netdev - Re: [PATCH net] iavf: Do not restart Tx queues after reset task failure

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Date:   Thu, 10 Nov 2022 23:07:21 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Ivan Vecera <ivecera@...hat.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "Piotrowski, Patryk" <patryk.piotrowski@...el.com>,
        SlawomirX Laba <slawomirx.laba@...el.com>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        intel-wired-lan@...ts.osuosl.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] iavf: Do not restart Tx queues after reset task
 failure

On Thu, Nov 10, 2022 at 12:24:18PM -0800, Jakub Kicinski wrote:
> On Thu, 10 Nov 2022 19:07:02 +0200 Leon Romanovsky wrote:
> > > > Yes I think you're right. A ton of people check it without the
> > > > lock but I think thats not strictly safe. Is dev_close safe to
> > > > call when netif_running is false? Why not just remove the check
> > > > and always call dev_close then.
> > > 
> > > Check for a bit value (like netif_runnning()) is much cheaper than
> > > unconditionally taking global lock like RTNL.  
> > 
> > This cheap operation is racy and performed in non-performance
> > critical path.
> 
> To be clear - the rtnl_lock around the entire if is still racy 
> in the grand scheme of things, no? What's stopping someone from
> bringing the device right back up after you drop the lock?

I want to believe what there is some sort of state machine that won't
allow simple toggling of dev_close/dev_open. If it doesn't, rtnl_lock
users should audit their code for possible toggling right after that
lock is dropped.

Anyway, this discussion reminds me our devl_lock debate where we had
completely opposite views if rtnl_lock model is the right one.
https://lore.kernel.org/netdev/20211101073259.33406da3@kicinski-fedora-PC1C0HJN/

Let's not start argue again, we had enough back then. :)

Thanks