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> 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.
