Multithreading – Can a Single Thread Produce a Deadlock?

multithreading

This expands on "Deadlock in Single threaded application" on StackOverflow.

That question was not really conclusive and I think it better belongs here.

By the definition of a deadlock, is it technically possible to produce a deadlock by using just a single thread?

This is the definition of a deadlock according to Wikipedia:

A deadlock situation on a resource can arise if and only if all of the
following conditions hold simultaneously in a system:

  1. Mutual exclusion: At least one resource must be held in a non-shareable mode. Otherwise, the processes would not be prevented
    from using the resource when necessary. Only one process can use the
    resource at any given instant of time.

  2. Hold and wait or resource holding: a process is currently holding at least one resource and requesting additional resources which are
    being held by other processes.

  3. No preemption: a resource can be released only voluntarily by the process holding it.
  4. Circular wait: each process must be waiting for a resource which is being held by another process, which in turn is waiting for the first
    process to release the resource.

I tried to create a deadlock with one thread in C#, where condition 4 may not literally be met, but it still looks like a deadlock to me. I am not posting this code since I know it would be off-topic for this site.

Best Answer

"Deadlock" just means you have two actions that are each locking each other so that neither can proceed.

Obviously in order to have two things running at the same time you would normally need more than one "thread".

But a thread is just an abstract layer over the actual CPU. You can easily* write you own abstraction layer and create deadlocks in it without using the language's thread objects.

*not so easily if you want it to work well

lets do some pseudo code

List<string> t1; //instruction set one
List<string> t2; //instruction set two

public void Run()
{
    while(1==1)
    {
        //attempt to run instructions one at a time, but timeout of they are 'locked'
        if(Execute(t1[i], timeout:1000)) { i++;}
        if(Execute(t2[j], timeout:1000)) { j++;}
    }
}

Now if I have some t1 instructions create a lock A and then B but also have t2 attempt to lock B and then A. I will have a deadlock with no System.Threads

Related Topic