scsi_debug: deadlock between completions and surprise module removal

A deadlock has been reported when the completion
of SCSI commands (simulated by a timer) was surprised
by a module removal. This patch removes one half of
the offending locks around timer deletions. This fix
is applied both to stop_all_queued() which is were
the deadlock was discovered and stop_queued_cmnd()
which has very similar logic.

This patch should be applied both to the lk 3.17 tree
and Christoph's drivers-for-3.18 tree.

Tested-and-reported-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/scsi_debug.c

@@ -2743,6 +2743,13 @@ static int stop_queued_cmnd(struct scsi_cmnd *cmnd)
 		if (test_bit(k, queued_in_use_bm)) {
 			sqcp = &queued_arr[k];
 			if (cmnd == sqcp->a_cmnd) {
+				devip = (struct sdebug_dev_info *)
+					cmnd->device->hostdata;
+				if (devip)
+					atomic_dec(&devip->num_in_q);
+				sqcp->a_cmnd = NULL;
+				spin_unlock_irqrestore(&queued_arr_lock,
+						       iflags);
 				if (scsi_debug_ndelay > 0) {
 					if (sqcp->sd_hrtp)
 						hrtimer_cancel(
@@ -2755,18 +2762,13 @@ static int stop_queued_cmnd(struct scsi_cmnd *cmnd)
 					if (sqcp->tletp)
 						tasklet_kill(sqcp->tletp);
 				}
-				__clear_bit(k, queued_in_use_bm);
-				devip = (struct sdebug_dev_info *)
-					cmnd->device->hostdata;
-				if (devip)
-					atomic_dec(&devip->num_in_q);
-				sqcp->a_cmnd = NULL;
-				break;
+				clear_bit(k, queued_in_use_bm);
+				return 1;
 			}
 		}
 	}
 	spin_unlock_irqrestore(&queued_arr_lock, iflags);
-	return (k < qmax) ? 1 : 0;
+	return 0;
 }
 
 /* Deletes (stops) timers or tasklets of all queued commands */
@@ -2782,6 +2784,13 @@ static void stop_all_queued(void)
 		if (test_bit(k, queued_in_use_bm)) {
 			sqcp = &queued_arr[k];
 			if (sqcp->a_cmnd) {
+				devip = (struct sdebug_dev_info *)
+					sqcp->a_cmnd->device->hostdata;
+				if (devip)
+					atomic_dec(&devip->num_in_q);
+				sqcp->a_cmnd = NULL;
+				spin_unlock_irqrestore(&queued_arr_lock,
+						       iflags);
 				if (scsi_debug_ndelay > 0) {
 					if (sqcp->sd_hrtp)
 						hrtimer_cancel(
@@ -2794,12 +2803,8 @@ static void stop_all_queued(void)
 					if (sqcp->tletp)
 						tasklet_kill(sqcp->tletp);
 				}
-				__clear_bit(k, queued_in_use_bm);
-				devip = (struct sdebug_dev_info *)
-					sqcp->a_cmnd->device->hostdata;
-				if (devip)
-					atomic_dec(&devip->num_in_q);
-				sqcp->a_cmnd = NULL;
+				clear_bit(k, queued_in_use_bm);
+				spin_lock_irqsave(&queued_arr_lock, iflags);
 			}
 		}
 	}