Re: [PATCH] doc/rcu: Correct field_count field naming in examples

Joel Fernandes
2019-05-08T16:26:40Z

On Mon, May 06, 2019 at 05:04:53PM -0700, Paul E. McKenney wrote:
> On Sat, May 04, 2019 at 10:03:10PM -0400, Joel Fernandes (Google) wrote:
> > I believe this field should be called field_count instead of file_count.
> > Correct the doc with the same.
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> But if we are going to update this, why not update it with the current
> audit_filter_task(), audit_del_rule(), and audit_add_rule() code?
>
> Hmmm... One reason is that some of them have changed beyond recognition.

It seems to me that these 3 functions are just structured differently but is
conceptually the same.

There is now an array of lists stored in audit_filter_list. Each list is a
set of rules. Versus in the listRCU.txt, there is only one global.

The other difference is there is a mutex held &audit_filter_mutex
audit_{add,del}_rule. Where as in listRCU, it says that is not needed since
another mutex is already held.

> And this example code predates v2.6.12. ;-)
>
> So good eyes, but I believe that this really does reflect the ancient
> code...
>
> On the other hand, would you have ideas for more modern replacement
> examples?

There are 3 cases I can see in listRCU.txt:
(1) action taken outside of read_lock (can tolerate stale data), no in-place update.
this is the best possible usage of RCU.
(2) action taken outside of read_lock, in-place updates
this is good as long as not too many in-place updates.
involves copying creating new list node and replacing the
node being updated with it.
(3) cannot tolerate stale data: here a deleted or obsolete flag can be used
protected by a per-entry lock. reader
aborts if object is stale.

Any replacement example must make satisfy (3) too?

The only example for (3) that I know of is sysvipc sempahores which you also
mentioned in the paper. Looking through this code, it hasn't changed
conceptually and it could be a fit for an example (ipc_valid_object() checks
for whether the object is stale).

The other example could be dentry look up which uses seqlocks for the
RCU-walk case? But that could be too complex. This is also something I first
learnt from the paper and then the excellent path-lookup.rst document in
kernel sources.

I will keep any eye out for other examples in the kernel code as well.

Let me know what you think, thanks!

- Joel

> > ---
> > Documentation/RCU/listRCU.txt | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/RCU/listRCU.txt b/Documentation/RCU/listRCU.txt
> > index adb5a3782846..190e666fc359 100644
> > --- a/Documentation/RCU/listRCU.txt
> > +++ b/Documentation/RCU/listRCU.txt
> > @@ -175,7 +175,7 @@ otherwise, the added fields would need to be filled in):
> > list_for_each_entry(e, list, list) {
> > if (!audit_compare_rule(rule, &e->rule)) {
> > e->rule.action = newaction;
> > - e->rule.file_count = newfield_count;
> > + e->rule.field_count = newfield_count;
> > write_unlock(&auditsc_lock);
> > return 0;
> > }
> > @@ -204,7 +204,7 @@ RCU ("read-copy update") its name. The RCU code is as follows:
> > return -ENOMEM;
> > audit_copy_rule(&ne->rule, &e->rule);
> > ne->rule.action = newaction;
> > - ne->rule.file_count = newfield_count;
> > + ne->rule.field_count = newfield_count;
> > list_replace_rcu(&e->list, &ne->list);
> > call_rcu(&e->rcu, audit_free_rule);
> > return 0;
> > --
> > 2.21.0.1020.gf2820cf01a-goog
> >
>