Re: [PATCH RFC 1/2] Add polling support to pidfd

[Joel Fernandes]

On Thu, Apr 18, 2019 at 12:14:58PM -0700, Linus Torvalds wrote:
> On Thu, Apr 18, 2019 at 11:58 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> >
> > On Thu, Apr 18, 2019 at 11:44 AM Jonathan Kowalski <bl0pbl33p@xxxxxxxxx> 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.

Sounds great and I agree with Linus and Daniel. So I am guessing you are Ok
with the current set of flags proposed this patch, so I will keep them intact
in future patch postings. But please let me know if you want me to change
something about the flags.

thanks!

 - Joel