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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 18 Apr 2019 12:14:58 -0700
Message-ID: <CAHk-=wi79KTxCYZgMZNapULR-ratPiZVt+fgh=DhgtYR_gYPAA@mail.gmail.com>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd
To:     Daniel Colascione <dancol@google.com>
Cc:     Jonathan Kowalski <bl0pbl33p@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Oleg Nesterov <oleg@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrei Vagin <avagin@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>, Michal Hocko <mhocko@suse.com>,
        Nadav Amit <namit@vmware.com>, Serge Hallyn <serge@hallyn.com>,
        Shuah Khan <shuah@kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Taehee Yoo <ap420073@gmail.com>, Tejun Heo <tj@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Android Kernel Team <kernel-team@android.com>,
        Tycho Andersen <tycho@tycho.ws>
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On Thu, Apr 18, 2019 at 11:58 AM Daniel Colascione <dancol@google.com> wrote:
>
> On Thu, Apr 18, 2019 at 11:44 AM Jonathan Kowalski <bl0pbl33p@gmail.com> wrote:
> >
> > Would using something other than POLLIN be an option (maybe POLLPRI)?
> > The convention is to use it to indicate readability on the descriptor,
> > and also possibly POLLHUP instead of POLLERR (the latter is less of a
> > problem, but FreeBSD also does the same, so it'd help with some
> > consistency for libraries wanting to use this, which aren't interested
> > in other sub states).
>
> Existing event loop libraries generally support checking only for
> readability and writability. Not setting POLLIN would make these FDs
> more difficult to use with existing event loop libraries. What
> advantage would compensate for this difficulty?

Right.

Usually you'd set POLLIN in _addition_ to any other more specialized poll flag.

For example, when a socket has shut down the read side, we do

        if (sk->sk_shutdown & RCV_SHUTDOWN)
                mask |= EPOLLRDHUP | EPOLLIN | EPOLLRDNORM;

because while it's true that EPOLLRDHUP is the most _specific_ poll
bit, it's _also_ true that a read shutdown means that the read() will
return immediately.

So generally a HUP condition should mean that POLLIN and POLLOUT also
get set. Not because there's any actual _data_ to be read, but simply
because the read will not block.

                   Linus