Re: [PATCH v2 1/4] pidfd: support PIDFD_NONBLOCK in pidfd_open()

[Josh Triplett <josh@joshtriplett.org>]

On Wed, Sep 02, 2020 at 12:21:27PM +0200, Christian Brauner wrote:
> Introduce PIDFD_NONBLOCK to support non-blocking pidfd file descriptors.
> 
> 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. In the
> following patch we will extend waitid() internally to support non-blocking
> pidfds.
> 
> This introduces a new flag PIDFD_NONBLOCK that is equivalent to O_NONBLOCK.
> This follows the same patterns we have for other (anon inode) file descriptors
> such as EFD_NONBLOCK, IN_NONBLOCK, SFD_NONBLOCK, TFD_NONBLOCK and the same for
> close-on-exec flags.
> 
> Link: https://lore.kernel.org/lkml/20200811181236.GA18763@localhost/
> Link: https://github.com/joshtriplett/async-pidfd
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Sargun Dhillon <sargun@sargun.me>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Suggested-by: Josh Triplett <josh@joshtriplett.org>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>