[PATCH v3 06/15] NTB: ntb_transport: Avoid losing QP link-up requests

[Koichiro Den <den@valinux.co.jp>]

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.

Fixes: fce8a7bb5b4b ("PCI-Express Non-Transparent Bridge Support")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://lore.kernel.org/r/20260907144701.702E41F00A3A@smtp.kernel.org/
Signed-off-by: Koichiro Den <den@valinux.co.jp>
---
Changes in v3:
  - Drop the *_ONCE changes. client_ready is now atomic_t.

v2: https://lore.kernel.org/r/20260910040836.3792333-6-den@valinux.co.jp/

 drivers/ntb/ntb_transport.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
index 51d9e9969065..d290e5869c21 100644
--- a/drivers/ntb/ntb_transport.c
+++ b/drivers/ntb/ntb_transport.c
@@ -1101,6 +1101,12 @@ static void ntb_transport_link_work(struct work_struct *work)
 	/* Publish the link only after every QP has been set up. */
 	atomic_set_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];
 
@@ -2400,6 +2406,9 @@ void ntb_transport_link_up(struct ntb_transport_qp *qp)
 
 	atomic_set(&qp->client_ready, true);
 
+	/* Pairs with the barrier in ntb_transport_link_work(). */
+	smp_mb();
+
 	ntb_transport_schedule_qp_link(qp, 0);
 }
 EXPORT_SYMBOL_GPL(ntb_transport_link_up);
-- 
2.51.0