Contents
What is lock ordering?
Lock Ordering Deadlock occurs when multiple threads need the same locks but obtain them in different order. Lock ordering is a simple yet effective deadlock prevention mechanism. However, it can only be used if you know about all locks needed ahead of taking any of the locks. This is not always the case.
How do you solve a multithreading deadlock?
The canonical technique for deadlock avoidance is to have a lock hierarchy. Make sure that all threads acquire locks or other resources in the same order. This avoids the deadlock scenario where thread 1 hold lock A and needs lock B while thread 2 holds lock B and needs lock A.
What does it mean to lock a thread?
Locks are one synchronization technique. A lock is an abstraction that allows at most one thread to own it at a time. Locks have two operations: acquire allows a thread to take ownership of a lock. If a thread tries to acquire a lock currently owned by another thread, it blocks until the other thread releases the lock.
Can a mutex be locked more than once?
Can a mutex be locked more than once? A mutex is a lock. Only one state (locked/unlocked) is associated with it. However, a recursive mutex can be locked more than once (POSIX compliant systems), in which a count is associated with it, yet retains only one state (locked/unlocked).
What happens when you lock a thread?
A lock allows you to force multiple threads to access a resource one at a time, rather than all of them trying to access the resource simultaneously. As you note, usually you do want threads to execute simultaneously. However, imagine that you have two threads and they are both writing to the same file.
Do threads share locks?
In the simplest scenario, two threads both own a separate lock, but additionally need to acquire the lock of the other thread. As no thread can advance without acquiring a second lock, both threads are blocked and cannot continue. Other locking issues include livelocks and lock starvations.