netdev - Re: [PATCH net-next v3 02/15] net: Implement netdev_nl_bind_queue

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Message-ID: <20251028164437.20b48513@kernel.org>
Date: Tue, 28 Oct 2025 16:44:37 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: David Wei <dw@...idwei.uk>
Cc: Daniel Borkmann <daniel@...earbox.net>, Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org, bpf@...r.kernel.org, davem@...emloft.net,
 razor@...ckwall.org, willemb@...gle.com, sdf@...ichev.me,
 john.fastabend@...il.com, martin.lau@...nel.org, jordan@...fe.io,
 maciej.fijalkowski@...el.com, magnus.karlsson@...el.com, toke@...hat.com,
 yangzhenze@...edance.com, wangdongdong.6@...edance.com
Subject: Re: [PATCH net-next v3 02/15] net: Implement
 netdev_nl_bind_queue_doit

On Tue, 28 Oct 2025 14:59:05 -0700 David Wei wrote:
> On 2025-10-23 19:08, Jakub Kicinski wrote:
> > On Thu, 23 Oct 2025 14:48:15 +0200 Daniel Borkmann wrote:  
> >> It is needed given we need to always ensure lock ordering for the two devices,
> >> that is, the order is always from the virtual to the physical device.  
> > 
> > You do seem to be taking the lock before you check if the device was
> > the type you expected tho.  
> 
> I believe this is okay. Let's say we have two netdevs, A that is real
> and B that is virtual. 

Now imagine they are both virtual.

> User calls netdev_nl_bind_queue_doit() twice in
> two different contexts, 1 with the correct order (A as src, B as dst)
> and 2 with the incorrect order (B as src, A as dst). We always try to
> lock dst first, then src.
> 
>          1                 2
> lock(dst == B)
>                    lock(dst == A)
>                    is not virtual...
>                    unlock(A)
> lock(src == A)
> 
> 
>          1                 2
>                    lock(dst == A)
> lock(dst == B)
>                    is not virtual...
>                    unlock(A)
> lock(src == A)
> 
> The check will prevent ABBA by never taking that final lock to complete
> the cycle. Please check and lmk if I'm off, stuff like this makes my
> brain hurt.