linux-kernel - [PATCH net-next 36/37] rxrpc: Fix request for an ACK when cwnd is minimum

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Message-ID: <20241202143057.378147-37-dhowells@redhat.com>
Date: Mon,  2 Dec 2024 14:30:54 +0000
From: David Howells <dhowells@...hat.com>
To: netdev@...r.kernel.org
Cc: David Howells <dhowells@...hat.com>,
	Marc Dionne <marc.dionne@...istor.com>,
	Yunsheng Lin <linyunsheng@...wei.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	linux-afs@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH net-next 36/37] rxrpc: Fix request for an ACK when cwnd is minimum

rxrpc_prepare_data_subpacket() sets the REQUEST-ACK flag on the outgoing
DATA packet under a number of circumstances, including, theoretically, when
the cwnd is at minimum (or less).  However, the minimum in this function is
hard-coded as 2, but the actual minimum is RXRPC_MIN_CWND (which is
currently 4) and so this never occurs.

Without this, we will miss the request of some ACKs, potentially leading to
a transmission stall until a timeout occurs on one side or the other that
leads to an ACK being generated.

Fix the function to use RXRPC_MIN_CWND rather than a hard-coded number.

Signed-off-by: David Howells <dhowells@...hat.com>
cc: Marc Dionne <marc.dionne@...istor.com>
cc: "David S. Miller" <davem@...emloft.net>
cc: Eric Dumazet <edumazet@...gle.com>
cc: Jakub Kicinski <kuba@...nel.org>
cc: Paolo Abeni <pabeni@...hat.com>
cc: linux-afs@...ts.infradead.org
cc: netdev@...r.kernel.org
---
 net/rxrpc/output.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/rxrpc/output.c b/net/rxrpc/output.c
index 891b85b3b9e7..c8ae59103c21 100644
--- a/net/rxrpc/output.c
+++ b/net/rxrpc/output.c
@@ -468,7 +468,7 @@ static size_t rxrpc_prepare_data_subpacket(struct rxrpc_call *call,
 		why = rxrpc_reqack_ack_lost;
 	else if (txb->flags & RXRPC_TXBUF_RESENT)
 		why = rxrpc_reqack_retrans;
-	else if (call->cong_ca_state == RXRPC_CA_SLOW_START && call->cong_cwnd <= 2)
+	else if (call->cong_ca_state == RXRPC_CA_SLOW_START && call->cong_cwnd <= RXRPC_MIN_CWND)
 		why = rxrpc_reqack_slow_start;
 	else if (call->tx_winsize <= 2)
 		why = rxrpc_reqack_small_txwin;