Sorry to be reviving this thread so long after.... but I seem to be running into similar issues as Michael S. did at the start.
In short, I'm using forkProcess with the threaded RTS, and see occasional hangs:
I see these only on Linux. On Mac OS X, I never do.
I'm using GHC 7.4.2
I noticed the warning in the doc for forkProcess, but assumed I was safe, as I wasn't holding any shared resources at the time of the fork, and no shared resources in the program are used in the child.
WIth gdb, I've traced the hang to here in the run-time: forkProcess > discardTasksExcept > freeTask > closeMutex(&task->lock) > pthread_mutex_destroy
The discussion in this thread leaves me with these questions:
Is there reason to think the situation has gotten better in 7.6 and later?
Isn't the only reason System.Process is safer because it does an immediate exec in the child? Alas, I really want to just fork() sometimes.
Is it really true that even if my program has no shared resources with the child, that the IO subsystem and FFI system do anyway? Surely the RTS would take care of doing the right thing with those, no?
There should be no concern with exec w.r.t. library invariants since exec is wholesale replacement - all the libraries will reinitialize. Is there a problem here I'm missing?
Alas, I've stopped using the threaded RTS until I understand this better.