powerpc/eeh: Clean up EEH PEs after recovery finishes

When the last device in an eeh_pe is removed the eeh_pe structure itself
(and any empty parents) are freed since they are no longer needed. This
results in a crash when a hotplug driver is involved since the following
may occur:

1. Device is suprise removed.
2. Driver performs an MMIO, which fails and queues and eeh_event.
3. Hotplug driver receives a hotplug interrupt and removes any
   pci_devs that were under the slot.
4. pci_dev is torn down and the eeh_pe is freed.
5. The EEH event handler thread processes the eeh_event and crashes
   since the eeh_pe pointer in the eeh_event structure is no
   longer valid.

Crashing is generally considered poor form. Instead of doing that use
the fact PEs are marked as EEH_PE_INVALID to keep them around until the
end of the recovery cycle, at which point we can safely prune any empty
PEs.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190903101605.2890-2-oohall@gmail.com

arch/powerpc/kernel/eeh_event.c

@@ -121,6 +121,14 @@ int __eeh_send_failure_event(struct eeh_pe *pe)
 	}
 	event->pe = pe;
 
+	/*
+	 * Mark the PE as recovering before inserting it in the queue.
+	 * This prevents the PE from being free()ed by a hotplug driver
+	 * while the PE is sitting in the event queue.
+	 */
+	if (pe)
+		eeh_pe_state_mark(pe, EEH_PE_RECOVERING);
+
 	/* We may or may not be called in an interrupt context */
 	spin_lock_irqsave(&eeh_eventlist_lock, flags);
 	list_add(&event->list, &eeh_eventlist);