Subject: [PATCH] net: fix hangup on napi_disable for threaded napi
From: Paolo Abeni <[email-masked]>
Date: Fri, 9 Apr 2021 17:24:17 +0200

napi_disable() is subject to an hangup, when the threaded
mode is enabled and the napi is under heavy traffic.

If the relevant napi has been scheduled and the napi_disable()
kicks in before the next napi_threaded_wait() completes - so
that the latter quits due to the napi_disable_pending() condition,
the existing code leaves the NAPI_STATE_SCHED bit set and the
napi_disable() loop waiting for such bit will hang.

This patch addresses the issue by dropping the NAPI_STATE_DISABLE
bit test in napi_thread_wait(). The later napi_threaded_poll()
iteration will take care of clearing the NAPI_STATE_SCHED.

This also addresses a related problem reported by Jakub:
before this patch a napi_disable()/napi_enable() pair killed
the napi thread, effectively disabling the threaded mode.
On the patched kernel napi_disable() simply stops scheduling
the relevant thread.

v1 -> v2:
  - let the main napi_thread_poll() loop clear the SCHED bit

Reported-by: Jakub Kicinski <[email-masked]>
Fixes: 29863d41bb6e ("net: implement threaded-able napi poll loop support")
Signed-off-by: Paolo Abeni <[email-masked]>
Reviewed-by: Eric Dumazet <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Jakub Kicinski <[email-masked]>
---

--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7023,7 +7023,7 @@ static int napi_thread_wait(struct napi_
 
 	set_current_state(TASK_INTERRUPTIBLE);
 
-	while (!kthread_should_stop() && !napi_disable_pending(napi)) {
+	while (!kthread_should_stop()) {
 		/* Testing SCHED_THREADED bit here to make sure the current
 		 * kthread owns this napi and could poll on this napi.
 		 * Testing SCHED bit is not enough because SCHED bit might be
@@ -7041,6 +7041,7 @@ static int napi_thread_wait(struct napi_
 		set_current_state(TASK_INTERRUPTIBLE);
 	}
 	__set_current_state(TASK_RUNNING);
+
 	return -1;
 }