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. --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -937,7 +937,7 @@ wakeup: /* Flush pending events that may still wait on event_wqh */ wake_up_all(&ctx->event_wqh); - wake_up_poll(&ctx->fd_wqh, EPOLLHUP); + wake_up_all(&ctx->fd_wqh); userfaultfd_ctx_put(ctx); return 0; }
