[PATCH] scsi_dh_alua: avoid crash during alua_bus_detach()

[Hannes Reinecke <hare@suse.de>]

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 would be 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.

Cc: Brian Bunker <brian@purestorage.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
---
 drivers/scsi/device_handler/scsi_dh_alua.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/device_handler/scsi_dh_alua.c b/drivers/scsi/device_handler/scsi_dh_alua.c
index f32da0ca529e..308bda2e9c00 100644
--- a/drivers/scsi/device_handler/scsi_dh_alua.c
+++ b/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);
 }
 
-- 
2.16.4