diff --git a/module/zfs/dmu_tx.c b/module/zfs/dmu_tx.c
index cbadcc86fc61..7d65e842ff03 100644
--- a/module/zfs/dmu_tx.c
+++ b/module/zfs/dmu_tx.c
@@ -925,6 +925,25 @@ dmu_tx_try_assign(dmu_tx_t *tx, uint64_t txg_how)
 	    txh = list_next(&tx->tx_holds, txh)) {
 		dnode_t *dn = txh->txh_dnode;
 		if (dn != NULL) {
+			/*
+			 * This thread can't hold the dn_struct_rwlock
+			 * while assigning the tx, because this can lead to
+			 * deadlock. Specifically, if this dnode is already
+			 * assigned to an earlier txg, this thread may need
+			 * to wait for that txg to sync (the ERESTART case
+			 * below).  The other thread that has assigned this
+			 * dnode to an earlier txg prevents this txg from
+			 * syncing until its tx can complete (calling
+			 * dmu_tx_commit()), but it may need to acquire the
+			 * dn_struct_rwlock to do so (e.g. via
+			 * dmu_buf_hold*()).
+			 *
+			 * Note that this thread can't hold the lock for
+			 * read either, but the rwlock doesn't record
+			 * enough information to make that assertion.
+			 */
+			ASSERT(!RW_WRITE_HELD(&dn->dn_struct_rwlock));
+
 			mutex_enter(&dn->dn_mtx);
 			if (dn->dn_assigned_txg == tx->tx_txg - 1) {
 				mutex_exit(&dn->dn_mtx);