netdev - [PATCH net-next v2 1/4] net: dev: Remove preempt_disable() and get_cpu() in netif_rx

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Message-ID: <Yf1EWFgPtjIq3Hzw@linutronix.de>
Date:   Fri, 4 Feb 2022 16:20:56 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Toke Høiland-Jørgensen <toke@...e.dk>
Cc:     Eric Dumazet <edumazet@...gle.com>, 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>
Subject: [PATCH net-next v2 1/4] net: dev: Remove preempt_disable() and
 get_cpu() in netif_rx_internal().

The preempt_disable() () section was introduced in commit
    cece1945bffcf ("net: disable preemption before call smp_processor_id()")

and adds it in case this function is invoked from preemtible context and
because get_cpu() later on as been added.

The get_cpu() usage was added in commit
    b0e28f1effd1d ("net: netif_rx() must disable preemption")

because ip_dev_loopback_xmit() invoked netif_rx() with enabled preemption
causing a warning in smp_processor_id(). The function netif_rx() should
only be invoked from an interrupt context which implies disabled
preemption. The commit
   e30b38c298b55 ("ip: Fix ip_dev_loopback_xmit()")

was addressing this and replaced netif_rx() with in netif_rx_ni() in
ip_dev_loopback_xmit().

Based on the discussion on the list, the former patch (b0e28f1effd1d)
should not have been applied only the latter (e30b38c298b55).

Remove get_cpu() and preempt_disable() since the function is supposed to
be invoked from context with stable per-CPU pointers. Bottom halves have
to be disabled at this point because the function may raise softirqs
which need to be processed.

Link: https://lkml.kernel.org/r/20100415.013347.98375530.davem@davemloft.net
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
v1…v2:
  - merge patch 1 und 2 from the series (as per Toke).
  - updated patch description and corrected the first commit number (as
    per Eric).

 net/core/dev.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 1baab07820f65..0d13340ed4054 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4796,7 +4796,6 @@ static int netif_rx_internal(struct sk_buff *skb)
 		struct rps_dev_flow voidflow, *rflow = &voidflow;
 		int cpu;
 
-		preempt_disable();
 		rcu_read_lock();
 
 		cpu = get_rps_cpu(skb->dev, skb, &rflow);
@@ -4806,14 +4805,12 @@ static int netif_rx_internal(struct sk_buff *skb)
 		ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
 
 		rcu_read_unlock();
-		preempt_enable();
 	} else
 #endif
 	{
 		unsigned int qtail;
 
-		ret = enqueue_to_backlog(skb, get_cpu(), &qtail);
-		put_cpu();
+		ret = enqueue_to_backlog(skb, smp_processor_id(), &qtail);
 	}
 	return ret;
 }
-- 
2.34.1