Re: [PATCH] kobject: record and send PREV_SEQNUM with uevents

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Oct 16, 2017 at 02:35:44PM +0200, Peter Rajnoha wrote:
> Record last uevent seqnum that was used with kobject's uevent and send
> it with next uevent as PREV_SEQNUM=<num> variable.
> 
> This enhances the way we are able to track and handle uevents in userspace
> if we are trying to catch up with all the uevents that were generated
> while we were not listening or the uevents which were not delivered.
> This may happen if the userspace listener is not running yet when the
> uevent is triggered or there's a period of time when it's not listening
> (e.g. because the userspace listener is being restarted while a new
> uevent fires).
> 
> A userspace listener can compare the last uevent seqnum it knows about
> with the last uevent seqnum the kernel reports through uevents delivered
> to userspace, both genuine and synthetic ones (the ones generated by
> writing uevent action name to /sys/.../uevent file). Then, if needed,
> userspace may execute appropriate actions based on that. We don't need
> to reevaluate and recheck all items, instead, we can concentrate only
> on items for which we really and actually need to reflect changed state
> in kernel while the userspace listener was not listening for uevents.
> 
> Signed-off-by: Peter Rajnoha <prajnoha@redhat.com>
> ---
>  include/linux/kobject.h |  1 +
>  lib/kobject.c           |  1 +
>  lib/kobject_uevent.c    | 15 +++++++++++++--
>  3 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kobject.h b/include/linux/kobject.h
> index e0a6205caa71..ee6db28b6186 100644
> --- a/include/linux/kobject.h
> +++ b/include/linux/kobject.h
> @@ -73,6 +73,7 @@ struct kobject {
>  #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
>  	struct delayed_work	release;
>  #endif
> +	u64			last_uevent_seqnum;
>  	unsigned int state_initialized:1;
>  	unsigned int state_in_sysfs:1;
>  	unsigned int state_add_uevent_sent:1;
> diff --git a/lib/kobject.c b/lib/kobject.c
> index 763d70a18941..2cc809f10ae7 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -190,6 +190,7 @@ static void kobject_init_internal(struct kobject *kobj)
>  		return;
>  	kref_init(&kobj->kref);
>  	INIT_LIST_HEAD(&kobj->entry);
> +	kobj->last_uevent_seqnum = 0;
>  	kobj->state_in_sysfs = 0;
>  	kobj->state_add_uevent_sent = 0;
>  	kobj->state_remove_uevent_sent = 0;
> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> index f237a09a5862..f1cbed06a395 100644
> --- a/lib/kobject_uevent.c
> +++ b/lib/kobject_uevent.c
> @@ -454,8 +454,19 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
>  	}
>  
>  	mutex_lock(&uevent_sock_mutex);
> -	/* we will send an event, so request a new sequence number */
> -	retval = add_uevent_var(env, "SEQNUM=%llu", (unsigned long long)++uevent_seqnum);
> +	/*
> +	 * We will send an event, so request a new sequence number.
> +	 * Also, record the number for next uevent's PREV_SEQNUM.
> +	 */
> +	kobj->last_uevent_seqnum = ++uevent_seqnum;
> +	retval = add_uevent_var(env, "SEQNUM=%llu",
> +				(unsigned long long)uevent_seqnum);
> +	if (retval) {
> +		mutext_unlock(&uevent_sock_mutex);
> +		goto exit;
> +	}
> +	retval = add_uevent_var(env, "PREV_SEQNUM=%llu",
> +				(unsigned long long)kobj->last_uevent_seqnum);

I'm confused, why are we storing "last_uevent_seqnum" here at all, if it
is only just the current one-1?  What else do you do with that value?

thanks,

greg k-h