What is a deadlock?

A lock occurs when multiple processes try to access the same resource at the same time.

One process loses out and must wait for the other to finish.

A deadlock occurs when the waiting process is still holding on to another resource that the first needs before it can finish.

So, an example:

Resource A and resource B are used by process X and process Y

• X starts to use A.
• X and Y try to start using B
• Y 'wins' and gets B first
• now Y needs to use A
• A is locked by X, which is waiting for Y

The best way to avoid deadlocks is to avoid having processes cross over in this way. Reduce the need to lock anything as much as you can.

In databases avoid making lots of changes to different tables in a single transaction, avoid triggers and switch to optimistic/dirty/nolock reads as much as possible.

Let me explain a real world (not actually real) example for a deadlock situation from the crime movies. Imagine a criminal holds an hostage and against that, a cop also holds an hostage who is a friend of the criminal. In this case, criminal is not going to let the hostage go if cop won't let his friend to let go. Also the cop is not going to let the friend of criminal let go, unless the criminal releases the hostage. This is an endless untrustworthy situation, because both sides are insisting the first step from each other.

Criminal & Cop Scene

So simply, when two threads needs two different resources and each of them has the lock of the resource that the other need, it is a deadlock.

Another High Level Explanation of Deadlock : Broken Hearts

You are dating with a girl and one day after an argument, both sides are heart-broken to each other and waiting for an I-am-sorry-and-I-missed-you call. In this situation, both sides want to communicate each other if and only if one of them receives an I-am-sorry call from the other. Because that neither of each is going to start communication and waiting in a passive state, both will wait for the other to start communication which ends up in a deadlock situation.

Deadlocks will only occur when you have two or more locks that can be aquired at the same time and they are grabbed in different order.

Ways to avoid having deadlocks are:

• avoid having locks (if possible),
• avoid having more than one lock
• always take the locks in the same order.