Re: kern/58933: panic in athn after wifi router rebooted, diagnostic assertion "ci->ci_mtx_count == -1" failed

["Taylor R Campbell via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR kern/58933; it has been noted by GNATS.

From: Taylor R Campbell <riastradh%NetBSD.org@localhost>
To: Jason Thorpe <thorpej%NetBSD.org@localhost>
Cc: gnats-bugs%NetBSD.org@localhost, netbsd-bugs%NetBSD.org@localhost,
	"T K Spindler (moof)" <dogcow%babymeat.com@localhost>
Subject: Re: kern/58933: panic in athn after wifi router rebooted,  diagnostic assertion "ci->ci_mtx_count == -1" failed
Date: Tue, 24 Dec 2024 22:46:01 +0000

 > #7  0xffffffff802e1d67 in athn_usb_wait_async (usc=3D0xffffd30030945000)
 >     at /usr/src/sys/dev/usb/if_athn_usb.c:907
 > ...
 > #11 athn_usb_media_change (ifp=3D0xffffd30030945d40)
 >     at /usr/src/sys/dev/usb/if_athn_usb.c:1391
 > #12 0xffffffff806406f7 in ifmedia_change (ifm=3Difm@entry=3D0xffffd300309=
 45a30,
 >     ifp=3Difp@entry=3D0xffffd30030945d40) at /usr/src/sys/net/if_media.c:=
 244
 
 This is happening because athn(4) still uses legacy ifmedia locking,
 which holds a spin lock.
 
 Fixing this properly requires teaching athn(4) to coordinate locking
 with ifmedia and ifmedia_init_with_lock, and that's going to be a pain
 to merge with the wifi branch (but we need to do it eventually,
 whether or not merge happens).
 
 But, athn-specific issues aside, I wonder whether ifmedia's approach
 to legacy drivers is right here.  Maybe for legacy drivers it should
 be the kernel lock (which is released across sleep, instead of
 forbidden to hold across sleep), like we usually do for legacy
 MP-unsafe subsystems.
 
 But I don't know enough about the ifmedia locking to know whether the
 lock has to serialize entire transactions (in which case the kernel
 lock is not good enough), or just has to make logic appear
 single-threaded.
 
 What do you think, thorpej?
 
 I think changing legacy ifmedia drivers from spin lock to kernel lock,
 if it's the right thing, should be pretty narrowly scoped and safe to
 pull up to release branches too.