[Bug 204298] xpt_release_device() panic: mutex CAM queue lock not owned

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=204298

            Bug ID: 204298
           Summary: xpt_release_device() panic: mutex CAM queue lock not
                    owned
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: smferris at gmail.com

Created attachment 162793
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=162793&action=edit
cam_xpt.c device callout patch

xpt_release_device() calls callout_stop() without holding the callout mutex
(devq->send_mtx).

When testing large-scale repeated FC device removal, we ran into panics where
xpt_release_device() was reaching refcount 0 while the callout was pending (or
at least active).

The attached patch will give the device callout a ref on the device while the
device callout is active, to ensure the device can't be destroyed until the
callout is either stopped or finishes executing.

Sponsored by: EMC / Isilon Storage Division

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