Who should set the priority of a select(2)ing thread being waken up?

[Seigo Tanimura]

In good old days, only a socket and a pipe were the major file
descriptors being select(2)ed.  As select(2) was just a socket
operation, it was sufficient to set the priority of select(2)ing
process to PSOCK(*1), I suppose.

Nowadays, quite a few drivers support select(2) as well, including
sound, usb, scsi controllers, and so on.  I am not convinced whether a
process should select(2) those devices at PSOCK as we do for a socket.

Suppose that a process select(2)s for a pcm device and a socket at
once.  If the process is waken up by the pcm driver at PSOCK, another
process at a better priority may preempt the first one, which can
result in dropping some pcm data.

Maybe it would be better if the caller of selwakeup() could determine
the priority of a process or a thread.  That would let us raise the
priority to PRIBIO if pcm data was ready, while the priority would
stay at PSOCK if the socket was ready.


(*1) I broke that in 5-CURRENT when I modified select(2) and poll(2)
to use a conditional variable.

-- 
Seigo Tanimura <tanimura at tanimura.dyndns.org> <tanimura at FreeBSD.org>