A blast from the past - strange sysctl behavior

[Paul Goyette <paul%whooppee.com@localhost>]

Nearly a year ago, I posted [1] on the current-users list.  Since that 
time I've been running with the attached patch, in an attempt to find 
out what was going on.  And as is usually the case, the debug code 
didn't get triggered, until now - 11 months later!

I haven't had a chance to investigate in any detail, but the problem 
(and associated KASSERT) occurred while starting firefox.  A bit more 
detail shows that it was installing the sysctl tree for the sysv_ipc 
module.  However, after the reboot I was able to successfully launch 
firefox without triggering the KASSERT.

If anyone has any clues about how this anomaly can happen (a newly 
created sysctl node has a wrong parent), I'd love to hear them.  I am 
guessing that there's some sort of locking botch in the sysctl code, 
however I haven't been able to spot it.


[1] https://mail-index.netbsd.org/current-users/2015/10/27/msg028285.html

+------------------+--------------------------+------------------------+
| Paul Goyette     | PGP Key fingerprint:     | E-mail addresses:      |
| (Retired)        | FA29 0E3B 35AF E8AE 6651 | paul at whooppee.com   |
| Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd.org |
+------------------+--------------------------+------------------------+

Index: kern_sysctl.c
===================================================================
RCS file: /cvsroot/src/sys/kern/kern_sysctl.c,v
retrieving revision 1.258
diff -u -p -r1.258 kern_sysctl.c
--- kern_sysctl.c	23 Oct 2015 01:58:43 -0000	1.258
+++ kern_sysctl.c	5 Sep 2016 09:26:26 -0000
@@ -1955,6 +1955,7 @@ sysctl_createv(struct sysctllog **log, i
 	const struct sysctlnode *root, *pnode;
 	struct sysctlnode nnode, onode, *dnode;
 	size_t sz;
+	const struct sysctlnode *snode __diagused;
 
 	/*
 	 * where are we putting this?
@@ -2111,6 +2112,7 @@ sysctl_createv(struct sysctllog **log, i
 		 */
 		pnode = root;
 		error = sysctl_locate(NULL, &name[0], namelen - 1, &pnode, &ni);
+		snode = pnode;
 
 		/*
 		 * manual scan of last layer so that aliased nodes
@@ -2131,6 +2133,8 @@ sysctl_createv(struct sysctllog **log, i
 		 * not expecting an error here, but...
 		 */
 		if (error == 0) {
+			KASSERTMSG(pnode->sysctl_parent == snode,
+			    "sysctl parent mis-match");
 			if (log != NULL)
 				sysctl_log_add(log, pnode);
 			if (cnode != NULL)