netdev - Re: [PATCH net] team: postpone features update to avoid deadlock

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Message-ID: <20210121183452.47f0cffa@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 21 Jan 2021 18:34:52 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Ivan Vecera <ivecera@...hat.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        saeed@...nel.org, Jiri Pirko <jiri@...nulli.us>
Subject: Re: [PATCH net] team: postpone features update to avoid deadlock

On Thu, 21 Jan 2021 11:29:37 +0100 Ivan Vecera wrote:
> On Wed, 20 Jan 2021 15:18:20 -0800
> Cong Wang <xiyou.wangcong@...il.com> wrote:
> > On Wed, Jan 20, 2021 at 4:56 AM Ivan Vecera <ivecera@...hat.com> wrote:  
> > > Team driver protects port list traversal by its team->lock mutex
> > > in functions like team_change_mtu(), team_set_rx_mode(),

The set_rx_mode part can't be true, set_rx_mode can't sleep and
team->lock is a mutex.

> > > To fix the problem __team_compute_features() needs to be postponed
> > > for these cases.    
> > 
> > Is there any user-visible effect after deferring this feature change?
>
> An user should not notice this change.

I think Cong is right, can you expand a little on your assertion?
User should be able to assume that the moment syscall returns the
features had settled.

What does team->mutex actually protect in team_compute_features()?
All callers seem to hold RTNL at a quick glance. This is a bit of 
a long shot but isn't it just tryin to protect the iteration over 
ports which could be under RCU?

More crude idea would be to wrap the mutex_unlock(&team->lock) into 
a helper which checks if something tried to change features while it
was locked. rtnl_unlock()-style.