net: Update threaded state in napi config in netif_set_threaded

Commit 2677010e77 ("Add support to set NAPI threaded for individual
NAPI") added support to enable/disable threaded napi using netlink. This
also extended the napi config save/restore functionality to set the napi
threaded state. This breaks netdev reset for drivers that use napi
threaded at device level and also use napi config save/restore on
napi_disable/napi_enable. Basically on netdev with napi threaded enabled
at device level, a napi_enable call will get stuck trying to stop the
napi kthread. This is because the napi->config->threaded is set to
disabled when threaded is enabled at device level.

The issue can be reproduced on virtio-net device using qemu. To
reproduce the issue run following,

  echo 1 > /sys/class/net/threaded
  ethtool -L eth0 combined 1

Update the threaded state in napi config in netif_set_threaded and add a
new test that verifies this scenario.

Tested on qemu with virtio-net:
 NETIF=eth0 ./tools/testing/selftests/drivers/net/napi_threaded.py
 TAP version 13
 1..2
 ok 1 napi_threaded.change_num_queues
 ok 2 napi_threaded.enable_dev_threaded_disable_napi_threaded
 # Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0

Fixes: 2677010e77 ("Add support to set NAPI threaded for individual NAPI")
Signed-off-by: Samiullah Khawaja <skhawaja@google.com>
Link: https://patch.msgid.link/20250804164457.2494390-1-skhawaja@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -6978,6 +6978,12 @@ int napi_set_threaded(struct napi_struct *napi,
 	if (napi->config)
 		napi->config->threaded = threaded;
 
+	/* Setting/unsetting threaded mode on a napi might not immediately
+	 * take effect, if the current napi instance is actively being
+	 * polled. In this case, the switch between threaded mode and
+	 * softirq mode will happen in the next round of napi_schedule().
+	 * This should not cause hiccups/stalls to the live traffic.
+	 */
 	if (!threaded && napi->thread) {
 		napi_stop_kthread(napi);
 	} else {
@@ -7011,23 +7017,9 @@ int netif_set_threaded(struct net_device *dev,
 
 	WRITE_ONCE(dev->threaded, threaded);
 
-	/* Make sure kthread is created before THREADED bit
-	 * is set.
-	 */
-	smp_mb__before_atomic();
-
-	/* Setting/unsetting threaded mode on a napi might not immediately
-	 * take effect, if the current napi instance is actively being
-	 * polled. In this case, the switch between threaded mode and
-	 * softirq mode will happen in the next round of napi_schedule().
-	 * This should not cause hiccups/stalls to the live traffic.
-	 */
-	list_for_each_entry(napi, &dev->napi_list, dev_list) {
-		if (!threaded && napi->thread)
-			napi_stop_kthread(napi);
-		else
-			assign_bit(NAPI_STATE_THREADED, &napi->state, threaded);
-	}
+	/* The error should not occur as the kthreads are already created. */
+	list_for_each_entry(napi, &dev->napi_list, dev_list)
+		WARN_ON_ONCE(napi_set_threaded(napi, threaded));
 
 	return err;
 }

tools/testing/selftests/drivers/net/Makefile

@@ -11,6 +11,7 @@ TEST_GEN_FILES := \
 
 TEST_PROGS := \
 	napi_id.py \
+	napi_threaded.py \
 	netcons_basic.sh \
 	netcons_cmdline.sh \
 	netcons_fragmented_msg.sh \

tools/testing/selftests/drivers/net/napi_threaded.py

@@ -0,0 +1,111 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+"""
+Test napi threaded states.
+"""
+
+from lib.py import ksft_run, ksft_exit
+from lib.py import ksft_eq, ksft_ne, ksft_ge
+from lib.py import NetDrvEnv, NetdevFamily
+from lib.py import cmd, defer, ethtool
+
+
+def _assert_napi_threaded_enabled(nl, napi_id) -> None:
+    napi = nl.napi_get({'id': napi_id})
+    ksft_eq(napi['threaded'], 'enabled')
+    ksft_ne(napi.get('pid'), None)
+
+
+def _assert_napi_threaded_disabled(nl, napi_id) -> None:
+    napi = nl.napi_get({'id': napi_id})
+    ksft_eq(napi['threaded'], 'disabled')
+    ksft_eq(napi.get('pid'), None)
+
+
+def _set_threaded_state(cfg, threaded) -> None:
+    cmd(f"echo {threaded} > /sys/class/net/{cfg.ifname}/threaded")
+
+
+def _setup_deferred_cleanup(cfg) -> None:
+    combined = ethtool(f"-l {cfg.ifname}", json=True)[0].get("combined", 0)
+    ksft_ge(combined, 2)
+    defer(ethtool, f"-L {cfg.ifname} combined {combined}")
+
+    threaded = cmd(f"cat /sys/class/net/{cfg.ifname}/threaded").stdout
+    defer(_set_threaded_state, cfg, threaded)
+
+
+def enable_dev_threaded_disable_napi_threaded(cfg, nl) -> None:
+    """
+    Test that when napi threaded is enabled at device level and
+    then disabled at napi level for one napi, the threaded state
+    of all napis is preserved after a change in number of queues.
+    """
+
+    napis = nl.napi_get({'ifindex': cfg.ifindex}, dump=True)
+    ksft_ge(len(napis), 2)
+
+    napi0_id = napis[0]['id']
+    napi1_id = napis[1]['id']
+
+    _setup_deferred_cleanup(cfg)
+
+    # set threaded
+    _set_threaded_state(cfg, 1)
+
+    # check napi threaded is set for both napis
+    _assert_napi_threaded_enabled(nl, napi0_id)
+    _assert_napi_threaded_enabled(nl, napi1_id)
+
+    # disable threaded for napi1
+    nl.napi_set({'id': napi1_id, 'threaded': 'disabled'})
+
+    cmd(f"ethtool -L {cfg.ifname} combined 1")
+    cmd(f"ethtool -L {cfg.ifname} combined 2")
+    _assert_napi_threaded_enabled(nl, napi0_id)
+    _assert_napi_threaded_disabled(nl, napi1_id)
+
+
+def change_num_queues(cfg, nl) -> None:
+    """
+    Test that when napi threaded is enabled at device level,
+    the napi threaded state is preserved after a change in
+    number of queues.
+    """
+
+    napis = nl.napi_get({'ifindex': cfg.ifindex}, dump=True)
+    ksft_ge(len(napis), 2)
+
+    napi0_id = napis[0]['id']
+    napi1_id = napis[1]['id']
+
+    _setup_deferred_cleanup(cfg)
+
+    # set threaded
+    _set_threaded_state(cfg, 1)
+
+    # check napi threaded is set for both napis
+    _assert_napi_threaded_enabled(nl, napi0_id)
+    _assert_napi_threaded_enabled(nl, napi1_id)
+
+    cmd(f"ethtool -L {cfg.ifname} combined 1")
+    cmd(f"ethtool -L {cfg.ifname} combined 2")
+
+    # check napi threaded is set for both napis
+    _assert_napi_threaded_enabled(nl, napi0_id)
+    _assert_napi_threaded_enabled(nl, napi1_id)
+
+
+def main() -> None:
+    """ Ksft boiler plate main """
+
+    with NetDrvEnv(__file__, queue_count=2) as cfg:
+        ksft_run([change_num_queues,
+                  enable_dev_threaded_disable_napi_threaded],
+                 args=(cfg, NetdevFamily()))
+    ksft_exit()
+
+
+if __name__ == "__main__":
+    main()