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Date:   Thu, 3 Sep 2020 16:58:55 -0700
From:   Josh Triplett <josh@joshtriplett.org>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     linux-kernel@vger.kernel.org,
        Christian Brauner <christian@brauner.io>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Sargun Dhillon <sargun@sargun.me>,
        Aleksa Sarai <cyphar@cyphar.com>,
        linux-kselftest@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
        linux-api@vger.kernel.org
Subject: Re: [PATCH v2 0/4] Support non-blocking pidfds
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On Wed, Sep 02, 2020 at 12:21:26PM +0200, Christian Brauner wrote:
> Hi,
> 
> 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.
> 
> A concrete use-case that was brought on-list (see [1]) 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.

Thanks for the patch series, Christian!

This will make it much easier to use pidfd in non-blocking event loops.

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

- Josh Triplett