[PATCH net-next v2 1/2] net/rds: reinitialize to_be_dropped on rds_send_xmit() restart

[Allison Henderson <achender@kernel.org>]

From: Sharath Srinivasan <sharath.srinivasan@oracle.com>

The to_be_dropped list is declared once at the top of rds_send_xmit()
but the function can loop via "goto restart" after each batch.  The
code currently relies on rds_send_remove_from_sock() having emptied
the list entry by entry (via list_del_init()) at the end of the
previous batch; nothing in rds_send_xmit() itself guarantees the list
head is empty when a new batch starts.

Re-initialize the list on every restart, and warn once if it is ever
found non-empty there: entries left on the list at that point would
keep their message reference, their RDS_MSG_ON_SOCK accounting and
their pending RDS_RDMA_DROPPED notification, so a silent re-init
would orphan them.  This is hardening: no user-visible bug is known
in the current code.

This mirrors Oracle UEK commit "net/rds: rds_send_xmit should
INIT_LIST_HEAD(&to_be_dropped) on restart".

Signed-off-by: Gerd Rausch <gerd.rausch@oracle.com>
Signed-off-by: Sharath Srinivasan <sharath.srinivasan@oracle.com>
[achender: port to net-next (keep the existing LIST_HEAD declaration and
 add only the restart re-init); warn if the restart invariant is
 violated; update commit message]
Assisted-by: Claude-Code:claude-fable-5
Signed-off-by: Allison Henderson <achender@kernel.org>
---
v2: name rds_send_remove_from_sock() in the comment; warn once if the
restart-time empty-list invariant is ever violated.
v1: https://lore.kernel.org/netdev/20260806072045.1092968-2-achender@kernel.org/
 net/rds/send.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/rds/send.c b/net/rds/send.c
index 309021e0cc9b..15a1b97f13e7 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c
@@ -200,6 +200,14 @@ int rds_send_xmit(struct rds_conn_path *cp)
 restart:
 	batch_count = 0;
 
+	/* The drop processing after over_batch relies on
+	 * rds_send_remove_from_sock() emptying to_be_dropped entry by
+	 * entry; warn if that post-condition ever stops holding, and
+	 * re-initialize the list head.
+	 */
+	WARN_ON_ONCE(!list_empty(&to_be_dropped));
+	INIT_LIST_HEAD(&to_be_dropped);
+
 	/*
 	 * sendmsg calls here after having queued its message on the send
 	 * queue.  We only have one task feeding the connection at a time.  If
-- 
2.25.1