bcachefs: c->ro_ref

Add a new refcount for async ops that don't necessarily need the fs to
be RW, with similar lifetime/rules otherwise as c->writes.

To be used by online fsck.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -710,6 +710,13 @@ struct bch_fs {
 #else
 	struct percpu_ref	writes;
 #endif
+	/*
+	 * Analagous to c->writes, for asynchronous ops that don't necessarily
+	 * need fs to be read-write
+	 */
+	refcount_t		ro_ref;
+	wait_queue_head_t	ro_ref_wait;
+
 	struct work_struct	read_only_work;
 
 	struct bch_dev __rcu	*devs[BCH_SB_MEMBERS_MAX];
@@ -1105,6 +1112,20 @@ static inline void bch2_write_ref_put(struct bch_fs *c, enum bch_write_ref ref)
 #endif
 }
 
+static inline bool bch2_ro_ref_tryget(struct bch_fs *c)
+{
+	if (test_bit(BCH_FS_stopping, &c->flags))
+		return false;
+
+	return refcount_inc_not_zero(&c->ro_ref);
+}
+
+static inline void bch2_ro_ref_put(struct bch_fs *c)
+{
+	if (refcount_dec_and_test(&c->ro_ref))
+		wake_up(&c->ro_ref_wait);
+}
+
 static inline void bch2_set_ra_pages(struct bch_fs *c, unsigned ra_pages)
 {
 #ifndef NO_BCACHEFS_FS

fs/bcachefs/super.c

@@ -599,6 +599,9 @@ void __bch2_fs_stop(struct bch_fs *c)
 	bch2_fs_debug_exit(c);
 	bch2_fs_chardev_exit(c);
 
+	bch2_ro_ref_put(c);
+	wait_event(c->ro_ref_wait, !refcount_read(&c->ro_ref));
+
 	kobject_put(&c->counters_kobj);
 	kobject_put(&c->time_stats);
 	kobject_put(&c->opts_dir);
@@ -729,6 +732,9 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 	mutex_init(&c->btree_root_lock);
 	INIT_WORK(&c->read_only_work, bch2_fs_read_only_work);
 
+	refcount_set(&c->ro_ref, 1);
+	init_waitqueue_head(&c->ro_ref_wait);
+
 	init_rwsem(&c->gc_lock);
 	mutex_init(&c->gc_gens_lock);
 	atomic_set(&c->journal_keys.ref, 1);