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Dead

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Dead

  1. 1. What is Deadlock?Deadlock occurs when each thread in a group of threads is waiting for a condition that canonly be caused by another member of the the group. For example, given threads T1 andT2, it is possible that T1 is holding a resource X while waiting for resource Y to becomeavailable. Meanwhile, T2 is holding resource Y and is waiting for resource X to becomeavailable. This condition is known, in the computing literature, as deadlock.How to Know When Deadlock Has Hit You?Aside from having to debug a distributed program, deadlock is perhaps one of the mostdifficult programming errors to correct. At times, deadlock appears to behave almost like aninfinite loop. Sometimes, the program will appear to run normally by not “locking up.” Often,the program will need to be run multiple times to even notice the problem. The tragedy ofit all is that the results are usually not reproducible nor consistent.There are four ways in which deadlock can occur. To eliminate deadlock, all you need to dois find a way to eliminate one of the conditions that can cause it. It is not always desirable toeliminate a given condition. We will return to a discussion of this point shortly; however, weconsider here the possibility of eliminating a circular wait condition.Four Conditions of DeadlockMutual exclusion: It must not be possible for more than one thread to use a resource atthe same time.Hold and wait: Threads must hold resources and wait for others to become available.Deadlock is not possible if no thread ever holds more than one resource at a time. Noris it possible if a thread can acquire more than one resource, but acquires all itsresources at one instant.No preemption: It must not be possible to remove a resource from a thread that holdsit. Only the thread that holds a resource can give it up.Circular wait: There must be a cycle of threads, each holding at least one resource andwaiting to acquire one of the resources that the next thread in the cycle holds.

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