linux-kernel - Re: [PATCH] doc/rcu: Correct field

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Message-Id: <20190508181638.GY3923@linux.ibm.com>
Date:   Wed, 8 May 2019 11:16:38 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Subject: Re: [PATCH] doc/rcu: Correct field_count field naming in examples

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@...lfernandes.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
> > > 
> > 
>