[src/trunk]: src/doc Add another known issue (if_flags)

[ozaki-r <ozaki-r%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/47f1e9caa8bf
branches:  trunk
changeset: 449876:47f1e9caa8bf
user:      ozaki-r <ozaki-r%NetBSD.org@localhost>
date:      Wed Mar 27 06:56:37 2019 +0000

description:
Add another known issue (if_flags)

diffstat:

 doc/TODO.smpnet |  23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)

diffs (34 lines):

diff -r d2f90a00fda3 -r 47f1e9caa8bf doc/TODO.smpnet
--- a/doc/TODO.smpnet   Wed Mar 27 06:56:19 2019 +0000
+++ b/doc/TODO.smpnet   Wed Mar 27 06:56:37 2019 +0000
@@ -1,4 +1,4 @@
-$NetBSD: TODO.smpnet,v 1.28 2019/03/26 05:17:17 ozaki-r Exp $
+$NetBSD: TODO.smpnet,v 1.29 2019/03/27 06:56:37 ozaki-r Exp $
 
 MP-safe components
 ==================
@@ -229,3 +229,24 @@
 first register an address to the hash table then create its routes.  Another
 solution is to use the routing table for the to-self check instead of using the
 global hash table, like IPv6.
+
+if_flags
+--------
+
+To avoid data race on if_flags it should be protected by a lock (currently it's
+IFNET_LOCK).  Thus, if_flags should not be accessed on packet processing to
+avoid performance degradation by lock contentions.  Traditionally IFF_RUNNING,
+IFF_UP and IFF_OACTIVE flags of if_flags are checked on packet processing.  If
+you make a driver MP-safe you must remove such checks.
+
+IFF_ALLMULTI can be set/unset via if_mcast_op.  To protect updates of the flag,
+we had added IFNET_LOCK around if_mcast_op.  However that was not a good
+approach because if_mcast_op is typically called in the middle of a call path
+and holding IFNET_LOCK such places is problematic.  Actually a deadlock is
+observed.  Probably we should remove IFNET_LOCK and manage IFF_ALLMULTI
+somewhere other than if_flags, for example ethercom or driver itself (or a
+common driver framework once it appears).  Such a change is feasible because
+IFF_ALLMULTI is only set/unset by a driver and not accessed from any common
+components such as network protocols.
+
+Also IFF_PROMISC is checked in ether_input and we should get rid of it somehow.