Subject: Re: [PATCH net v2 1/2] net: netdevsim: fix inconsistent carrier state after link/unlink

From: Yohei Kojima <[email-masked]>
Date: Thu, 1 Jan 2026 01:06:56 +0900

On Wed, Dec 31, 2025 at 02:16:53AM -0800, Breno Leitao wrote:
> Hello Yohei,
> 
> On Wed, Dec 31, 2025 at 01:03:29AM +0900, [email-masked] wrote:
> > From: Yohei Kojima <[email-masked]>
> > 
> > This patch fixes the edge case behavior on ifup/ifdown and
> > linking/unlinking two netdevsim interfaces:
> > 
> > 1. unlink two interfaces netdevsim1 and netdevsim2
> > 2. ifdown netdevsim1
> > 3. ifup netdevsim1
> > 4. link two interfaces netdevsim1 and netdevsim2
> 
> > 5. (Now two interfaces are linked in terms of netdevsim peer, but
> >     carrier state of the two interfaces remains DOWN.)
> 
> That seems a real issue, in fact. The carriers are only getting up when
> opening the device, not when linking. Thus, this patch makes sense to
> me.
> 
> > This inconsistent behavior is caused by the current implementation,
> > which only cares about the "link, then ifup" order, not "ifup, then
> > link" order. This patch fixes the inconsistency by calling
> > netif_carrier_on() when two netdevsim interfaces are linked.
> > 
> > This patch fixes buggy behavior on NetworkManager-based systems which
> > causes the netdevsim test to fail with the following error:
> > 
> >   # timeout set to 600
> >   # selftests: drivers/net/netdevsim: peer.sh
> >   # 2025/12/25 00:54:03 socat[9115] W address is opened in read-write mode but only supports read-only
> >   # 2025/12/25 00:56:17 socat[9115] W connect(7, AF=2 192.168.1.1:1234, 16): Connection timed out
> >   # 2025/12/25 00:56:17 socat[9115] E TCP:192.168.1.1:1234: Connection timed out
> >   # expected 3 bytes, got 0
> >   # 2025/12/25 00:56:17 socat[9109] W exiting on signal 15
> >   not ok 13 selftests: drivers/net/netdevsim: peer.sh # exit=1
> > 
> > This patch also solves timeout on TCP Fast Open (TFO) test in
> > NetworkManager-based systems because it also depends on netdevsim's
> > carrier consistency.
> > 
> > Fixes: 1a8fed52f7be ("netdevsim: set the carrier when the device goes up")
> > Signed-off-by: Yohei Kojima <[email-masked]>
> 
> Reviewed-by: Breno Leitao <[email-masked]>

Thank you for the review!  

> Thanks for the fix!

When I encountered this bug, I could easily identify where to fix thanks
to your previous commit. Thanks again for your support!

Best regards,
Yohei Kojima