On Tue, Apr 14, 2020 at 05:45:16PM -0400, Peter Xu wrote: > On Sun, Apr 12, 2020 at 01:10:40PM -0700, Brian Geffon wrote: > > Hi, > > It seems that userfaultfd isn't woken from a poll when the file > > descriptor is closed. It seems that it should be from the code in > > userfault_ctx_release, but it appears that's not actually called > > immediately. I have a simple standalone example that shows this > > behavior. It's straight forward: one thread creates a userfaultfd and > > then closes it after a second thread has entered a poll syscall, some > > abbreviated strace output is below showing this and the code can be > > seen here: https://gist.github.com/bgaff/9a8fbbe8af79c0e18502430d416df77e > > > > Given that it's probably very common to have a dedicated thread remain > > blocked indefinitely in a poll(2) waiting for faults there must be a > > way to break it out early when it's closed. Am I missing something? > > Hi, Brian, > > I might be wrong below, just to share my understanding... > > IMHO a well-behaved userspace should not close() on a file descriptor > if it's still in use within another thread. In this case, the poll() > thread is still using the userfaultfd handle I also don't think concurrant close() on a file descriptor that is under poll() is well defined, or should be relied upon. > IIUC userfaultfd_release() is only called when the file descriptor > destructs itself. But shouldn't the poll() take a refcount of that > file descriptor too before waiting? Not sure userfaultfd_release() is > the place to kick then, because if so, close() will only decrease the > fd refcount from 2->1, and I'm not sure userfaultfd_release() will be > triggered. This is most probably true. eventfd, epoll and pthread_join is the robust answer to these problems. Jason
