[PATCH v2 05/14] NTB: ntb_transport: Avoid losing QP link-up requests

[Koichiro Den]

ntb_netdev_open() can call ntb_transport_link_up() while the transport
worker is completing setup on another CPU. Concurrent transport setup
and a client link-up request can both read the other's flag as false and
leave QP link work unqueued. The QP then stays down until another link
event or client link-up request.

This is the store-buffering pattern described in
tools/memory-model/Documentation/recipes.txt ("Store buffering").

Add a full barrier between the store and load on each side, and
mark the client_ready accesses with READ_ONCE()/WRITE_ONCE().

Fixes: fce8a7bb5b4b ("PCI-Express Non-Transparent Bridge Support")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20260907144701.702E41F00A3A@xxxxxxxxxxxxxxx/
Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
---
Changes in v2:
  - New patch (Sashiko)

 drivers/ntb/ntb_transport.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
index 75d4a2e021f4..1332d53bcfe7 100644
--- a/drivers/ntb/ntb_transport.c
+++ b/drivers/ntb/ntb_transport.c
@@ -1104,10 +1104,16 @@ static void ntb_transport_link_work(struct work_struct *work)
 	/* Publish the link only after every QP has been set up. */
 	smp_store_release(&nt->link_is_up, true);
 
+	/*
+	 * Prevent both sides from missing each other's flag. Pairs with
+	 * the barrier in ntb_transport_link_up().
+	 */
+	smp_mb();
+
 	for (i = 0; i < nt->qp_count; i++) {
 		struct ntb_transport_qp *qp = &nt->qp_vec[i];
 
-		if (qp->client_ready)
+		if (READ_ONCE(qp->client_ready))
 			ntb_transport_schedule_qp_link(qp, 0);
 	}
 
@@ -2401,7 +2407,10 @@ void ntb_transport_link_up(struct ntb_transport_qp *qp)
 	if (!qp)
 		return;
 
-	qp->client_ready = true;
+	WRITE_ONCE(qp->client_ready, true);
+
+	/* Pairs with the barrier in ntb_transport_link_work(). */
+	smp_mb();
 
 	ntb_transport_schedule_qp_link(qp, 0);
 }
-- 
2.51.0