linux-kernel - Re: [PATCH v2 2/4] exit: support non-blocking pidfds

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Message-ID: <20200903153847.zvi5dzwj6v4eap6i@wittgenstein>
Date:   Thu, 3 Sep 2020 17:38:47 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Christian Brauner <christian@...uner.io>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Sargun Dhillon <sargun@...gun.me>,
        Aleksa Sarai <cyphar@...har.com>,
        linux-kselftest@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Jens Axboe <axboe@...nel.dk>, linux-api@...r.kernel.org,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v2 2/4] exit: support non-blocking pidfds

On Thu, Sep 03, 2020 at 04:22:42PM +0200, Oleg Nesterov wrote:
> On 09/02, Christian Brauner wrote:
> >
> > 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.
> 
> What I personally do not like is that waitid(WNOHANG) returns zero or EAGAIN
> depending on f_flags & O_NONBLOCK... This doesn't match recvmsg(MSG_DONTWAIT)
> and doesn't look consistent to me.

It's not my favorite thing either but async event loops are usually
modeled around EAGAIN so I think this has benefits. Josh can speak more
to that.

Christian