The following reply was made to PR kern/140313; it has been noted by GNATS.

From: dfil...@freebsd.org (dfilter service)
To: bug-follo...@freebsd.org
Cc:  
Subject: Re: kern/140313: commit references a PR
Date: Sun, 26 Dec 2010 13:05:48 +0000 (UTC)

 Author: cperciva
 Date: Sun Dec 26 13:05:43 2010
 New Revision: 216703
 URL: http://svn.freebsd.org/changeset/base/216703
 
 Log:
   Lock the vm page queue mutex in pmap_pte_release around the call
   to PMAP_SET_VA; this fixes a mutex-not-held panic when a process
   which called mlock(2) exits, and parallels a change made in
   pmap_pte 10 months ago (svn r204160).
   
   Note: The locking in this code is utterly broken.  We should not
   be using the VM page queue mutex to protect the queue of pending
   Xen page mapping hypervisor calls.  Even if it made sense to do
   so, this commit and r204160 introduce LORs between the vm page
   queue mutex and PMAP2mutex.
   
   (However, a possible deadlock is better than a guaranteed panic,
   and this change will hopefully make life easier for whoever fixes
   the Xen pmap locking in the future.)
   
   PR:          kern/140313
   MFC after:   3 days
 
 Modified:
   head/sys/i386/xen/pmap.c
 
 Modified: head/sys/i386/xen/pmap.c
 ==============================================================================
 --- head/sys/i386/xen/pmap.c   Sun Dec 26 01:42:52 2010        (r216702)
 +++ head/sys/i386/xen/pmap.c   Sun Dec 26 13:05:43 2010        (r216703)
 @@ -1015,7 +1015,9 @@ pmap_pte_release(pt_entry_t *pte)
        if ((pt_entry_t *)((vm_offset_t)pte & ~PAGE_MASK) == PADDR2) {
                CTR1(KTR_PMAP, "pmap_pte_release: pte=0x%jx",
                    *PMAP2);
 +              vm_page_lock_queues();
                PT_SET_VA(PMAP2, 0, TRUE);
 +              vm_page_unlock_queues();
                mtx_unlock(&PMAP2mutex);
        }
  }
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