Le vendredi 26 février 2010 à 14:18 +1100, Simon Horman a écrit :
> 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.
The code you copy/pasted seems really complex, I would suggest to make
it as simple as possible (using spinlocks for example instead of
rwlocks) before considering RCU conversion.
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