System reboot hangs due to race against devices_kset->listtriggered by SCSI FC workqueue

[Hugh Daschbach]

The system may fail to boot when the kernel's devices_kset->list gets
written by another thread while device_shutdown() is traversing the
list.  Though not common, this is fairly reproducible for some SCSI
Fibre Channel topologies; particularly so with FCoE configurations.

The reboot thread calls device_shutdown() as part of system shutdown.
device_shutdown() loops through devices_kset->list, shutting down each
system device.  But devices_kset->list isn't protected from other
writers while device_shutdown() traverses the list.

One such secondary writer is the SCI Fibre Channel workqueue.  When
fc_wq_N removes a device that device_shutdown() holds in it's "devn"
(list traversal iterator) variable, device_shutdown() stalls, chasing
what is essentially a broken link.

This is not a common occurrence.  But FC SCSI devices associated with a
link that has gone down cause a race between device_shutdown() running
in reboot's process and scsi_remove_target() running in a SCSI FC
workqueue (fc_wq_N).

Network attached FC devices are particularly vulnerable because SysV
init scripts shut network interfaces down before proceeding with the
reboot request.  So by the time reboot is called, the link to the FC
devices is already down.

When the link is down device_shutdown() stalls (in sd_shutdown() --
which issues cache flush CDBs to what are, by that time, inaccessible
devices).  The stall ends when the fc rport timer expires.  But the
timer expiration also initiates fc_starget_delete() in the fc workqueue,
causing the race with device_shutdown().

The attached patch detects and attempts to recover from the
corruption.  But this can hardly be considered a fix, as it does not
address the race between device_shutdown() and scsi_remove_target().

Perhaps converting the list_for_each_entry_safe_reverse() to something
like.

        while (!list_empty(&devices_kset->list)) {
                dev = list_last_entry(...);
                ...
        }

might be appropriate.  But I have no idea if any devices don't fully
remove themselves from the list when shutdown.

Does anyone have any guidance for what would make a more appropriate
fix?

Thanks,
Hugh