ubi: fastmap: Implement produce_free_peb()

If fastmap requests a free PEB for a pool and UBI is busy
with erasing PEBs we need to offer a function to wait for one.
We can reuse produce_free_peb() from the non-fastmap WL code
but with different locking semantics.

Cc: stable@vger.kernel.org # 4.1.x-
Reported-and-tested-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/fastmap-wl.c

@@ -171,6 +171,30 @@ void ubi_refill_pools(struct ubi_device *ubi)
 	spin_unlock(&ubi->wl_lock);
 }
 
+/**
+ * produce_free_peb - produce a free physical eraseblock.
+ * @ubi: UBI device description object
+ *
+ * This function tries to make a free PEB by means of synchronous execution of
+ * pending works. This may be needed if, for example the background thread is
+ * disabled. Returns zero in case of success and a negative error code in case
+ * of failure.
+ */
+static int produce_free_peb(struct ubi_device *ubi)
+{
+	int err;
+
+	while (!ubi->free.rb_node && ubi->works_count) {
+		dbg_wl("do one work synchronously");
+		err = do_work(ubi);
+
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
 /**
  * ubi_wl_get_peb - get a physical eraseblock.
  * @ubi: UBI device description object
@@ -213,6 +237,11 @@ int ubi_wl_get_peb(struct ubi_device *ubi)
 		}
 		retried = 1;
 		up_read(&ubi->fm_eba_sem);
+		ret = produce_free_peb(ubi);
+		if (ret < 0) {
+			down_read(&ubi->fm_eba_sem);
+			goto out;
+		}
 		goto again;
 	}