bcachefs: Fix a deadlock in the RO path

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

fs/bcachefs/btree_gc.c

@@ -932,7 +932,12 @@ int bch2_gc_gens(struct bch_fs *c)
 	unsigned i;
 	int ret;
 
-	down_read(&c->state_lock);
+	/*
+	 * Ideally we would be using state_lock and not gc_lock here, but that
+	 * introduces a deadlock in the RO path - we currently take the state
+	 * lock at the start of going RO, thus the gc thread may get stuck:
+	 */
+	down_read(&c->gc_lock);
 
 	for_each_member_device(ca, c, i) {
 		down_read(&ca->bucket_lock);
@@ -959,7 +964,7 @@ int bch2_gc_gens(struct bch_fs *c)
 		up_read(&ca->bucket_lock);
 	}
 err:
-	up_read(&c->state_lock);
+	up_read(&c->gc_lock);
 	return ret;
 }
 

fs/bcachefs/buckets.c

@@ -2003,6 +2003,7 @@ int bch2_dev_buckets_resize(struct bch_fs *c, struct bch_dev *ca, u64 nbuckets)
 	bch2_copygc_stop(ca);
 
 	if (resize) {
+		down_write(&c->gc_lock);
 		down_write(&ca->bucket_lock);
 		percpu_down_write(&c->mark_lock);
 	}
@@ -2025,8 +2026,10 @@ int bch2_dev_buckets_resize(struct bch_fs *c, struct bch_dev *ca, u64 nbuckets)
 
 	swap(ca->buckets_nouse, buckets_nouse);
 
-	if (resize)
+	if (resize) {
 		percpu_up_write(&c->mark_lock);
+		up_write(&c->gc_lock);
+	}
 
 	spin_lock(&c->freelist_lock);
 	for (i = 0; i < RESERVE_NR; i++) {