured, meaning there's no tell-tale stack trace to help you find the source of the problem. Unlike a crash, an application hang may not produce a crash dump or trigger custom failure logic. They're terribly difficult to find before shipping, and even more difficult to debug after an application has been deployed.
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This article uses the following technologies:Ĭode download available at: Deadlocks.exe(188 KB)Īpplication hangs are one of the most frustrating situations a user can experience.
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