[src/trunk]: src/sys/net Note that IFNET_LOCK must not be held in softint

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

details:   https://anonhg.NetBSD.org/src/rev/ed3d9c5180dc
branches:  trunk
changeset: 358230:ed3d9c5180dc
user:      ozaki-r <ozaki-r%NetBSD.org@localhost>
date:      Mon Dec 18 08:24:29 2017 +0000

description:
Note that IFNET_LOCK must not be held in softint

diffstat:

 sys/net/if.h |  12 +++++++++++-
 1 files changed, 11 insertions(+), 1 deletions(-)

diffs (26 lines):

diff -r 236d0a95c902 -r ed3d9c5180dc sys/net/if.h
--- a/sys/net/if.h      Mon Dec 18 07:57:54 2017 +0000
+++ b/sys/net/if.h      Mon Dec 18 08:24:29 2017 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: if.h,v 1.256 2017/12/15 04:07:20 ozaki-r Exp $ */
+/*     $NetBSD: if.h,v 1.257 2017/12/18 08:24:29 ozaki-r Exp $ */
 
 /*-
  * Copyright (c) 1999, 2000, 2001 The NetBSD Foundation, Inc.
@@ -448,6 +448,16 @@
  *   - Must be updated with holding IFNET_LOCK
  *   - You cannot use the flag in Tx/Rx paths anymore because there is no
  *     synchronization on the flag except for IFNET_LOCK
+ *   - Note that IFNET_LOCK can't be taken in softint because it's known
+ *     that it causes a deadlock
+ *     - Some synchronization mechanisms such as pserialize_perform are called
+ *       with IFNET_LOCK and also require context switches on every CPUs
+ *       that mean softints finish so trying to take IFNET_LOCK in softint
+ *       might block on IFNET_LOCK and prevent such synchronization mechanisms
+ *       from being completed
+ *     - Currently the deadlock occurs only if NET_MPSAFE is enabled, however,
+ *       we should deal with the restriction because NET_MPSAFE will be enabled
+ *       by default in the future
  * - if_watchdog and if_timer
  *   - The watchdog framework works only for non-IFEF_MPSAFE interfaces
  *     that rely on KERNEL_LOCK