[PATCH] kobject: record and send PREV_SEQNUM with uevents

[Peter Rajnoha]

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@xxxxxxxxxx>
---
 include/linux/kobject.h |  1 +
 lib/kobject.c           |  1 +
 lib/kobject_uevent.c    | 12 +++++++++++-
 3 files changed, 13 insertions(+), 1 deletion(-)

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..b1d301bab3e6 100644
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -454,12 +454,22 @@ 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 */
+	/*
+	 * We will send an event, so request a new sequence number.
+	 * Also, record the number for next uevent's PREV_SEQNUM.
+	 */
 	retval = add_uevent_var(env, "SEQNUM=%llu", (unsigned long long)++uevent_seqnum);
 	if (retval) {
 		mutex_unlock(&uevent_sock_mutex);
 		goto exit;
 	}
+	retval = add_uevent_var(env, "PREV_SEQNUM=%llu",
+				(unsigned long long)kobj->last_uevent_seqnum);
+	kobj->last_uevent_seqnum = uevent_seqnum;
+	if (retval) {
+		mutex_unlock(&uevent_sock_mutex);
+		goto exit;
+	}
 
 #if defined(CONFIG_NET)
 	/* send netlink message */
-- 
2.13.6