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Date:   Thu, 3 Sep 2020 17:38:47 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Christian Brauner <christian@brauner.io>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
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        Josh Triplett <josh@joshtriplett.org>,
        Jens Axboe <axboe@kernel.dk>, linux-api@vger.kernel.org,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH v2 2/4] exit: support non-blocking pidfds
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 <20200903142241.GI4386@redhat.com>
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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