This problem only occurs on versions of FreeBSD prior to the recent CAM
locking changes. (i.e. stable/9 and older versions of stable/10) This
change should be a no-op for head and stable/10.
If a path isn't specified, xpt_register_async() will create a fully
wildcarded path and acquire a lock (the XPT lock in older versions,
and via xpt_path_lock() in newer versions) to call xpt_action() for the
XPT_SASYNC_CB CCB. It will then drop the lock and if the requested event
includes AC_FOUND_DEVICE or AC_PATH_REGISTERED, it will get the caller up
to date with any device arrivals or path registrations.
The issue is that before the locking changes, each SIM lock would get
acquired in turn during the EDT tree traversal process. If a path is
specified for xpt_register_async(), it won't acquire and drop its own lock,
but instead expects the caller to hold its own SIM lock. That works for
the first part of xpt_register_async(), but causes a recursive lock
acquisition once the EDT traversal happens and it comes to the SIM in
question. And it isn't possible to call xpt_action() without holding a SIM
lock.
The locking changes fix this by using the XPT topology lock for EDT
traversal, so it is no longer an issue to hold the SIM lock while calling
xpt_register_async().
The solution for FreeBSD versions before the locking changes is to request
notification of all device arrivals (so we pass a NULL path into
xpt_register_async()) and then filter out the arrivals that are not ours.
MFC After: 3 days
Sponsored by: Spectra Logic Corporation
07aa4de131