scsi: scsi_dh_alua: Avoid crash during alua_bus_detach()

alua_bus_detach() might be running concurrently with alua_rtpg_work(), so
we might trip over h->sdev == NULL and call BUG_ON().  The correct way of
handling it is to not set h->sdev to NULL in alua_bus_detach(), and call
rcu_synchronize() before the final delete to ensure that all concurrent
threads have left the critical section.  Then we can get rid of the
BUG_ON() and replace it with a simple if condition.

Link: https://lore.kernel.org/r/1600167537-12509-1-git-send-email-jitendra.khasdev@oracle.com
Link: https://lore.kernel.org/r/20200924104559.26753-1-hare@suse.de
Cc: Brian Bunker <brian@purestorage.com>
Acked-by: Brian Bunker <brian@purestorage.com>
Tested-by: Jitendra Khasdev <jitendra.khasdev@oracle.com>
Reviewed-by: Jitendra Khasdev <jitendra.khasdev@oracle.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/device_handler/scsi_dh_alua.c

@@ -658,8 +658,8 @@ static int alua_rtpg(struct scsi_device *sdev, struct alua_port_group *pg)
 					rcu_read_lock();
 					list_for_each_entry_rcu(h,
 						&tmp_pg->dh_list, node) {
-						/* h->sdev should always be valid */
-						BUG_ON(!h->sdev);
+						if (!h->sdev)
+							continue;
 						h->sdev->access_state = desc[0];
 					}
 					rcu_read_unlock();
@@ -705,7 +705,8 @@ static int alua_rtpg(struct scsi_device *sdev, struct alua_port_group *pg)
 			pg->expiry = 0;
 			rcu_read_lock();
 			list_for_each_entry_rcu(h, &pg->dh_list, node) {
-				BUG_ON(!h->sdev);
+				if (!h->sdev)
+					continue;
 				h->sdev->access_state =
 					(pg->state & SCSI_ACCESS_STATE_MASK);
 				if (pg->pref)
@@ -1147,7 +1148,6 @@ static void alua_bus_detach(struct scsi_device *sdev)
 	spin_lock(&h->pg_lock);
 	pg = rcu_dereference_protected(h->pg, lockdep_is_held(&h->pg_lock));
 	rcu_assign_pointer(h->pg, NULL);
-	h->sdev = NULL;
 	spin_unlock(&h->pg_lock);
 	if (pg) {
 		spin_lock_irq(&pg->lock);
@@ -1156,6 +1156,7 @@ static void alua_bus_detach(struct scsi_device *sdev)
 		kref_put(&pg->kref, release_port_group);
 	}
 	sdev->handler_data = NULL;
+	synchronize_rcu();
 	kfree(h);
 }