为什么TABLOCKX没有死锁

Question

I need help to understand why tablockx does have deadlock in multithreaded application.

I tried tablockx with SqlQueryStress will multiple threads still i am not getting any deadlock

Answer 1

Because deadlocks can pretty much only occur when you use row-level locking. The most basic example of a deadlock is:

• Transaction A is updating Rows 1,2,3
• Transaction B is updating Rows 3,2,1
• A modifies row 1, opens (locks) row 2, and waits for row 3
• B locks row 3, and waits for row 2

Since A has 2 and needs 3, and B has 3 and needs 2, they will wait forever unless the query engine catches it, which is what causes a deadlock. Basically it's an infinite logical loop of waits that get cancelled.

When you use a TABLOCK , which indicates to the engine to use table locking instead of row locking, Transaction A would either wait for the whole table to be available if B already had it open, or would place a lock on the whole table if it were available, blocking any other transaction from modifying the table until it is complete.