Submitter: Torvald Riegel [email protected]
Submission Date: 2014-10-07
Summary
If a mutex M is acquired by a thread T, and afterwards T terminates without releasing ownership of M, then the resulting state after termination of T seems to be unspecified.
Specifically, N1570 7.26.4.5p2 states:
The mtx_trylock function endeavors to lock the mutex pointed to by mtx. If the mutex is already locked, the function returns without blocking.
However, there is no statement about whether a mutex whose owner has terminated remains locked. This seems to be a source of confusion, and it affects implementations. C++11 specifies that such a case results in undefined behavior (see 30.4.1.2.1p5). On the other hand, POSIX wants (PThreads) mutexes to remain locked in this case (see Austin Group Bug 755).
From an implementation perspective, the C++11 semantics are more practical because they do not require implementations to maintain identities of threads that do not exist any more. For example, with C++11 semantics, an implementation can just use a thread ID to identify an owner, even if another thread eventually reuses the same ID (e.g., a process ID) after the former owning thread terminated. In contrast, the POSIX semantics require an implementation to avoid ABA issues on the thread identities (i.e., the same value representing different states of ownership). This effectively results in a higher runtime overhead for lock acquisition or for lock initialization of at least recursive mutexes, or address space leakage (or other workarounds).
Suggested Technical Corrigendum
I would like the expected behavior to be explicitly specified. To me, C should do what C++11 states. In particular, add the following or a similar sentence at an appropriate place:
The behavior of a program is undefined if a thread terminates while owning a mutex.