bcachefs: Fix assertion popping in transaction commit path

We can't be holding read locks on btree nodes when we go to take write
locks: this would deadlock if another thread is holding an intent lock
on the node we have a read lock on, and it tries to commit and upgrade
to a write lock.

But instead of triggering an assertion, if this happens we can just
upgrade the read lock to an intent lock.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_update_leaf.c

@@ -503,6 +503,10 @@ static inline int do_bch2_trans_commit(struct btree_trans *trans,
 
 	/*
 	 * Can't be holding any read locks when we go to take write locks:
+	 * another thread could be holding an intent lock on the same node we
+	 * have a read lock on, and it'll block trying to take a write lock
+	 * (because we hold a read lock) and it could be blocking us by holding
+	 * its own read lock (while we're trying to to take write locks).
 	 *
 	 * note - this must be done after bch2_trans_journal_preres_get_cold()
 	 * or anything else that might call bch2_trans_relock(), since that
@@ -510,9 +514,15 @@ static inline int do_bch2_trans_commit(struct btree_trans *trans,
 	 */
 	trans_for_each_iter(trans, iter) {
 		if (iter->nodes_locked != iter->nodes_intent_locked) {
-			EBUG_ON(iter->flags & BTREE_ITER_KEEP_UNTIL_COMMIT);
-			EBUG_ON(trans->iters_live & (1ULL << iter->idx));
-			bch2_btree_iter_unlock_noinline(iter);
+			if ((iter->flags & BTREE_ITER_KEEP_UNTIL_COMMIT) ||
+			    (trans->iters_live & (1ULL << iter->idx))) {
+				if (!bch2_btree_iter_upgrade(iter, 1)) {
+					trace_trans_restart_upgrade(trans->ip);
+					return -EINTR;
+				}
+			} else {
+				bch2_btree_iter_unlock_noinline(iter);
+			}
 		}
 	}