On 08/07/2014 12:53 PM, Ken Brown wrote: > On 8/7/2014 11:30 AM, Eric Blake wrote: >> On 08/07/2014 05:51 AM, Ken Brown wrote: >>> >>> I think I found the problem with NORMAL mutexes. emacs calls >>> pthread_atfork after initializing the mutexes, and the resulting >>> 'prepare' handler locks the mutexes. (The parent and child handlers >>> unlock them.) So when emacs calls fork, the mutexes are locked, and >>> shortly thereafter the Cygwin DLL calls calloc, leading to a deadlock. >>> Here's a gdb backtrace showing the sequence of calls: >> >> Arguably, that's an upstream bug in emacs. POSIX has declared >> pthread_atfork to be fundamentally useless; it is broken by design, >> because you cannot use it for anything that is not async-signal-safe >> without risking deadlock. And (except for sem_post()), NONE of the >> standardized locking functions are async-signal-safe. >> >> http://austingroupbugs.net/view.php?id=858 >> >> That said, it would still be nice to support this, since even though the >> theory says it is broken, there are still lots of (broken) >> programs/libraries still trying to use it. > > So what do you think emacs should do instead of using pthread_atfork? Or > is it better to just remove it? I don't know how likely it is that this > would cause a problem.
The POSIX recommendation is that multithreaded apps limit themselves solely to async-signal-safe functions in the window between fork and exec (or to use pthread_spawn instead of fork/exec). I don't know what emacs is trying to do in that window, but at this point, it's certainly worth reporting it upstream. If you need a pointer to the full list of async-signal-safe functions: http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04 and search for "The following table defines a set of functions that shall be async-signal-safe." The most common deadlocks when violating async-signal-safety rules look like this in single-threaded programs: function calls malloc() malloc() grabs a non-recursive mutex async signal arrives signal handler called signal handler calls malloc() malloc() can't grab the mutex - deadlock and this counterpart in multithreaded programs: thread1 calls malloc() malloc() grabs a non-recursive mutex thread 2 gains control and calls fork() because of the fork, thread1 no longer exists to release the lock child process calls malloc() malloc() tries to grab mutex, but it is locked with no thread to release it Switching malloc() to a recursive lock may or may not "solve" the single-threaded deadlock (in that malloc can now obtain the mutex), but it is probably NOT what you want to happen (unless malloc is fully re-entrant, the inner instance will see incomplete data and either be totally clobbered itself, or else totally clobber the outer instance when it returns). So it's GOOD that malloc does NOT use a recursive mutex by default. In the multithreaded case, you are flat out hosed. Switching to a recursive lock does not change the picture - you are still deadlocked waiting on thread1 to release the lock, but thread1 doesn't exist. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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