bcachefs: fix a deadlock on startup

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -1678,7 +1678,6 @@ int bch2_fs_allocator_start(struct bch_fs *c)
 {
 	struct bch_dev *ca;
 	unsigned i;
-	bool wrote;
 	int ret;
 
 	down_read(&c->gc_lock);
@@ -1697,8 +1696,7 @@ int bch2_fs_allocator_start(struct bch_fs *c)
 	}
 
 	set_bit(BCH_FS_ALLOCATOR_RUNNING, &c->flags);
-
-	return bch2_alloc_write(c, false, &wrote);
+	return 0;
 }
 
 void bch2_fs_allocator_background_init(struct bch_fs *c)

fs/bcachefs/journal_io.c

@@ -888,9 +888,18 @@ int bch2_journal_replay(struct bch_fs *c, struct list_head *list)
 static unsigned journal_dev_buckets_available(struct journal *j,
 					      struct journal_device *ja)
 {
+	struct bch_fs *c = container_of(j, struct bch_fs, journal);
 	unsigned next = (ja->cur_idx + 1) % ja->nr;
 	unsigned available = (ja->last_idx + ja->nr - next) % ja->nr;
 
+	/*
+	 * Allocator startup needs some journal space before we can do journal
+	 * replay:
+	 */
+	if (available &&
+	    test_bit(BCH_FS_ALLOCATOR_STARTED, &c->flags))
+		available--;
+
 	/*
 	 * Don't use the last bucket unless writing the new last_seq
 	 * will make another bucket available: