Hi,
I have been looking at converting the rwlocks in IPVS over to use RCU.
A problem that I am facing is that the lblcr scheduler uses
a write lock on list A and then taking a write lock on list B.
Where list B is basically part of one of the elements of list A.
This problem is present in ip_vs_lblcr_schedule() and
the key code looks like this.
/* First look in our cache */
read_lock(&svc->sched_lock);
en = ip_vs_lblcr_get(svc->af, tbl, &iph.daddr);
if (en) {
...
/* Get the least loaded destination */
read_lock(&en->set.lock);
dest = ip_vs_dest_set_min(&en->set);
read_unlock(&en->set.lock);
...
write_lock(&en->set.lock);
m = ip_vs_dest_set_max(&en->set);
if (m)
ip_vs_dest_set_erase(&en->set, m);
write_unlock(&en->set.lock);
...
/* Update our cache entry */
write_lock(&en->set.lock);
ip_vs_dest_set_insert(&en->set, dest);
write_unlock(&en->set.lock);
}
read_unlock(&svc->sched_lock);
dest is referenced counted and doesn't seem to need to be guarded
by svc->sched_lock.
It seems to me that this is quite difficult to convert over to RCU
as there are write-side critical sections inside a read-side critical
section.
I investigated reference counting the return value of
ip_vs_lblcr_get() or the return value of ip_vs_dest_set_max() and
ip_vs_dest_set_insert(). But this seems to be difficult,
especially at rmmod time.
I also considered just making the whole thing a write-side critical section.
Which seems to be somewhat of a sledge-hammer and result in
a critical section that is much larger than I would like. Though
no bigger than the existing area covered by the read-lock on
svc->sched_lock.
Any suggestions would be appreciated.
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