Thread pools: memory barriers in task completion notifications. The ngx_thread_pool_done object isn't volatile, and at least some compilers assume that it is permitted to reorder modifications of volatile and non-volatile objects. Added appropriate ngx_memory_barrier() calls to make sure all modifications will happen before the lock is released. Reported by Mindaugas Rasiukevicius, http://mailman.nginx.org/pipermail/nginx-devel/2016-April/008160.html.