net: fix hangup on napi_disable for threaded napi

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 <kuba@kernel.org>
Fixes: 29863d41 ("net: implement threaded-able napi poll loop support")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/883923fa22745a9589e8610962b7dc59df09fb1f.1617981844.git.pabeni@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -6992,7 +6992,7 @@ static int napi_thread_wait(struct napi_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
@@ -7010,6 +7010,7 @@ static int napi_thread_wait(struct napi_struct *napi)
 		set_current_state(TASK_INTERRUPTIBLE);
 	}
 	__set_current_state(TASK_RUNNING);
+
 	return -1;
 }