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

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Date:   Sun, 12 May 2019 00:11:26 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        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

Hi Paul, Joel,

> > > 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?

Sorry to take you down here..., but what do you mean by "the paper"?  ;-/

Thanx,
  Andrea