[PATCH net v3] net: Use a dedicated flag to track disabled napi instances

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Trying to disable the napi threaded state while the napi instance is
disabled will hang forever in napi_stop_kthread(), as it waits for
NAPIF_STATE_SCHED_THREADED to clear.

This won't happen until the next ____napi_schedule() call, which won't
happen as it's disabled.

This happens in 2 instances :

 - Drivers that create but don't use napi instances (stmmac's rxtx napi
   for AF_XDP for example), here it's a driver bug

 - Drivers that create their napi instances in .probe(). Here, setting
   threaded off while the interface is down will hang :

   ip link set eth0 down
   echo 1 > /sys/class/net/eth0/threaded
   echo 0 > /sys/class/net/eth0/threaded
   -> hang

Checking on netif_running() isn't enough, as napi instances may be
transiently disabled when changing the MTU for example, let's use a
dedicated flag to track if the instance is disabled.

This allows immediately disabling the kthread when the instance is
disabled.

This was tested on mvpp2 (probe-time napi_add) and stmmac.

Fixes: 689883de94dd ("net: stop napi kthreads when THREADED napi is disabled")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
---
V3:
 - Use a new flag to keep track of the napi instance state, if it's
   disabled or not. It was tested on the same set of boards as before.

   I guess if this approach doesn't work either, we'll have to start
   fixing all the drivers :)

V2:
https://lore.kernel.org/r/20261006141119.85562-1-maxime.chevallier@bootlin.com
 - Different approach as per Jakub's review, just don't allow setting
   the napi threaded mode while interface is down. Uses the NAPI_STATE_NPSVC
   flag to check the napi state, but it wasn't enough.

V1:
https://lore.kernel.org/r/20260926194714.648819-1-maxime.chevallier@bootlin.com

 include/linux/netdevice.h |  2 ++
 net/core/dev.c            | 15 +++++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index ac257221c7c5..d410c7303d1a 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -433,6 +433,7 @@ enum {
 	NAPI_STATE_SCHED_THREADED,	/* Napi is currently scheduled in threaded mode */
 	NAPI_STATE_HAS_NOTIFIER,	/* Napi has an IRQ notifier */
 	NAPI_STATE_THREADED_BUSY_POLL,	/* The threaded NAPI poller will busy poll */
+	NAPI_STATE_DISABLED,		/* NAPI is disabled */
 };
 
 enum {
@@ -448,6 +449,7 @@ enum {
 	NAPIF_STATE_SCHED_THREADED	= BIT(NAPI_STATE_SCHED_THREADED),
 	NAPIF_STATE_HAS_NOTIFIER	= BIT(NAPI_STATE_HAS_NOTIFIER),
 	NAPIF_STATE_THREADED_BUSY_POLL	= BIT(NAPI_STATE_THREADED_BUSY_POLL),
+	NAPIF_STATE_DISABLED		= BIT(NAPI_STATE_DISABLED),
 };
 
 enum gro_result {
diff --git a/net/core/dev.c b/net/core/dev.c
index e76762e29360..a6d66af8947a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7253,6 +7253,13 @@ static void napi_stop_kthread(struct napi_struct *napi)
 {
 	unsigned long val, new;
 
+	/* Instance is disabled, we can immediately stop the kthread */
+	if (test_bit(NAPI_STATE_DISABLED, &napi->state)) {
+		clear_bit(NAPI_STATE_THREADED, &napi->state);
+		clear_bit(NAPI_STATE_THREADED_BUSY_POLL, &napi->state);
+		goto stop;
+	}
+
 	/* Wait until the napi STATE_THREADED is unset. */
 	while (true) {
 		val = READ_ONCE(napi->state);
@@ -7283,6 +7290,7 @@ static void napi_stop_kthread(struct napi_struct *napi)
 		msleep(20);
 	}
 
+stop:
 	kthread_stop(napi->thread);
 	napi->thread = NULL;
 }
@@ -7678,6 +7686,7 @@ void netif_napi_add_weight_locked(struct net_device *dev,
 	napi->list_owner = -1;
 	set_bit(NAPI_STATE_SCHED, &napi->state);
 	set_bit(NAPI_STATE_NPSVC, &napi->state);
+	set_bit(NAPI_STATE_DISABLED, &napi->state);
 	netif_napi_dev_list_add(dev, napi);
 
 	/* default settings from sysfs are applied to all NAPIs. any per-NAPI
@@ -7714,7 +7723,8 @@ void napi_disable_locked(struct napi_struct *n)
 			val = READ_ONCE(n->state);
 		}
 
-		new = val | NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC;
+		new = val | NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC |
+		      NAPIF_STATE_DISABLED;
 		new &= ~(NAPIF_STATE_THREADED |
 			 NAPIF_STATE_THREADED_BUSY_POLL |
 			 NAPIF_STATE_PREFER_BUSY_POLL);
@@ -7759,7 +7769,8 @@ void napi_enable_locked(struct napi_struct *n)
 	do {
 		BUG_ON(!test_bit(NAPI_STATE_SCHED, &val));
 
-		new = val & ~(NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC);
+		new = val & ~(NAPIF_STATE_SCHED | NAPIF_STATE_NPSVC |
+			      NAPIF_STATE_DISABLED);
 		if (n->dev->threaded && n->thread)
 			new |= NAPIF_STATE_THREADED;
 	} while (!try_cmpxchg(&n->state, &val, new));
-- 
2.56.0