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

Andrea Parri
2019-05-12T01:09:54Z

On Sat, May 11, 2019 at 05:41:31PM -0700, Paul E. McKenney wrote:
> On Sun, May 12, 2019 at 12:11:26AM +0200, Andrea Parri wrote:
> > 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"? ;-/
>
> One or both of these two:
>
> http://www2.rdrop.com/~paulmck/techreports/survey.2012.09.17a.pdf
> http://www2.rdrop.com/~paulmck/techreports/RCUUsage.2013.02.24a.pdf

Oh, these are familiar. ;-) Thank you!

Andrea