bcachefs: Fix copygc dying on startup

The copygc threads errors out and makes the filesystem go RO if it ever
tries to run and discovers it has no reserve allocated - which is a
problem if it races with the allocator thread and its reserve hasn't
been filled yet.

The allocator thread doesn't start filling the copygc reserve until
after BCH_FS_STARTED has been set, so make sure to wake up the allocator
threads after setting that and before starting copygc.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.h

@@ -61,8 +61,10 @@ static inline void bch2_wake_allocator(struct bch_dev *ca)
 
 	rcu_read_lock();
 	p = rcu_dereference(ca->alloc_thread);
-	if (p)
+	if (p) {
 		wake_up_process(p);
+		ca->allocator_state = ALLOCATOR_RUNNING;
+	}
 	rcu_read_unlock();
 }
 

fs/bcachefs/super.c

@@ -888,6 +888,13 @@ int bch2_fs_start(struct bch_fs *c)
 
 	set_bit(BCH_FS_STARTED, &c->flags);
 
+	/*
+	 * Allocator threads don't start filling copygc reserve until after we
+	 * set BCH_FS_STARTED - wake them now:
+	 */
+	for_each_online_member(ca, c, i)
+		bch2_wake_allocator(ca);
+
 	if (c->opts.read_only || c->opts.nochanges) {
 		bch2_fs_read_only(c);
 	} else {