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

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Message-ID: <20210311145230.5f368151@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 11 Mar 2021 14:52:30 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Xie He <xie.he.0141@...il.com>
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, 11 Mar 2021 13:12:25 -0800 Xie He wrote:
> 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.

Normally driver's ndo_stop() calls netif_tx_disable() which takes TX
locks, so unless your driver is lockless (LLTX) there should be no xmit
calls after that point.