kern/153672: i386/XEN panics under heavy fork load

[Colin Percival]

>Number:         153672
>Category:       kern
>Synopsis:       i386/XEN panics under heavy fork load
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Jan 04 11:10:09 UTC 2011
>Closed-Date:
>Last-Modified:
>Originator:     Colin Percival
>Release:        FreeBSD 9.0-CURRENT i386/XEN
>Organization:
>Environment:
>Description:

Running 'make index', i386/XEN frequently panics.  There are three
common backtraces:
  vmspace_fork -> pmap_copy -> pmap_qenter
  vmspace_fork -> pmap_copy -> pmap_zero_page
  vmspace_fork -> pmap_pinit -> pmap_qenter

Notably, in every vmcore I've examined, a different thread was inside
pmap_release, suggesting that there is a race happening between
pmap_release and pmap_{copy, pinit}.

>How-To-Repeat:

Launch an EC2 instance running ami-f4db2a9d; portsnap fetch extract;
cd /usr/ports && make index.

On average there is one panic per 100 minutes.

>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted: