[Bug 263704] Panic because device went away with XPT_ATA_IO pending

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263704

Warner Losh <imp@FreeBSD.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|New                         |In Progress
           Assignee|bugs@FreeBSD.org            |imp@FreeBSD.org

--- Comment #1 from Warner Losh <imp@FreeBSD.org> ---
On further examination, it appears to be a bug in the dynamic I/O
scheduler. We can call xpt_schedule(CAM_PRI_NORMAL) during recovery
which causes all existing periphs that use it to schedule their recovery
operation a second time, but at a bad priority. It has nothing to do
with dropping references to the device, but rather causing extra
I/O to be scheduled that can persist after the periph is invalidated
(because the periph driver knows nothing of the extra CCBs) leading
to accessing the path after it's been freed which leads to a number of
different pathologies depending on where in the CCB lifecycle we wakeup. It
only affects the dynamic scheduler (all the time, but that points out another
bug, the timeout ticker shouldn't run all the time if there's nothing to
be controlled like bandwidth or iops).

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