ports/183512: mdnsd from net/mDNSResponder wakes up every 976us

[Bradley T. Hughes]

>Number:         183512
>Category:       ports
>Synopsis:       mdnsd from net/mDNSResponder wakes up every 976us
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-ports-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Thu Oct 31 06:00:03 UTC 2013
>Closed-Date:
>Last-Modified:
>Originator:     Bradley T. Hughes
>Release:        10.0-BETA2
>Organization:
>Environment:
FreeBSD freebsd10.local 10.0-BETA2 FreeBSD 10.0-BETA2 #0 r257166: Sat Oct 26 19:23:22 UTC 2013     root at snap.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64
>Description:
The mdnsd daemon from net/mDNSResponder, once started, works correctly, but never idles. This can be seen as constant load on the machine, with constant wake ups. After examining and tracing the code, in particular mDNSPosix/mDNSPosix.c and mDNSPosix/PosixDaemon.c, I noticed that most of the load comes from very load timeout values passed to select(), but once select() returned and mDNS_Execute(), from mDNSCore/mDNS.c, never actually did any work. Using the ShowTaskSchedulingError() method from mDNSCore/DNSCommon.c, I noticed that the m->clearIgnoreNA flag was never cleared. After finding the relevant code in mDNS_Execute(), I noticed that the flag is cleared inside an #ifdef APPLE_OSX_mDNSResponder/#endif block. A small oversight, I think. I've attached a patch that moves the #ifdef to just around the OSX specific function, allowing the clearIgnoreNA flag to be properly set, and mdnsd now idles when there is nothing more to do.
>How-To-Repeat:
1. Install net/mDNSResponder
2. # echo 'mdnsd_enable="YES"' >> /etc/rc.conf
3. # service mdnsd start
4. Observe system activity, CPU and load rises, 
>Fix:
See the attached patch. FYI, I have not reported this upstream. I am unsure if my patch is the correct fix or not. An alternative would be to only set the clearIgnoreNA flag inside a similar #ifdef APPLE_OSX_mDNSResponder block.

Patch attached with submission follows:

--- mDNSCore/mDNS.c.orig	2013-10-31 06:41:04.868158806 +0100
+++ mDNSCore/mDNS.c	2013-10-31 06:41:21.089955155 +0100
@@ -4508,13 +4508,13 @@
 
         // After two seconds after the owner option is set, call the ioctl to clear the
         // ignore neighbor advertisement flag.
-#if APPLE_OSX_mDNSResponder
         if (m->clearIgnoreNA &&  m->timenow - m->clearIgnoreNA >= 0)
         {
+#if APPLE_OSX_mDNSResponder
             mDNSPlatformToggleInterfaceAdvt(m, mDNSfalse);
+#endif
             m->clearIgnoreNA = 0;
         }
-#endif
         // Clear AnnounceOwner if necessary. (Do this *before* SendQueries() and SendResponses().)
         if (m->AnnounceOwner && m->timenow - m->AnnounceOwner >= 0)
         {


>Release-Note:
>Audit-Trail:
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