Re: [RFC PATCH 04/12] rcu_txn: Add CONFIG_DEBUG_RCU_TXN

[Puranjay Mohan <puranjay@kernel.org>]

On Fri, Oct 16, 2026 at 09:33:11AM -0400, Mathieu Desnoyers wrote:
[...]

> @@ -261,6 +291,7 @@ static inline bool rcu_txn_is_proxy(const void *v, unsigned long tag)
>   */
>  static inline struct rcu_txn_record *__rcu_txn_untag(void *v, unsigned long tag)
>  {
> +	RCU_TXN_WARN_ON(!rcu_txn_is_proxy(v, tag));
>  	return (struct rcu_txn_record *)((unsigned long)v - tag);
>  }

This can never fire: the only caller, rcu_txn_resolve(), has just
checked rcu_txn_is_proxy(v, tag).  I'd drop it.

[...]

> +	RCU_TXN_WARN_ON(__rcu_txn_holds_proxy(old_ptr, tag));
> +	RCU_TXN_WARN_ON(__rcu_txn_holds_proxy(new_ptr, tag));

[...]

> @@ -715,6 +893,9 @@ static inline bool __rcu_txn_chain(struct rcu_txn *t, void __rcu **slot,
>  	r = __rcu_txn_find_ryw(t, slot);
>  	if (!r)
>  		return rcu_txn_record(t, slot, old_ptr, new_ptr, tag);
> +	RCU_TXN_WARN_ON(r->tag != tag);
> +	/* The caller must have read its own write. */
> +	RCU_TXN_WARN_ON(r->ptr[1] != old_ptr);
>  	r->ptr[1] = new_ptr;		/* committed old preserved */
>  	return true;
>  }

The append path checks new_ptr for the tag, but the chain path doesn't.
With CONFIG_DEBUG_RCU_TXN, chaining a tagged value onto an
already-recorded slot of a non-disjoint handle and committing gives no
warning, and leaves the tagged value in the slot, where every later
reader treats it as a proxy.  Passing the same slot, value and tag
through the append path warns once:

    # rcu_txn_test_exp_chain_tagged: append path: 1 warnings
    # rcu_txn_test_exp_chain_tagged: chain path + commit: 0 warnings, slot now 0000000000000301 (tagged: 1)

The same check in the chain path catches it:

--- a/include/linux/rcu_txn.h
+++ b/include/linux/rcu_txn.h
@@ -905,6 +905,7 @@ static inline bool __rcu_txn_chain(struct rcu_txn *t, void __rcu **slot,
 	RCU_TXN_WARN_ON(r->tag != tag);
 	/* The caller must have read its own write. */
 	RCU_TXN_WARN_ON(r->ptr[1] != old_ptr);
+	RCU_TXN_WARN_ON(__rcu_txn_holds_proxy(new_ptr, tag));
 	r->ptr[1] = new_ptr;		/* committed old preserved */
 	return true;
 }

Thanks,
Puranjay