linux-kernel - Re: [RFC] rcu: doc: update example about stale data

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Message-Id: <20181030235039.GX4170@linux.ibm.com>
Date:   Tue, 30 Oct 2018 16:50:39 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC] rcu: doc: update example about stale data

On Sun, Oct 28, 2018 at 06:16:31PM -0700, Joel Fernandes wrote:
> On Sun, Oct 28, 2018 at 10:21:42AM -0700, Paul E. McKenney wrote:
> > On Sat, Oct 27, 2018 at 07:16:53PM -0700, Joel Fernandes (Google) wrote:
> > > The RCU example for 'rejecting stale data' on system-call auditting
> > > stops iterating through the rules if a deleted one is found. It makes
> > > more sense to continue looking at other rules once a deleted one is
> > > rejected. Although the original example is fine, this makes it more
> > > meaningful.
> > > 
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > 
> > Does the actual audit code that this was copied from now include the
> > continue statement?  If so, please update the commit log to state that
> > and then I will take the resulting patch.  (This example was inspired
> > by a long-ago version of the actual audit code.)
> 
> The document talks of a situation that could be but is not really in the
> implementation. It says "If the system-call audit module were to ever need to
> reject stale data". So its not really something implemented. I was just
> correcting the example you had there since it made more sense to me to
> continue looking for other rules in the list once a rule was shown to be
> stale. It just makes the example more correct.
> 
> But I'm Ok if you want to leave that alone ;-) Hence, the RFC tag to this
> patch ;-)

Well, I do agree that there are situations where you need to keep
going.  But in the common case where only one instance of a given key
is allowed, and where the list is either (1) sorted and/or (2) added
to at the beginning, if you find a deleted element with a given key,
you are guaranteed that you won't find another with that key even if
you continue scanning the list.  After all, if you did find a deleted
element, the duplicate either is not on the list in the sorted case
or is behind you in the add-at-front case.

And in the more complex cases where persistent searching is required,
you usually have to restart the search instead of continuing it.  Besides,
things like the Issaquah Challenge don't seem to belong in introductory
documentation on RCU.  ;-)

							Thanx, Paul