[lttng-dev] urcu/rculist.h clarifications - for implementing LRU

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Mon Mar 13 10:29:31 EDT 2023

On 2023-03-11 01:04, Ondřej Surý via lttng-dev wrote:
> Hey,
> so, we are integrating userspace-rcu to BIND 9 (yay!) and as experiment,
> I am rewriting the internal address database (keeps the infrastructure
> information about names and addresses).

That's indeed very interesting !

> There's a hashtable to keep the entries and there's associated LRU list.
> For the hashtable the cds_lfht seems to work well, but I am kind of struggling
> with cds_list (the urcu/rculist.h variant).
> The names and entries works in pretty much similar way, so I am going to
> describe just one.
> The workhorse is get_attached_and_locked_name() function (I am going to
> skip the parts where we create keys, checks if LRU needs to be updated, etc.)

It would help if you could share a git branch of your prototype. In order to reason
about RCU, we typically need to look at both the update-side and the read-side.
For instance I don't see the read-side of the LRU linked-list in the code snippets
below. We also need to have a complete picture of the object lifetime, from allocation
to reclaim/reuse. I don't see where the grace periods (either synchronize_rcu or
call_rcu) are before reclaim or reuse in the code snippets below.

> So this is part with the hashtable lookup which seems to work well:
>          rcu_read_lock();
>          struct cds_lfht_iter iter;
>          struct cds_lfht_node *ht_node;
>          cds_lfht_lookup(adb->names_ht, hashval, names_match, &key, &iter);
>          ht_node = cds_lfht_iter_get_node(&iter);
>          bool unlink = false;
>          if (ht_node == NULL) {
>                  /* Allocate a new name and add it to the hash table. */
>                  adbname = new_adbname(adb, name, start_at_zone);
>                  ht_node = cds_lfht_add_unique(adb->names_ht, hashval,
>                                                names_match, &key,
>                                                &adbname->ht_node);
>                  if (ht_node != &adbname->ht_node) {
>                          /* ISC_R_EXISTS */
>                          destroy_adbname(adbname);
>                          adbname = NULL;
>                  }
>          }
>          if (adbname == NULL) {
>                  INSIST(ht_node != NULL);
>                  adbname = caa_container_of(ht_node, dns_adbname_t, ht_node);
>                  unlink = true;
>          }
>          dns_adbname_ref(adbname);

What is this dns_adbname_ref() supposed to do ? And is there a reference to adbname
that is still used after rcu_read_unlock() ? What guarantees the existence of the
adbname after rcu_read_unlock() ?

>          rcu_read_unlock();
> and here's the part where LRU gets updated:
>          LOCK(&adbname->lock); /* Must be unlocked by the caller */

I suspect you use a scheme where you hold the RCU read-side to perform the lookup, and
then you use the object with an internal lock held. But expecting the object to still
exist after rcu read unlock is incorrect, unless some other reference counting scheme
is used.

>          if (NAME_DEAD(adbname)) {
>                  UNLOCK(&adbname->lock);
>                  dns_adbname_detach(&adbname);
>                  goto again;
>          }
>          if (adbname->last_used + ADB_CACHE_MINIMUM <= last_update) {
>                  adbname->last_used = now;
>                  LOCK(&adb->names_lru_lock);
>                  if (unlink) {
>                          cds_list_del_rcu(&adbname->list_node);
>                  }

This looks odd. I don't see the code implementing traversal of this list, but
I would expect a grace period between unlink of the node from a list and insertion
into another list, otherwise if there are RCU readers traversing the list
concurrently, they can observe an inconsistent state.

>                  cds_list_add_tail_rcu(&adbname->list_node, &adb->names_lru);
>                  UNLOCK(&adb->names_lru_lock);
>          }
> The NAME_DEAD gets updated under the adbname->lock in expire_name():
>          if (!NAME_DEAD(adbname)) {
>                  adbname->flags |= NAME_IS_DEAD;
>                  /* Remove the adbname from the hashtable... */
>                  (void)cds_lfht_del(adb->names_ht, &adbname->ht_node);

I don't have the full context here, but AFAIR cds_lfht_del() allows two removals
of the same ht_node to be done concurrently, and only one will succeed (which is
probably what happens here). cds_list_del_rcu() however does not allow concurrent
removals of a list_node. So if you somehow get two RCU lookups to find the same
node in expire_name, one will likely do an extra unexpected cds_list_del_rcu().

>                  /* ... and LRU list */
>                  LOCK(&adb->names_lru_lock);
>                  cds_list_del_rcu(&adbname->list_node);
>                  UNLOCK(&adb->names_lru_lock);
>          }
> So, now the problem is that sometimes I get a crash under load:
> (gdb) bt
> #0  0x00007fae87a34c96 in cds_list_del_rcu (elem=0x7fae37e78880) at /usr/include/x86_64-linux-gnu/urcu/rculist.h:71
> #1  get_attached_and_locked_name (adb=adb at entry=0x7fae830142a0, name=name at entry=0x7fae804fc9b0, start_at_zone=true, now=<optimized out>) at adb.c:1446
> #2  0x00007fae87a392bf in dns_adb_createfind (adb=0x7fae830142a0, loop=0x7fae842c3a20, cb=cb at entry=0x7fae87b28d9f <fctx_finddone>, cbarg=0x7fae7c679000, name=name at entry=0x7fae804fc9b0, qname=0x7fae7c679010, qtype=1, options=63, now=<optimized out>, target=0x0, port=53, depth=1, qc=0x7fae7c651060, findp=0x7fae804fc698) at adb.c:2149
> (gdb) frame 0
> #0  0x00007fae87a34c96 in cds_list_del_rcu (elem=0x7fae37e78880) at /usr/include/x86_64-linux-gnu/urcu/rculist.h:71
> 71		elem->next->prev = elem->prev;
> (gdb) print elem->next
> $1 = (struct cds_list_head *) 0x0
> (gdb) print elem
> $2 = (struct cds_list_head *) 0x7fae37e78880
> So, I suspect, I am doing something wrong when updating the position of the the name in the LRU list.
> There are couple of places where we iterate through the LRU list (overmem cleaning can kick-in, the user initiated cleaning can start, shutdown can be happening...)

It gets me to wonder whether you really need RCU for the LRU list ? Are those lookups
very frequent ? And do they typically end up needing to grab a lock to protect against
concurrent list modifications ?

> Is there perhaps already some LRU implementation using Userspace-RCU that I can take look at?

I don't have an example implementing an LRU with a linked list specifically, but this is not
different from other linked-list uses.



> Thank you!
> Ondrej
> --
> Ondřej Surý (He/Him)
> ondrej at sury.org
> _______________________________________________
> lttng-dev mailing list
> lttng-dev at lists.lttng.org
> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev

Mathieu Desnoyers
EfficiOS Inc.

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