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From:   Daniel Colascione <dancol@google.com>
Date:   Fri, 19 Apr 2019 16:36:24 -0700
Message-ID: <CAKOZuesaWK1Y7c85jNv71LNx7sKoGgn_eLzQVPnoi6e4eUx4aQ@mail.gmail.com>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christian Brauner <christian@brauner.io>,
        Joel Fernandes <joel@joelfernandes.org>,
        Jann Horn <jannh@google.com>, Oleg Nesterov <oleg@redhat.com>,
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        kernel list <linux-kernel@vger.kernel.org>,
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        Alexey Dobriyan <adobriyan@gmail.com>,
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On Fri, Apr 19, 2019 at 4:33 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Apr 19, 2019 at 4:20 PM Christian Brauner <christian@brauner.io> wrote:
> >
> > On Sat, Apr 20, 2019 at 1:11 AM Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > > It's also worth noting that POLLERR/POLLHUP/POLLNVAL cannot be masked
> > > for "poll()". Even if you only ask for POLLIN/POLLOUT, you will always
> > > get POLLERR/POLLHUP notification. That is again historical behavior,
> > > and it's kind of a "you can't poll a hung up fd". But it once again
> > > means that you should consider POLLHUP to be something *exceptional*
> > > and final, where no further or other state changes can happen or are
> > > relevant.
> >
> > Which kind of makes sense for process exit. So the historical behavior
> > here is in our favor and having POLLIN | POLLHUP rather fitting.
> > It just seems right that POLLHUP indicates "there can be
> > no more state transitions".
>
> Note that that is *not* true of process exit.
>
> The final state transition isn't "exit", it is actually "process has
> been reaped".  That's the point where data no longer exists.

FWIW, I think the exit status should be available via pidfd even after
process reaping. A non-parent holder of a pidfd has no ability to
control when the parent reaps the child, or even if reaping is
necessary at all --- the parent could make SIGCHLD SIG_IGN. Someone
trying to read exit status via a pidfd shouldn't fail to get that exit
status just because he lost the race with a concurrent waitpid().