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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Apr 19, 2019 at 2:20 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> According to Linus, POLLHUP usually indicates that something is readable:

Note that if you use the legacy interfaces (ie "select()"), then even
just a plain POLLHUP will always just show as "readable".

So for a lot of applications, it won't even matter. Returning POLLHUP
will mean it's readable whether POLLIN is set or not (and EPOLLERR
will automatically mean that it's marked both readable and writable).

In fact, I'd argue that not a lot of people care about the more
esoteric bits at all. If your poll function does POLLIN and POLLOUT,
it's fine. Anything else is specialized enough that most people simply
won't care. Don't overdesign things too much. You need to have a major
reason to care about the other POLL bits.

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.

That may well work fine for pidfd when the task is gone, but it's
really worth noting that any *normal* state should be about
POLLIN/POLLOUT. People should not think that "POLLHUP sounds like the
appropriate name", they should realize that POLLHUP is basically a
terminal error condition, not a "input is available".

So just keep that in mind.

                Linus