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

[Jakub Kicinski <kuba@kernel.org>]

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.