linux-kernel - [PATCH net v3] net: thunderx: avoid direct MTU assignment after WRITE

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Message-ID: <20250706194327.1369390-1-alok.a.tiwari@oracle.com>
Date: Sun,  6 Jul 2025 12:43:21 -0700
From: Alok Tiwari <alok.a.tiwari@...cle.com>
To: sgoutham@...vell.com, andrew+netdev@...n.ch, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        horms@...nel.org, netdev@...r.kernel.org
Cc: alok.a.tiwari@...cle.com, linux-arm-kernel@...ts.infradead.org,
        darren.kenny@...cle.com, linux-kernel@...r.kernel.org
Subject: [PATCH net v3] net: thunderx: avoid direct MTU assignment after WRITE_ONCE()

The current logic in nicvf_change_mtu() writes the new MTU to
netdev->mtu using WRITE_ONCE() before verifying if the hardware
update succeeds. However on hardware update failure, it attempts
to revert to the original MTU using a direct assignment
(netdev->mtu = orig_mtu)
which violates the intended of WRITE_ONCE protection introduced in
commit 1eb2cded45b3 ("net: annotate writes on dev->mtu from
ndo_change_mtu()")

Additionally, WRITE_ONCE(netdev->mtu, new_mtu) is unnecessarily
performed even when the device is not running.

Fix this by:
  Only writing netdev->mtu after successfully updating the hardware.
  Skipping hardware update when the device is down, and setting MTU
  directly. Remove unused variable orig_mtu.

This ensures that all writes to netdev->mtu are consistent with
WRITE_ONCE expectations and avoids unintended state corruption
on failure paths.

Signed-off-by: Alok Tiwari <alok.a.tiwari@...cle.com>
---
v2 -> v3
https://lore.kernel.org/all/20250630111836.GE41770@horms.kernel.org/
Simplify code as suggested by Simon
---
 drivers/net/ethernet/cavium/thunder/nicvf_main.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/cavium/thunder/nicvf_main.c b/drivers/net/ethernet/cavium/thunder/nicvf_main.c
index aebb9fef3f6eb..1be2dc40a1a63 100644
--- a/drivers/net/ethernet/cavium/thunder/nicvf_main.c
+++ b/drivers/net/ethernet/cavium/thunder/nicvf_main.c
@@ -1578,7 +1578,6 @@ int nicvf_open(struct net_device *netdev)
 static int nicvf_change_mtu(struct net_device *netdev, int new_mtu)
 {
 	struct nicvf *nic = netdev_priv(netdev);
-	int orig_mtu = netdev->mtu;

 	/* For now just support only the usual MTU sized frames,
 	 * plus some headroom for VLAN, QinQ.
@@ -1589,15 +1588,10 @@ static int nicvf_change_mtu(struct net_device *netdev, int new_mtu)
 		return -EINVAL;
 	}

-	WRITE_ONCE(netdev->mtu, new_mtu);
-
-	if (!netif_running(netdev))
-		return 0;
-
-	if (nicvf_update_hw_max_frs(nic, new_mtu)) {
-		netdev->mtu = orig_mtu;
+	if (netif_running(netdev) && nicvf_update_hw_max_frs(nic, new_mtu))
 		return -EINVAL;
-	}
+
+	WRITE_ONCE(netdev->mtu, new_mtu);

 	return 0;
 }
-- 
2.46.0