Re: [PATCH 1/2] liveupdate: Reference count outgoing FLB data

[Pratyush Yadav <pratyush@kernel.org>]

Hi David,

On Thu, May 28 2026, David Matlack wrote:

> Increment the outgoing FLB refcount in liveupdate_flb_get_outgoing() so
> that the FLB structure cannot be freed while the caller is actively
> using it. Add an additional liveupdate_flb_put_outgoing() function so
> the caller can explicitly indicate when it is done using the outgoing
> FLB.
>
> During a Live Update, the kernel may need to fetch the outgoing FLB
> outside of the scope of a file handler's preserve() and unpreserve()
> callbacks. In that situation there is no way for the caller to protect
> itself against the outgoing FLB from being freed while it is using it.
> Incrementing the reference count in liveupdate_flb_get_outgoing()
> ensures it cannot be freed.

We grab a reference to the FLB's module when the first file using the
FLB is preserved. So the FLB should never go away while preserved files
exist. Once all preserved files go away, you normally shouldn't be doing
anything with the FLB anyway.

Can you please elaborate on the use case and why this is a problem?
Using the FLB outside of the standard LUO file callbacks sounds
problematic.

>
> This change also aligns the outgoing FLB lifecycle management with the
> incoming FLB, since the latter uses the same get/put semantics.
>
> Fixes: cab056f2aae7 ("liveupdate: luo_flb: introduce File-Lifecycle-Bound global state")
> Assisted-by: Gemini:gemini-3-pro-preview
> Signed-off-by: David Matlack <dmatlack@google.com>
[...]

-- 
Regards,
Pratyush Yadav