[PATCH] Fix possible deadlock in stdio locking code

[Jonathan Nieder]

Siddhesh Poyarekar wrote:

> 	while(1) {
> 		int child = fork();
>
> 		if (child == 0)
> 			exit(0);

POSIX explains (under fork()):

	If a multi-threaded process calls fork( ), the new process
	shall contain a replica of the calling thread and its entire
	address space, possibly including the states of mutexes and
	other resources. Consequently, to avoid errors, the child
	process may only execute async-signal-safe operations until
	such time as one of the exec functions is called.

exit() is not async-signal-safe, though _exit() is.  I don't think
that glibc is the right tool to diagnose or work around application
problems like this.  (Maybe helgrind could be tweaked to make the
diagnosis.)