netdev - Re: [RFC PATCH 01/13] net: dsa: add plumbing for custom netdev statistics

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Message-ID: <20201018225820.b2vhgzyzwk7vy62j@skbuf>
Date:   Sun, 18 Oct 2020 22:58:21 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Heiner Kallweit <hkallweit1@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "vivien.didelot@...il.com" <vivien.didelot@...il.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        Christian Eggers <ceggers@...i.de>,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: Re: [RFC PATCH 01/13] net: dsa: add plumbing for custom netdev
 statistics

On Sun, Oct 18, 2020 at 04:13:28PM +0200, Heiner Kallweit wrote:
> On a side note, because I'm also just dealing with it: this_cpu_ptr()
> is safe only if preemption is disabled. Is this the case here? Else
> there's get_cpu_ptr/put_cpu_ptr.

lockdep would shout about using smp_processor_id() in preemptible
context? Because it doesn't do that, and never did, but I don't really
understand why, coming to think of it.

> Also, if irq's aren't disabled there might be a need to use
> u64_stats_update_begin_irqsave() et al. See: 2695578b896a ("net:
> usbnet: fix potential deadlock on 32bit hosts") But I don't know dsa
> good enough to say whether this is applicable here.

DSA xmit and receive path do not run from hardirq context, just softirq,
so I believe the issue reported to Eric there does not apply here.