[PATCH] Avoid hang when multiple raids on shared drives are trying to sync.

I think this is finally right:
 - we always wake_up when curr_resync decreases.
 - we wait for the other guys curr_resync to be less than ours
 - if we ever wait for someone who might yield, we start checking
   again from the start.

drivers/md/md.c

@@ -2838,16 +2838,18 @@ static void md_do_sync(void *data)
 				       mdidx(mddev), mdidx(mddev2));
 				if (mddev < mddev2) {/* arbitrarily yield */
 					mddev->curr_resync = 1;
-					yield();
+					wake_up(&resync_wait);
 				}
 				if (wait_event_interruptible(resync_wait,
-							     mddev2->curr_resync < 2)) {
+							     mddev2->curr_resync < mddev->curr_resync)) {
 					flush_curr_signals();
 					err = -EINTR;
 					mddev_put(mddev2);
 					goto skip;
 				}
 			}
+			if (mddev->curr_resync == 1)
+				break;
 		}
 	} while (mddev->curr_resync < 2);