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
