netdev - Re: [BUG] deadlock in nl80211_vendor

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Date:   Fri, 25 Mar 2022 14:48:39 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     William McVicker <willmcvicker@...gle.com>,
        linux-wireless@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Amitkumar Karwar <amitkarwar@...il.com>,
        Xinming Hu <huxinming820@...il.com>, kernel-team@...roid.com,
        Paolo Abeni <pabeni@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Cong Wang <cwang@...pensource.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [BUG] deadlock in nl80211_vendor_cmd

On Fri, 25 Mar 2022 22:25:05 +0100 Johannes Berg wrote:
> > > With some suitable commentary, that might also be a reasonable thing?
> > > __rtnl_unlock() is actually rather pretty rare, and not exported.  
> > 
> > The main use for it seems to be re-locking before loading a module,
> > which TBH I have no idea why, is it just a cargo cult or a historical
> > thing :S  I don't see how letting netdevs leave before _loading_ 
> > a module makes any difference whatsoever.  
> 
> Indeed.
> 
> > The WARN_ON() you suggested up front make perfect sense to me.
> > You can also take the definition of net_unlink_todo() out of
> > netdevice.h while at it because o_0  
> 
> Heh indeed, what?

To be clear - I just meant that it's declaring a static variable in 
a header, so I doubt that it'll do the right thing unless it's only
called from one compilation unit.

> But (and now I'll CC even more people...) if we can actually have an
> invariant that while RTNL is unlocked the todo list is empty, then we
> also don't need rtnl_lock_unregistering_all(), and can remove the
> netdev_unregistering_wq, etc., no?
> 
> IOW, I'm not sure why we needed commit 50624c934db1 ("net: Delay
> default_device_exit_batch until no devices are unregistering v2"), but I
> also have little doubt that we did.
> 
> Ah, no. This isn't about locking in this case, it's literally about
> ensuring that free_netdev() has been called in netdev_run_todo()?

Yup, multiple contexts sitting independently in netdev_run_todo() and
chewing on netdevs is slightly different. destructors of those netdevs
could be pointing at memory of a module being unloaded.

> Which we don't care about in cfg80211 - we just care about the list
> being empty so there's no chance we'll reacquire the RTNL.