Question

Thread 1:

putQ
setFlag

Thread 2：

while (1) {
waitFlag
processQ
clearFlag
}

This is an interview question. I am not sure whether there is a while loop for Thread 1.
But the answer is like when Thread 2 re-enter the while loop, two threads go into deadlock.

Can anyone tell me what’s condition for deadlock? Thanks.

Answer 1

I think the OP’s comments are correct:

1. @Gray: This is most likely NOT a dead-lock, but a race condition. Yet the application is stuck not processing the queue, so it’s a lock.

2. @Wug: It is probably the “trivial case” where while thread 1 has put in the queue and updated the flag, thread 2 will never notice. See course of action below.

Potential lock situation:

• T2: ProcessQ Element 0 (processing a former element)
• T1: PutQ Element 1
• T1: SetFlag
• T2: ClearFlag
• T2: waitFlag

Element 1 might never be processed.

EDIT:

Of course, the next question would be: how do you fix that?

The answer would be with a Conditional Variable (see pthread_cond_init and others for details):

Thread 1:

PutQ
LockMutex
SetFlag
CondSignal
UnlockMutex

Thread 2:

while (1) {
    LockMutex
    while (FlagIsUnset)
       CondWait
    FlagUnset
    UnlockMutex
    ProcessQ
}