[PATCH 1/2] igb: Disable threaded IRQ for igb_msix_other

[Wander Lairson Costa <wander@redhat.com>]

During testing of SR-IOV, Red Hat QE encountered an issue where the
ip link up command intermittently fails for the igbvf interfaces when
using the PREEMPT_RT variant. Investigation revealed that
e1000_write_posted_mbx returns an error due to the lack of an ACK
from e1000_poll_for_ack.

The underlying issue arises from the fact that IRQs are threaded by
default under PREEMPT_RT. While the exact hardware details are not
available, it appears that the IRQ handled by igb_msix_other must
be processed before e1000_poll_for_ack times out. However,
e1000_write_posted_mbx is called with preemption disabled, leading
to a scenario where the IRQ is serviced only after the failure of
e1000_write_posted_mbx.

To resolve this, we set IRQF_NO_THREAD for the affected interrupt,
ensuring that the kernel handles it immediately, thereby preventing
the aforementioned error.

Reproducer:

    #!/bin/bash

    # echo 2 > /sys/class/net/ens14f0/device/sriov_numvfs
    ipaddr_vlan=3
    nic_test=ens14f0
    vf=${nic_test}v0

    while true; do
	    ip link set ${nic_test} mtu 1500
	    ip link set ${vf} mtu 1500
	    ip link set $vf up
	    ip link set ${nic_test} vf 0 vlan ${ipaddr_vlan}
	    ip addr add 172.30.${ipaddr_vlan}.1/24 dev ${vf}
	    ip addr add 2021:db8:${ipaddr_vlan}::1/64 dev ${vf}
	    if ! ip link show $vf | grep 'state UP'; then
		    echo 'Error found'
		    break
	    fi
	    ip link set $vf down
    done

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reported-by: Yuying Ma <yuma@redhat.com>
---
 drivers/net/ethernet/intel/igb/igb_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index 1ef4cb871452..8a1696d7289f 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -907,7 +907,7 @@ static int igb_request_msix(struct igb_adapter *adapter)
 	int i, err = 0, vector = 0, free_vector = 0;
 
 	err = request_irq(adapter->msix_entries[vector].vector,
-			  igb_msix_other, 0, netdev->name, adapter);
+			  igb_msix_other, IRQF_NO_THREAD, netdev->name, adapter);
 	if (err)
 		goto err_out;
 
-- 
2.46.1