netdev - Issues in error queue polling

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Date:   Thu, 18 Oct 2018 15:26:44 -0300
From:   Ricardo Biehl Pasquali <pasqualirb@...il.com>
To:     netdev@...r.kernel.org
Cc:     jacob.e.keller@...el.com, vinicius.gomes@...el.com,
        davem@...emloft.net
Subject: Issues in error queue polling

The commit 7d4c04fc170087119727 ("net: add option to enable
error queue packets waking select") (2013-03-28) introduced
SO_SELECT_ERR_QUEUE, which masks POLLPRI with POLLERR event
return in some socket poll callbacks.

POLLERR event issued with sock_queue_err_skb() did not wake
up a poll when POLLERR is the only requested event because
sk_data_ready() (sock_def_readable()) was used and it
doesn't mask POLLERR in poll wake up:

wake_up_interruptible_sync_poll(&wq->wait,
                                EPOLLIN | EPOLLPRI |
                                EPOLLRDNORM | EPOLLRDBAND);

If POLLIN or POLLPRI are requested, for example, poll does
wake up.

POLLERR wakeup by requesting POLLPRI is possible without
set SO_SELECT_ERR_QUEUE. All the option does is masking
POLLPRI as a returned event before poll returns. poll
would return anyway because of POLLERR.

Also, the sentence "[...] enable software to wait on error
queue packets without waking up for regular data on the
socket." from the above commit is not true.

A POLLIN event issued via sock_def_readable() wakes up
threads waiting for POLLPRI, and vice versa. However,
poll() does not return, sleeping again, as the requested
events do not match events.

The commit 6e5d58fdc9bedd0255a8 ("skbuff: Fix not waking
applications when errors are enqueued") (2018-03-14) make
POLLERR alone wake up poll. It replaces sk_data_ready()
(sock_def_readable()) with sk_error_report()
(sock_def_error_report()). This makes "POLLERR wake up by
requesting POLLPRI" obsolete.

Rationale:

POLLIN-only and POLLERR-only wake up are useful when there
is a receiving thread, a sending thread, and a thread that
get transmit timestamps. The thread polling on POLLERR will
not wake up when regular data arrives (POLLIN). The thread
polling on POLLIN will not wake up when tx timestamps are
ready (POLLERR).

One solution is adding an option that disable POLLERR as
requested event. This is in the Virtual File System
subsystem, not in the network, though.

This solves the problem of waking up other threads that
not interested in error queue. Thus allowing a separate
thread take care of error queue (useful for receiving
transmit timestamps).

However, this may not be a good solution as it depends on
polling behavior. Thoughts?

By the way, as the kernel development shouldn't break user
space, SO_SELECT_ERR_QUEUE can become a compatibility
option.

P.S.: The kernel is slowly getting bigger (and perhaps
      messy) with compatibility code. One day, the
      compatibility code should be moved to a
      compatibility-only place, or completely dropped
      (unlikely).

	pasquali