btrfs: reada: avoid undone reada extents in btrfs_reada_wait

Reada background works is not designed to finish all jobs
completely, it will break in following case:
1: When a device reaches workload limit (MAX_IN_FLIGHT)
2: Total reads reach max limit (10000)
3: All devices don't have queued more jobs, often happened in DUP case

And if all background works exit with remaining jobs,
btrfs_reada_wait() will wait indefinetelly.

Above problem is rarely happened in old code, because:
1: Every work queues 2x new works
   So many works reduced chances of undone jobs.
2: One work will continue 10000 times loop in case of no-jobs
   It reduced no-thread window time.

But after we fixed above case, the "undone reada extents" frequently
happened.

Fix:
 Check to ensure we have at least one thread if there are undone jobs
 in btrfs_reada_wait().
Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/reada.c

@@ -953,8 +953,11 @@ struct reada_control *btrfs_reada_add(struct btrfs_root *root,
 int btrfs_reada_wait(void *handle)
 {
 	struct reada_control *rc = handle;
+	struct btrfs_fs_info *fs_info = rc->root->fs_info;
 
 	while (atomic_read(&rc->elems)) {
+		if (!atomic_read(&fs_info->reada_works_cnt))
+			reada_start_machine(fs_info);
 		wait_event_timeout(rc->wait, atomic_read(&rc->elems) == 0,
 				   5 * HZ);
 		dump_devs(rc->root->fs_info,
@@ -971,9 +974,13 @@ int btrfs_reada_wait(void *handle)
 int btrfs_reada_wait(void *handle)
 {
 	struct reada_control *rc = handle;
+	struct btrfs_fs_info *fs_info = rc->root->fs_info;
 
 	while (atomic_read(&rc->elems)) {
-		wait_event(rc->wait, atomic_read(&rc->elems) == 0);
+		if (!atomic_read(&fs_info->reada_works_cnt))
+			reada_start_machine(fs_info);
+		wait_event_timeout(rc->wait, atomic_read(&rc->elems) == 0,
+				   (HZ + 9) / 10);
 	}
 
 	kref_put(&rc->refcnt, reada_control_release);