netdev - Re: [BUG] deadlock in nl80211_vendor

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Message-ID: <20220325094952.10c46350@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Fri, 25 Mar 2022 09:49:52 -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>,
        Ganapathi Bhat <ganapathi.bhat@....com>,
        Xinming Hu <huxinming820@...il.com>, kernel-team@...roid.com,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [BUG] deadlock in nl80211_vendor_cmd

On Fri, 25 Mar 2022 13:04:23 +0100 Johannes Berg wrote:
> So we can avoid the potential deadlock in cfg80211 in a few ways:
> 
>  1) export rtnl_lock_unregistering_all() or maybe a variant after
>     refactoring the two versions, to allow cfg80211 to use it, that way
>     netdev_run_todo() can never have a non-empty todo list
> 
>  2) export __rtnl_unlock() so cfg80211 can avoid running
>     netdev_run_todo() in the unlock, personally I like this less because
>     it might encourage random drivers to use it
> 
>  3) completely rework cfg80211's locking, adding a separate mutex for
>     the wiphy list so we don't need to acquire the RTNL at all here
>     (unless the ops need it, but there's no issue if we don't drop it),
>     something like https://p.sipsolutions.net/27d08e1f5881a793.txt
> 
> 
> I think I'm happy with 3) now (even if it took a couple of hours), so I
> think we can go with it, just need to go through all the possibilities.

I like 3) as well. FWIW a few places (e.g. mlx5, devlink, I think I've
seen more) had been converting to xarray for managing the "registered"
objects. It may be worth looking into if you're re-doing things, anyway.