md/raid: raid5 preserve the writeback action after the parity check

The problem is that any 'uptodate' vs 'disks' check is not precise
in this path. Put a "WARN_ON(!test_bit(R5_UPTODATE, &dev->flags)" on the
device that might try to kick off writes and then skip the action.
Better to prevent the raid driver from taking unexpected action *and* keep
the system alive vs killing the machine with BUG_ON.

Note: fixed warning reported by kbuild test robot <lkp@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Nigel Croxon <ncroxon@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>

drivers/md/raid5.c

@@ -4191,7 +4191,7 @@ static void handle_parity_checks6(struct r5conf *conf, struct stripe_head *sh,
 		/* now write out any block on a failed drive,
 		 * or P or Q if they were recomputed
 		 */
-		BUG_ON(s->uptodate < disks - 1); /* We don't need Q to recover */
+		dev = NULL;
 		if (s->failed == 2) {
 			dev = &sh->dev[s->failed_num[1]];
 			s->locked++;
@@ -4216,6 +4216,14 @@ static void handle_parity_checks6(struct r5conf *conf, struct stripe_head *sh,
 			set_bit(R5_LOCKED, &dev->flags);
 			set_bit(R5_Wantwrite, &dev->flags);
 		}
+		if (WARN_ONCE(dev && !test_bit(R5_UPTODATE, &dev->flags),
+			      "%s: disk%td not up to date\n",
+			      mdname(conf->mddev),
+			      dev - (struct r5dev *) &sh->dev)) {
+			clear_bit(R5_LOCKED, &dev->flags);
+			clear_bit(R5_Wantwrite, &dev->flags);
+			s->locked--;
+		}
 		clear_bit(STRIPE_DEGRADED, &sh->state);
 
 		set_bit(STRIPE_INSYNC, &sh->state);