Re: Re: [PATCH stable 6.1] devlink: Fix RCU stall when unregistering a devlink instance

[Aleksandr Nogikh <nogikh@google.com>]

Hi Ido,

On Tue, 1 Oct 2024 16:38:39 +0300, Ido Schimmel wrote:
> 
> On Tue, Oct 01, 2024 at 02:11:27PM +0200, Greg KH wrote:
> > On Tue, Oct 01, 2024 at 02:20:35PM +0300, Ido Schimmel wrote:
> > > I read the stable rules and I am not providing an "upstream commit ID"
> > > since the code in upstream has been reworked, making this fix
> > > irrelevant. The only affected stable kernel is 6.1.y.
> > 
> > You need to document the heck out of why this is only relevant for this
> > one specific kernel branch IN the changelog text, so that we understand
> > what is going on, AND you need to get acks from the relevant maintainers
> > of this area of the kernel to accept something that is not in Linus's
> > tree.
> > 
> > But first of, why?  Why not just take the upstrema commits instead?
> 
> There were a lot of changes as part of the 6.3 cycle to completely
> rework the semantics of the devlink instance reference count. As part of
> these changes, commit d77278196441 ("devlink: bump the instance index
> directly when iterating") inadvertently fixed the bug mentioned in this
> patch. This commit cannot be applied to 6.1.y as-is because a prior
> commit (also in 6.3) moved the code to a different file (leftover.c ->
> core.c). There might be more dependencies that I'm currently unaware of.
> 
> The alternative, proposed in this patch, is to provide a minimal and
> contained fix for the bug introduced in upstream commit c2368b19807a
> ("net: devlink: introduce "unregistering" mark and use it during
> devlinks iteration") as part of the 6.0 cycle.
> 
> The above explains why the patch is only relevant to 6.1.y.

Thanks for bringing up this topic!

For what it's worth, syzbot would also greatly benefit from your fix:
https://github.com/google/syzkaller/issues/5328

I've built a kernel locally with your changes, run syzkaller against it,
and I can confirm that the kernel no longer crashes due to devlink.

-- 
Aleksandr

> 
> Jakub / Jiri, what is your preference here? This patch or cherry picking
> a lot of code from 6.3?