Thanks everyone for the insights! On Wed, Apr 15, 2020 at 7:25 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > On Wed, Apr 15, 2020 at 11:16:02AM +0800, Hillf Danton wrote: > > > > On Tue, 14 Apr 2020 19:34:10 -0300 Jason Gunthorpe wrote: > > > > > > 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. > > > > > > > See the difference EPOLLHUP makes. > > The whole idea is completely racey: > > CPU1 CPU2 CPU3 > fds[i]->fd = userfaultfd; > while() > close(userfaultfd) > pthread_join() > someother_fd = open() > userfaultfd == someother_fd > poll(fds) // <- Still sleeps > > The kernel should not be trying to wake poll from fd release, and > userspace should not close a FD that is currently under poll. > > Besides, it really does look like poll holds the fget while doing its > work (see poll_freewait), so fops release() won't be called anyhow.. > > Jason
