md: don't call ->add_disk unless there is good reason.

Commit 7bfec5f

   md/raid5: If there is a spare and a want_replacement device, start replacement.

cause md_check_recovery to call ->add_disk much more often.
Instead of only when the array is degraded, it is now called whenever
md_check_recovery finds anything useful to do, which includes
updating the metadata for clean<->dirty transition.
This causes unnecessary work, and causes info messages from ->add_disk
to be reported much too often.

So refine md_check_recovery to only do any actual recovery checking
(including ->add_disk) if MD_RECOVERY_NEEDED is set.

This fix is suitable for 3.3.y:

Cc: [email protected]
Reported-by: Jan Ceuleers <[email protected]>
Signed-off-by: NeilBrown <[email protected]>

drivers/md/md.c

@@ -7560,14 +7560,14 @@ void md_check_recovery(struct mddev *mddev)
 		 * any transients in the value of "sync_action".
 		 */
 		set_bit(MD_RECOVERY_RUNNING, &mddev->recovery);
-		clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
 		/* Clear some bits that don't mean anything, but
 		 * might be left set
 		 */
 		clear_bit(MD_RECOVERY_INTR, &mddev->recovery);
 		clear_bit(MD_RECOVERY_DONE, &mddev->recovery);
 
-		if (test_bit(MD_RECOVERY_FROZEN, &mddev->recovery))
+		if (!test_and_clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery) ||
+		    test_bit(MD_RECOVERY_FROZEN, &mddev->recovery))
 			goto unlock;
 		/* no recovery is running.
 		 * remove any failed drives, then