Re: [RFC PATCH] liveupdate: Allow multiple openers for /dev/liveupdate

[Pratyush Yadav <pratyush@kernel.org>]

Hi David,

On Tue, Jul 14 2026, David Matlack wrote:

> Remove the single-opener restriction for /dev/liveupdate by removing the
> atomic in_use tracking and the exclusive open check in luo_open() that
> returned -EBUSY. Protect luo_session_deserialize() with a mutex guard so
> that concurrent open attempts by multiple processes safely executes
> deserialization only once. Update liveupdate selftest to verify that
> multiple concurrent openers succeed.
>
> LUO does not inherently require a single opener. There is some
> documentation about it simplifying state management, but the only thing
> it actually protects is the session deserialization during first open,
> which can be easily handled with a mutex.
>
> Relaxing the single-opener requirement avoids the kernel forcing a
> design pattern on userspace that it itself does not require, e.g.
> allowing multiple userspace processes to create and manage sessions.

Agreed. When the kernel had a global state machine in the early versions
of LUO, this might have been more relevant. With sessions, even if we
later add a state machine, it likely will be per-session instead of
being global. So I think letting userspace open /dev/liveupdate multiple
times makes a lot of sense.

Also, today's systemd only supports preserving individual files, and
does not hand out sessions. To get sessions, userspace must open
/dev/liveupdate and create a session. This opens up room for one bad
process to block every other process from creating sessions. It also
imposes a need for userspace to add a polling/retry logic for getting
sessions and serializes their execution around this point.

I don't see any architectural reasons for doing so from kernel's side.
If userspace wants to only have one owner of /dev/liveupdate, they are
free to do so by unlinking the device from devtmpfs after opening or
restricting its permissions.

So the idea has my vote :-)

Acked-by: Pratyush Yadav (Google) <pratyush@kernel.org>

That said, a comment on the code below.

>
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
[...]
> diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
> index b79b2a488974..ca4d0639d39a 100644
> --- a/kernel/liveupdate/luo_session.c
> +++ b/kernel/liveupdate/luo_session.c
> @@ -584,13 +584,17 @@ static int luo_session_deserialize_one(struct luo_session_header *sh,
>  
>  int luo_session_deserialize(void)
>  {
> -	struct luo_session_header *sh = &luo_session_global.incoming;
> +	static DEFINE_MUTEX(luo_session_deserialize_lock);
>  	static bool is_deserialized;
> +	static int saved_err;
> +
> +	struct luo_session_header *sh = &luo_session_global.incoming;
>  	struct luo_session_ser *ser;
>  	struct kho_block_set_it it;
> -	static int saved_err;
>  	int err;
>  
> +	guard(mutex)(&luo_session_deserialize_lock);

Do we really need a new lock? Can we re-use sh->rwsem instead?

It can block session retrieve (but not file retrieve) for a short time
though since luo_session_retrieve() also takes it. But the block will be
short since only the first open of /dev/liveupdate does work. After that
it just checks is_deserialized and returns. And session retrieval should
not be very frequent anyway.

I don't have very strong opinion on this, but I reckon the less locks to
keep track of the better.

> +
>  	/* If has been deserialized, always return the same error code */
>  	if (is_deserialized)
>  		return saved_err;
[...]

-- 
Regards,
Pratyush Yadav