[PATCH net v2 3/3] net: prevent deadlocks when enabling NAPIs with mixed kthread config

[Jakub Kicinski <kuba@kernel.org>]

The following order of calls currently deadlocks if:
 - device has threaded=1; and
 - NAPI has persistent config with threaded=0.

  netif_napi_add_weight_config()
    dev->threaded == 1
      napi_kthread_create()

  napi_enable()
    napi_restore_config()
      napi_set_threaded(0)
        napi_stop_kthread()
	  while (NAPIF_STATE_SCHED)
	    msleep(20)

We deadlock because disabled NAPI has STATE_SCHED set.
Creating a thread in netif_napi_add() just to destroy it in
napi_disable() is fairly ugly in the first place. Let's read
both the device config and the NAPI config in netif_napi_add().

Fixes: e6d76268813d ("net: Update threaded state in napi config in netif_set_threaded")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 net/core/dev.h | 8 ++++++++
 net/core/dev.c | 5 +++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.h b/net/core/dev.h
index ab69edc0c3e3..d6b08d435479 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -323,6 +323,14 @@ static inline enum netdev_napi_threaded napi_get_threaded(struct napi_struct *n)
 	return NETDEV_NAPI_THREADED_DISABLED;
 }
 
+static inline enum netdev_napi_threaded
+napi_get_threaded_config(struct net_device *dev, struct napi_struct *n)
+{
+	if (n->config)
+		return n->config->threaded;
+	return dev->threaded;
+}
+
 int napi_set_threaded(struct napi_struct *n,
 		      enum netdev_napi_threaded threaded);
 
diff --git a/net/core/dev.c b/net/core/dev.c
index f180746382a1..5a3c0f40a93f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7357,8 +7357,9 @@ void netif_napi_add_weight_locked(struct net_device *dev,
 	 * Clear dev->threaded if kthread creation failed so that
 	 * threaded mode will not be enabled in napi_enable().
 	 */
-	if (dev->threaded && napi_kthread_create(napi))
-		dev->threaded = NETDEV_NAPI_THREADED_DISABLED;
+	if (napi_get_threaded_config(dev, napi))
+		if (napi_kthread_create(napi))
+			dev->threaded = NETDEV_NAPI_THREADED_DISABLED;
 	netif_napi_set_irq_locked(napi, -1);
 }
 EXPORT_SYMBOL(netif_napi_add_weight_locked);
-- 
2.50.1