[PATCH net v5 4/7] net/rds: tcp: don't force RDS_CONN_RESETTING over a concurrent shutdown

[Allison Henderson <achender@kernel.org>]

From: Gerd Rausch <gerd.rausch@oracle.com>

rds_tcp_reset_callbacks() resolves a duelling SYN by storing
RDS_CONN_RESETTING into cp_state unconditionally.  Nothing serializes
that store against the shutdown path: rds_tcp_accept_one() checks
for RDS_CONN_CONNECTING or RDS_CONN_ERROR under t_conn_path_lock, but
neither rds_conn_path_drop(), which forces RDS_CONN_ERROR, nor
rds_conn_shutdown(), which moves the path to RDS_CONN_DISCONNECTING
under cp_cm_lock, takes that lock.  The store can therefore land on
top of a shutdown that is already in progress, or that gets queued
right after the accept-side check.

When it does, the shutdown worker's final DISCONNECTING -> DOWN
transition fails and the path goes through rds_conn_path_error() and
a second drop/shutdown cycle instead of a clean reconnect, tearing
down the socket the accept path has just installed.  Before commit
ad22d24be635 ("net/rds: No shortcut out of RDS_CONN_ERROR") a path
found in RDS_CONN_RESETTING even made rds_conn_shutdown() bail out
altogether.

Make the transition conditional: move CONNECTING -> RESETTING (or
stay in RESETTING from an earlier duel), and drop the path in any
other state.  The drop has side effects of its own: it replaces the
shutdown's RDS_CONN_DISCONNECTING (or RDS_CONN_ERROR) with
RDS_CONN_ERROR and queues one more cp_down_w run.  The difference is
that rds_conn_shutdown() accepts RDS_CONN_ERROR in its final
transition to RDS_CONN_DOWN, so the shutdown in flight completes
normally instead of through rds_conn_path_error(); the extra
down-work pass then finds the path already down and falls through to
the reconnect check, or catches a reconnect that has already started
and restarts it.  The accept path still installs the new socket,
rds_connect_path_complete() then fails its RESETTING -> UP transition
and drops it: the raced socket ends up torn down as it does today.
The comment at that call site, which promised that
rds_connect_path_complete() marks the path RDS_CONN_UP, is updated to
name this outcome as well.

The state can change again between the failed transitions and the
drop.  That is inherent to rds_conn_path_drop(), which the socket
state-change callbacks also call unconditionally, and costs at most
one extra drop/reconnect cycle.

Based on Oracle UEK commit "net/rds: Don't force state
RDS_CONN_RESETTING" by Gerd Rausch.

Fixes: 9c79440e2c5e ("RDS: TCP: fix race windows in send-path quiescence by rds_tcp_accept_one()")
Signed-off-by: Gerd Rausch <gerd.rausch@oracle.com>
[achender: port to net-next: use the two-argument
 rds_conn_path_transition()/rds_conn_path_drop() and rewrite the
 changelog for the upstream shutdown path]
Assisted-by: Claude-Code:claude-fable-5
Signed-off-by: Allison Henderson <achender@kernel.org>
---
    v5: the rds_tcp_accept_one() call-site comment no longer promises
    an unconditional transition to RDS_CONN_UP.  No other change.
 net/rds/tcp.c        | 17 +++++++++++++++--
 net/rds/tcp_listen.c |  6 +++++-
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index b263634ac750..ad14217867a4 100644
--- a/net/rds/tcp.c
+++ b/net/rds/tcp.c
@@ -150,9 +150,22 @@ void rds_tcp_reset_callbacks(struct socket *sock,
 	 * end up deadlocking with tcp_sendmsg(), and the RDS_IN_XMIT
 	 * would not get set. As a result, we set c_state to
 	 * RDS_CONN_RESETTTING, to ensure that rds_tcp_state_change
-	 * cannot mark rds_conn_path_up() in the window before lock_sock()
+	 * cannot mark rds_conn_path_up() in the window before lock_sock().
+	 *
+	 * Only make that transition if the path is still connecting
+	 * (or already resetting from an earlier duel).  A path in any
+	 * other state - typically RDS_CONN_DISCONNECTING or
+	 * RDS_CONN_ERROR with a shutdown in flight - is dropped
+	 * instead.  That still replaces its state, with RDS_CONN_ERROR,
+	 * and queues one more shutdown pass, but rds_conn_shutdown()
+	 * accepts RDS_CONN_ERROR in its final transition to
+	 * RDS_CONN_DOWN, so the shutdown in flight completes normally.
 	 */
-	atomic_set(&cp->cp_state, RDS_CONN_RESETTING);
+	if (!rds_conn_path_transition(cp, RDS_CONN_CONNECTING,
+				      RDS_CONN_RESETTING) &&
+	    !rds_conn_path_transition(cp, RDS_CONN_RESETTING,
+				      RDS_CONN_RESETTING))
+		rds_conn_path_drop(cp, 0);
 	wait_event(cp->cp_waitq, !test_bit(RDS_IN_XMIT, &cp->cp_flags));
 	/* reset receive side state for rds_tcp_data_recv() for osock  */
 	cancel_delayed_work_sync(&cp->cp_send_w);
diff --git a/net/rds/tcp_listen.c b/net/rds/tcp_listen.c
index a3db9b057084..13fa60c1985b 100644
--- a/net/rds/tcp_listen.c
+++ b/net/rds/tcp_listen.c
@@ -295,7 +295,11 @@ int rds_tcp_accept_one(struct rds_tcp_net *rtn)
 	if (rs_tcp->t_sock) {
 		/* Duelling SYN has been handled in rds_tcp_accept_one() */
 		rds_tcp_reset_callbacks(new_sock, cp);
-		/* rds_connect_path_complete() marks RDS_CONN_UP */
+		/* rds_connect_path_complete() marks RDS_CONN_UP, or,
+		 * if a concurrent shutdown won the duel, drops the
+		 * path again and the pass that drop queues reaps the
+		 * socket installed above.
+		 */
 		rds_connect_path_complete(cp, RDS_CONN_RESETTING);
 	} else {
 		rds_tcp_set_callbacks(new_sock, cp);
-- 
2.25.1