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

Paul E. McKenney
2019-05-08T18:16:44Z

On Wed, May 08, 2019 at 12:26:35PM -0400, Joel Fernandes wrote:
> 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.

Agreed.

> > 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?

It would be OK to have a separate example for (3). It would of course
be nicer to have one example for all three, but not all -that- important.

> 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).

That is indeed the classic canonical example. ;-)

> 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.

This is a great example, but it would need serious simplification for
use in the Documentation/RCU directory. Note that dcache uses it to
gain very limited and targeted consistency -- only a few types of updates
acquire the write-side of that seqlock.

Might be quite worthwhile to have a simplified example, though!
Perhaps a trivial hash table where write-side sequence lock is acquired
only when moving an element from one chain to another?

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

Very good!

Thanx, Paul

> 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
> > >
> >
>