netdev - Re: [PATCH net] net: lapbether: Prevent racing when checking whether the netif is running

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Message-ID: <CAJht_EMToKj2OeeE1fMfwAVYvhbgZpENkv0C7ac+XHnWcTe2Tg@mail.gmail.com>
Date:   Thu, 11 Mar 2021 13:12:25 -0800
From:   Xie He <xie.he.0141@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Martin Schiller <ms@....tdt.de>,
        "David S. Miller" <davem@...emloft.net>,
        Linux X25 <linux-x25@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] net: lapbether: Prevent racing when checking whether
 the netif is running

On Thu, Mar 11, 2021 at 12:43 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> Is this a theoretical issues or do you see a path where it triggers?
>
> Who are the callers sending frames to a device which went down?

This is a theoretical issue. I didn't see this issue in practice.

When "__dev_queue_xmit" and "sch_direct_xmit" call
"dev_hard_start_xmit", there appears to be no locking mechanism
preventing the netif from going down while "dev_hard_start_xmit" is
doing its work.

David once confirmed in an email that a driver's "ndo_stop" function
would indeed race with its "ndo_start_xmit" function:
https://lore.kernel.org/netdev/20190520.200922.2277656639346033061.davem@davemloft.net/