Messages in this thread |  | | From | Linus Torvalds <> | Date | Thu, 18 Apr 2019 12:14:58 -0700 | Subject | Re: [PATCH RFC 1/2] Add polling support to pidfd |
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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
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