Re: [PATCH v6 05/20] liveupdate: luo_ioctl: add user interface

[Pasha Tatashin]

On Thu, Nov 20, 2025 at 1:38 PM David Matlack <dmatlack@xxxxxxxxxx> wrote:
>
> On Sat, Nov 15, 2025 at 3:34 PM Pasha Tatashin
> <pasha.tatashin@xxxxxxxxxx> wrote:
> > The idea is that there is going to be a single userspace agent driving
> > the live update, therefore, only a single process can ever hold this
> > device opened at a time.
> ...
> > +static int luo_open(struct inode *inodep, struct file *filep)
> > +{
> > +       struct luo_device_state *ldev = container_of(filep->private_data,
> > +                                                    struct luo_device_state,
> > +                                                    miscdev);
> > +
> > +       if (atomic_cmpxchg(&ldev->in_use, 0, 1))
> > +               return -EBUSY;
>
> Can you remind me why the kernel needs to enforce this? What would be
> wrong or unsafe from the kernel perspective if there were multiple
> userspace agents holding open files for /dev/liveupdate, each with
> their own sessions?

By enforcing a singleton, we will ensure a consistent view for tooling
like luoadm (which will track incoming/outgoing sessions, UUIDs, etc.)
and prevent conflicting commands regarding the transition state.

This is not a bottleneck because the vast majority of the work
(preserving devicse/memory) is handled via the individual Session FDs.
Also, since sessions persist even if /dev/liveupdate is closed, we
allow the agent upgrade, or crashing without requiring concurrent
access.

Pasha