[Bug 201421] locking issues prevent new process creation

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=201421

            Bug ID: 201421
           Summary: locking issues prevent new process creation
           Product: Base System
           Version: 10.2-STABLE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: threads
          Assignee: freebsd-threads at FreeBSD.org
          Reporter: FreeBSD at ShaneWare.Biz

Current system is - FreeBSD leader.local 10.2-PRERELEASE FreeBSD
10.2-PRERELEASE #13 r285123: Sat Jul  4 15:30:32 ACST 2015    
root at leader.local:/usr/obj/usr/src/sys/GENERIC  amd64

For some time I have had repeated trouble forcing me to restart. I have had a
couple of comments from freebsd-stable mailing list to suggest it is locking
related but no suggestions for gathering more useful data. This isn't something
I can say 'do this and it breaks', I can only say it has been a long time since
I have had more than a few days uptime. Before changing to 10.1 (from 9.2) I
only restarted about once a month after installing updates.

While most programs already running will keep running ok, many new processes
fail to start. An instance of top started earlier will stop updating(usually my
first confirmation of needing to restart), ps and proctstat will always fail. I
can run `xterm -e tcsh -f`, to bypass my shell login script.

The only info I know to collect is `thread apply all bt` from kgdb. The info I
have collected can be found with a copy of boot.dmesg at
http://shaneware.biz/freebsddebugdata/

For the last two months I have included the uptime output and you will find 6
occasions I have made it to more than one day uptime. While I have had other
restarts, these are occasions when I have been able to get kgdb output, others
have usually been a complete lockup mostly when using poudriere. 

At least two occasions I have restarted, logged in and run startxfce4 then left
it to sit while I had something else to do only to return maybe one or two
hours later and have to restart again before I can do anything.

The few times I have got an output from `procstat -kk -a` has been when
inserting a usb memstick has failed to create a new device entry, preventing me
from mounting the filesystem. Over the last 2 months I have had this happen on
three occasions, each of which I was able to continue running without use of
adding usb devices for between three and seven days uptime which is better than
any other I have had. This makes me think that the usb or device creation may
be a location of this issue.

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