Thread Deadlock

In an operating system that supports multithreading, thread deadlock refers to a deadlock state caused by multiple threads waiting for resources. Thread deadlock can be divided into two types:

1. Thread deadlock in the same process
   • In the same process, different threads may deadlock because they share the resources of the same process (such as variables, memory, etc.). In order to synchronize access to these resources, threads usually use mutex to perform mutual exclusion operations.

2. Deadlock scenario:

   • There are two mutexes: L1 and L2.
   • Thread T1 obtains L1 and requests L2;
   • Thread T2 obtains L2 and requests L1.
   • Because the two threads wait for each other to release the lock, deadlock occurs and the two threads cannot continue to execute.

3. Deadlock cause: A thread requests another lock while holding one lock, and the lock is held by another thread. This circular waiting situation causes deadlock.

4. Thread deadlock in different processes

   • Thread deadlock in different processes means that threads in two or more processes wait for each other and cannot continue to execute. Assume there are two processes P1 and P2, each with multiple threads:
   • Threads P11, P12, ... in process P1, threads P21, P22, ... in process P2;
   • Thread P11 of process P1 obtains resource R1, thread P21 of process P2 obtains resource R2;
   • If P11 requests R2 but cannot obtain it, it enters a blocked state, waiting for P21 to release R2;
   • At the same time, P21 requests R1 but cannot obtain it, enters a blocked state, waiting for P11 to release R1;
   • At this time, P11 and P21 wait for each other to release resources, resulting in deadlock.

5. Deadlock cause: Threads between processes hold resources needed by each other, and form a circular wait while waiting.

Solution

The solution to thread deadlock is similar to that to process deadlock, and can be handled in the following ways:

1. Deadlock prevention: Prevent deadlock by limiting the order of resource application or designing resource allocation rules.

   • For example: Use resource sorting strategy to require threads to apply for resources in a fixed order to avoid circular waiting.

2. Deadlock avoidance: Avoid deadlock by dynamically checking the resource requests of threads to ensure that the system is in a safe state after resource allocation. For example, use banker's algorithm to avoid deadlock.

3. Deadlock detection and release: Allow the system to deadlock, but run the deadlock detection program regularly to identify deadlocks and take measures to resolve them. For example:

   • Terminate process: End a deadlocked process and release resources.
   • Roll back process: Roll back a process to a previous safe state and release resources.

Summary

• Thread deadlock in the same process: Threads deadlock due to incorrect application order or competition for mutexes.
• Deadlock of threads in different processes: Threads in different processes deadlock because they are waiting for each other's resources.