net: Fix a data-race around sysctl_net_busy_poll.

stable inclusion
from stable-v5.10.140
commit 2c7dae6c45112ee7ead62155fed375cb2e7d7cf8
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I63FTT

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c7dae6c45112ee7ead62155fed375cb2e7d7cf8

--------------------------------

[ Upstream commit c42b7cdd ]

While reading sysctl_net_busy_poll, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its reader.

Fixes: 06021292 ("net: add low latency socket poll")
Signed-off-by: NKuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Reviewed-by: NWei Li <liwei391@huawei.com>

include/net/busy_poll.h

@@ -31,7 +31,7 @@ extern unsigned int sysctl_net_busy_poll __read_mostly;
 
 static inline bool net_busy_loop_on(void)
 {
-	return sysctl_net_busy_poll;
+	return READ_ONCE(sysctl_net_busy_poll);
 }
 
 static inline bool sk_can_busy_loop(const struct sock *sk)