drbd: fix race between handshake and admin disconnect/down

conn_try_disconnect() could potentialy hit the BUG_ON()
in _conn_set_state() where it iterates over _drbd_set_state()
and "asserts" via BUG_ON() that the latter was successful.

If the STATE_SENT bit was not yet visible to conn_is_valid_transition()
early in _conn_request_state(), but became visible before conn_set_state()
later in that call path, we could hit the BUG_ON() after _drbd_set_state(),
because it returned SS_IN_TRANSIENT_STATE.

To avoid that race, we better protect set_bit(SENT_STATE) with the spinlock.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_receiver.c

@@ -1100,7 +1100,10 @@ randomize:
 	idr_for_each_entry(&connection->peer_devices, peer_device, vnr)
 		mutex_lock(peer_device->device->state_mutex);
 
+	/* avoid a race with conn_request_state( C_DISCONNECTING ) */
+	spin_lock_irq(&connection->resource->req_lock);
 	set_bit(STATE_SENT, &connection->flags);
+	spin_unlock_irq(&connection->resource->req_lock);
 
 	idr_for_each_entry(&connection->peer_devices, peer_device, vnr)
 		mutex_unlock(peer_device->device->state_mutex);