linux-kernel - Re: [PATCH v6 05/20] liveupdate: luo

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Message-ID: <CA+CK2bAXaMdp7rw8RVWNpfunu-XW59RjmdhwmEwZtCZohrdrGA@mail.gmail.com>
Date: Thu, 20 Nov 2025 15:13:47 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: David Matlack <dmatlack@...gle.com>
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Subject: Re: [PATCH v6 05/20] liveupdate: luo_ioctl: add user interface

On Thu, Nov 20, 2025 at 2:43 PM David Matlack <dmatlack@...gle.com> wrote:
>
> On Thu, Nov 20, 2025 at 11:23 AM Pasha Tatashin
> <pasha.tatashin@...een.com> wrote:
> >
> > On Thu, Nov 20, 2025 at 1:38 PM David Matlack <dmatlack@...gle.com> wrote:
> > >
> > > On Sat, Nov 15, 2025 at 3:34 PM Pasha Tatashin
> > > <pasha.tatashin@...een.com> 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.
>
> Yeah, I'm not concerned about bottlenecking. It just seems like an
> artificial constraint to impose on userspace at this point. The only
> ioctls on /dev/liveupdate are to create a session and retreive a
> session. Neither of those will conflict with having multiple open
> files for /dev/liveupdate.

Enforcing tooling consistency, and improving security for global
state. Otherwise, it can be relaxed.

Pasha