![]() ![]() This means that A must never be acquired after B or C, and B must not be acquired after C. If A is taken before B at one point and B before C at another, the hierarchy is A-B-C. Whenever it is necessary to have more than one lock acquired at a time, each lock should have a clear precedence. The common denominator among all deadlocks is that lock hierarchy is not respected. Single-thread deadlocks can occur when a thread attempts to take a lock that it already owns. However, the farther apart the lock acquisitions are, the more likely these become. These deadlocks require a certain degree of "bad luck" since they rely on a number of things happening simultaneously. Three-way deadlocks between three threads and three locks are common - and even five-part or six-part deadlocks occur occasionally. At best this causes the threads involved to halt, and at worst causes the system to stop responding.ĭeadlocks are not limited to two threads and two resources. ![]() If both sequences happen at the same time, Thread 1 will never get Lock B because it is owned by Thread 2, and Thread 2 will never get Lock A because it is owned by Thread 1. The most common form of deadlock occurs when two or more threads wait for a resource that is owned by the other thread. Causes of DeadlocksĪ deadlock is caused when two or more threads come into conflict over some resource, in such a way that no execution is possible. The Deadlock Detection option of Driver Verifier, along with the !deadlock kernel debugger extension, is an effective tool for making sure your code avoids poor use of these resources.ĭeadlock Detection is supported only in Windows XP and later versions of Windows. This Driver Verifier option will detect code logic that has the potential to cause a deadlock at some future point. You can change the default behavior by updating the deadlock priority.Deadlock Detection monitors the driver's use of resources which need to be locked - spin locks, mutexes, and fast mutexes. When a deadlock occurs, by default, SQL Server chooses a deadlock "victim" by identifying which of the two processes will use the least resources to roll back, and then returns error message 1205. If appropriate, use the lowest possible isolation level for the user connection running the transaction.If the data being locked is not modified very frequently, consider using NOLOCK to prevent locking.If appropriate, reduce lock escalation by using ROWLOCK or PAGLOCK. Develop applications that obtain locks at the latest possible time, and release them at the earliest possible time. If you do need to read the same data more than once, cache it by storing it in a variable or an array, and then re-reading it from there. Reduce the number of round trips between your application and SQL Server by using stored procedures or by keeping transactions within a single batch.Keep transactions as short as possible.Do not allow any user input during transactions. ![]()
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