On linux, how to make sure to unlock a mutex which was locked in a thread that dies/terminates?

Question

This is an interview question.

On linux, how to make sure to unlock a POSIX mutex which was locked in a POSIX thread that dies/terminates?

My idea:

Answer 1

If it's not a process-shared mutex, it doesn't matter. When one thread dies, the process dies, and the mutex goes away.

If it's a process-shared mutex, you're asking the wrong question. You wouldn't want to unlock the mutex if a thread died while holding it. The reason a thread holds a mutex is so that it can manipulate shared data through states that must not be seen by other threads. If a thread dies while holding a mutex, it is likely that the data was left in such an inconsistent state. Unlocking the mutex would just allow other threads to see the invalid/corrupt data.

Answer 2

A robust mutex can be used to handle the case where the owner of the mutex is terminated while holding the mutex lock, so that a deadlock does not occur. These have more overhead than a regular mutex, and require that all clients locking the mutex be prepared to handle the error code EOWNERDEAD. This indicates that the former owner has died and that the client receiving this error code is the new owner and is responsible for cleaning up any inconsistent state.

A robust mutex is a mutex with the robust attribute set. It is set using the POSIX.1-2008 standard function pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST).

Further details and example code can be found on the Linux manual page for pthread_mutexattr_getrobust.