I spent the past few days trying to reproduce a deadlock in LMDB I've been seeing on a recently updated Arch Linux system. It turns out there's a bug in mutex locking in glibc 2.25. https://sourceware.org/bugzilla/show_bug.cgi?id=21778

Be aware, if your system has upgraded to glibc 2.25 you really need to get onto glib 2.26 otherwise you're going to run into a lot of mysterious hangs.