When running multiple nfsd processes, there is one master process and several slave processes.
As soon as the master process receives a SIGUSR1 all nfsd will terminate. This results in a
race condition when shutting down nfsd. The kill signal is not send to a list of PIDs, but
a list of PIDs is iterated and kill(1) called for every PID. 

As soon as the kill-loop hits the master nfsd, all subsequent kills will go to stale PIDs
eventually killing of other innocent processes.

Fix: This is only a proof of concept as I'm not sure if this should be kludged into rc.subr or
if we should provide a minimal override in rc.d/nfsd.

You might wanna diff the two functions to see what I did. (Change _fp_args, extend the if test
to if-master-and-same-jailid).

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--- nfsd	2006-10-18 11:15:50.000000000 +0200
+++ nfsd.new	2006-10-18 11:15:47.000000000 +0200
@@ -44,4 +44,42 @@
 	return 0
 }
 
+# Overwrite the _find_processes() function. We are only interested in the
+# nfsd master process. Only this one should get the kill signal.
+
+_find_processes()
+{
+	if [ $# -ne 3 ]; then
+		err 3 'USAGE: _find_processes procname interpreter psargs'
+	fi
+	_procname=$1
+	_interpreter=$2
+	_psargs=$3
+
+	_pref=
+		_procnamebn=${_procname##*/}
+		_fp_args='_arg0 _argv _x'
+		_fp_match='case "$_arg0" in
+		    $_procname|$_procnamebn|${_procnamebn}:|"(${_procnamebn})"|"[${_procnamebn}]")'
+
+	_proccheck='
+		ps 2>/dev/null -o "pid,jid,command" '"$_psargs"' |
+		while read _npid _jid '"$_fp_args"'; do
+			case "$_npid" in
+			PID)
+				continue;;
+			esac; '"$_fp_match"'
+				if [ "$_argv" = "master" -a "$JID" -eq "$_jid" ];
+				then echo -n "$_pref$_npid";
+				_pref=" ";
+				fi
+				;;
+			esac
+		done'
+
+#	debug "in _find_processes: proccheck is ($_proccheck)."
+	eval $_proccheck
+}
+
+
 run_rc_command "$1"
How-To-Repeat: Crank up the number of processes to make it more obvious:
nfs_server_enable="YES"
nfs_server_flags="-u -t -n 64"

run /etc/rc.d/nfsd start and /etc/rc.d/nfsd stop. You will most probably see several errors
from kill. Check $?