From 20ab39d13e2e9f916cf570fc834f2cadd6e5dc4a Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 1 Oct 2021 16:58:41 +0200
Subject: [PATCH] net/core: disable NET_RX_BUSY_POLL on PREEMPT_RT
Git-commit: 20ab39d13e2e9f916cf570fc834f2cadd6e5dc4a
Patch-mainline: v5.16-rc1
References: bsc#1189998

napi_busy_loop() disables preemption and performs a NAPI poll. We can't acquire
sleeping locks with disabled preemption which would be required while
__napi_poll() invokes the callback of the driver.

A threaded interrupt performing the NAPI-poll can be preempted on PREEMPT_RT.
A RT thread on another CPU may observe NAPIF_STATE_SCHED bit set and busy-spin
until it is cleared or its spin time runs out. Given it is the task with the
highest priority it will never observe the NEED_RESCHED bit set.
In this case the time is better spent by simply sleeping.

The NET_RX_BUSY_POLL is disabled by default (the system wide sysctls for
poll/read are set to zero). Disabling NET_RX_BUSY_POLL on PREEMPT_RT to avoid
wrong locking context in case it is used.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20211001145841.2308454-1-bigeasy@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

---
 net/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/Kconfig b/net/Kconfig
index fb13460c6dab..074472dfa94a 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -294,7 +294,7 @@ config CGROUP_NET_CLASSID
 
 config NET_RX_BUSY_POLL
 	bool
-	default y
+	default y if !PREEMPT_RT
 
 config BQL
 	bool
-- 
2.26.2