On Thu, Sep 03, 2020 at 04:56:59PM -0700, Josh Triplett wrote: > On Wed, Sep 02, 2020 at 12:21:28PM +0200, Christian Brauner wrote: > > Passing a non-blocking pidfd to waitid() currently has no effect, i.e. is not > > supported. There are users which would like to use waitid() on pidfds that are > > O_NONBLOCK and mix it with pidfds that are blocking and both pass them to > > waitid(). > > The expected behavior is to have waitid() return -EAGAIN for non-blocking > > pidfds and to block for blocking pidfds without needing to perform any > > additional checks for flags set on the pidfd before passing it to waitid(). > > Non-blocking pidfds will return EAGAIN from waitid() when no child process is > > ready yet. Returning -EAGAIN for non-blocking pidfds makes it easier for event > > loops that handle EAGAIN specially. > > > > It also makes the API more consistent and uniform. In essence, waitid() is > > treated like a read on a non-blocking pidfd or a recvmsg() on a non-blocking > > socket. > > With the addition of support for non-blocking pidfds we support the same > > functionality that sockets do. For sockets() recvmsg() supports MSG_DONTWAIT > > for pidfds waitid() supports WNOHANG. Both flags are per-call options. In > > contrast non-blocking pidfds and non-blocking sockets are a setting on an open > > file description affecting all threads in the calling process as well as other > > processes that hold file descriptors referring to the same open file > > description. Both behaviors, per call and per open file description, have > > genuine use-cases. > > > > The implementation should be straightforward, we simply raise the WNOHANG flag > > when a non-blocking pidfd is passed and when do_wait() returns without finding > > an eligible task and the pidfd is non-blocking we set EAGAIN. If no child > > process exists non-blocking pidfd users will continue to see ECHILD but if > > child processes exist but have not yet exited users will see EAGAIN. > > > > A concrete use-case that was brought on-list was Josh's async pidfd library. > > Ever since the introduction of pidfds and more advanced async io various > > programming languages such as Rust have grown support for async event > > libraries. These libraries are created to help build epoll-based event loops > > around file descriptors. A common pattern is to automatically make all file > > descriptors they manage to O_NONBLOCK. > > > > For such libraries the EAGAIN error code is treated specially. When a function > > is called that returns EAGAIN the function isn't called again until the event > > loop indicates the the file descriptor is ready. Supporting EAGAIN when > > waiting on pidfds makes such libraries just work with little effort. > > > > Link: https://lore.kernel.org/lkml/20200811181236.GA18763@localhost/ > > Link: https://github.com/joshtriplett/async-pidfd > > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > > Cc: Sargun Dhillon <sargun@xxxxxxxxx> > > Cc: Jann Horn <jannh@xxxxxxxxxx> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx> > > Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx> > > Suggested-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx> > > Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx> > > With or without the discussed change to WNOHANG behavior for > compatibility: > Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx> I think that WNOHANG compatibility change might be a good idea. So I've changed this to: ret = do_wait(&wo); if (!ret && !(options & WNOHANG) && (f_flags & O_NONBLOCK)) ret = -EAGAIN; > > Also, I think you should flip the order of patches 1 and 2, so that > there isn't a one-patch window in kernel history where you can create an > O_NONBLOCK pidfd with pidfd_open but it has no effect. I'd expect > userspace to use pidfd_open accepting or EINVAL-ing the flag as an > indication of whether it'll work. Good point! I've changed the order now. Thanks! Christian
