bcachefs: Fix erasure coding locking

This adds a new helper, bch2_trans_mutex_lock(), for locking a mutex -
dropping and retaking btree locks as needed.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_foreground.c

@@ -1073,7 +1073,7 @@ static bool try_decrease_writepoints(struct bch_fs *c,
 	return true;
 }
 
-static void bch2_trans_mutex_lock(struct btree_trans *trans,
+static void bch2_trans_mutex_lock_norelock(struct btree_trans *trans,
 				  struct mutex *lock)
 {
 	if (!mutex_trylock(lock)) {
@@ -1091,7 +1091,7 @@ static struct write_point *writepoint_find(struct btree_trans *trans,
 
 	if (!(write_point & 1UL)) {
 		wp = (struct write_point *) write_point;
-		bch2_trans_mutex_lock(trans, &wp->lock);
+		bch2_trans_mutex_lock_norelock(trans, &wp->lock);
 		return wp;
 	}
 
@@ -1100,7 +1100,7 @@ static struct write_point *writepoint_find(struct btree_trans *trans,
 	wp = __writepoint_find(head, write_point);
 	if (wp) {
 lock_wp:
-		bch2_trans_mutex_lock(trans, &wp->lock);
+		bch2_trans_mutex_lock_norelock(trans, &wp->lock);
 		if (wp->write_point == write_point)
 			goto out;
 		mutex_unlock(&wp->lock);
@@ -1113,8 +1113,8 @@ static struct write_point *writepoint_find(struct btree_trans *trans,
 		if (!oldest || time_before64(wp->last_used, oldest->last_used))
 			oldest = wp;
 
-	bch2_trans_mutex_lock(trans, &oldest->lock);
-	bch2_trans_mutex_lock(trans, &c->write_points_hash_lock);
+	bch2_trans_mutex_lock_norelock(trans, &oldest->lock);
+	bch2_trans_mutex_lock_norelock(trans, &c->write_points_hash_lock);
 	if (oldest >= c->write_points + c->write_points_nr ||
 	    try_increase_writepoints(c)) {
 		mutex_unlock(&c->write_points_hash_lock);

fs/bcachefs/btree_iter.h

@@ -197,6 +197,15 @@ struct bkey_i *bch2_btree_journal_peek_slot(struct btree_trans *,
 
 void bch2_btree_path_level_init(struct btree_trans *, struct btree_path *, struct btree *);
 
+int __bch2_trans_mutex_lock(struct btree_trans *, struct mutex *);
+
+static inline int bch2_trans_mutex_lock(struct btree_trans *trans, struct mutex *lock)
+{
+	return mutex_trylock(lock)
+		? 0
+		: __bch2_trans_mutex_lock(trans, lock);
+}
+
 #ifdef CONFIG_BCACHEFS_DEBUG
 void bch2_trans_verify_paths(struct btree_trans *);
 void bch2_assert_pos_locked(struct btree_trans *, enum btree_id,

fs/bcachefs/btree_locking.c

@@ -725,6 +725,19 @@ bool bch2_trans_locked(struct btree_trans *trans)
 	return false;
 }
 
+int __bch2_trans_mutex_lock(struct btree_trans *trans,
+			    struct mutex *lock)
+{
+	int ret;
+
+	bch2_trans_unlock(trans);
+	mutex_lock(lock);
+	ret = bch2_trans_relock(trans);
+	if (ret)
+		mutex_unlock(lock);
+	return ret;
+}
+
 /* Debug */
 
 #ifdef CONFIG_BCACHEFS_DEBUG

fs/bcachefs/ec.c

@@ -1231,7 +1231,7 @@ ec_new_stripe_head_alloc(struct bch_fs *c, unsigned target,
 		return NULL;
 
 	mutex_init(&h->lock);
-	mutex_lock(&h->lock);
+	BUG_ON(!mutex_trylock(&h->lock));
 
 	h->target	= target;
 	h->algo		= algo;
@@ -1280,23 +1280,18 @@ struct ec_stripe_head *__bch2_ec_stripe_head_get(struct btree_trans *trans,
 	if (!redundancy)
 		return NULL;
 
-	if (!mutex_trylock(&c->ec_stripe_head_lock)) {
-		bch2_trans_unlock(trans);
-		mutex_lock(&c->ec_stripe_head_lock);
-
-		ret = bch2_trans_relock(trans);
-		if (ret) {
-			mutex_unlock(&c->ec_stripe_head_lock);
-			return ERR_PTR(ret);
-		}
-	}
+	ret = bch2_trans_mutex_lock(trans, &c->ec_stripe_head_lock);
+	if (ret)
+		return ERR_PTR(ret);
 
 	list_for_each_entry(h, &c->ec_stripe_head_list, list)
 		if (h->target		== target &&
 		    h->algo		== algo &&
 		    h->redundancy	== redundancy &&
 		    h->copygc		== copygc) {
-			mutex_lock(&h->lock);
+			ret = bch2_trans_mutex_lock(trans, &h->lock);
+			if (ret)
+				h = ERR_PTR(ret);
 			goto found;
 		}