netdev - Re: [PATCH net-next 3/4] net: dev: Makes sure netif

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Date:   Thu, 3 Feb 2022 16:40:35 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH net-next 3/4] net: dev: Makes sure netif_rx() can be
 invoked in any context.

On 2022-02-03 07:25:01 [-0800], Eric Dumazet wrote:
> 
> No, the loopback device (ifconfig log) I am referring to is in
> drivers/net/loopback.c
> 
> loopback_xmit() calls netif_rx() directly, while bh are already disabled.

ah okay. Makes sense.

> Instead of adding a local_bh_disable()/local_bh_enable() in netif_rx()
> I suggested
> to rename current netif_rx() to __netif_rx() and add a wrapper, eg :

So we still end up with two interfaces. Do I move a few callers like the
one you already mentioned over to the __netif_rx() interface or will it
be the one previously mentioned for now?

Would something like 

diff --git a/include/linux/bottom_half.h b/include/linux/bottom_half.h
index fc53e0ad56d90..561cbca431ca6 100644
--- a/include/linux/bottom_half.h
+++ b/include/linux/bottom_half.h
@@ -30,7 +30,12 @@ static inline void local_bh_enable_ip(unsigned long ip)
 
 static inline void local_bh_enable(void)
 {
-	__local_bh_enable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);
+	if (unlikely(softirq_count() == SOFTIRQ_DISABLE_OFFSET)) {
+		__local_bh_enable_ip(_THIS_IP_, SOFTIRQ_DISABLE_OFFSET);
+	} else {
+		preempt_count_sub(SOFTIRQ_DISABLE_OFFSET);
+		barrier();
+	}
 }
 
 #ifdef CONFIG_PREEMPT_RT

lower the overhead to acceptable range? (I still need to sell this to
peterz first).

Sebastian