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

Andrea Parri
2019-05-11T22:11:39Z

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