On Wed, May 8, 2019 at 9:16 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote: [snip] > > > And this example code predates v2.6.12. ;-) > > > > > > So good eyes, but I believe that this really does reflect the ancient > > > code... > > > > > > 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. ;-) FWIW just want to mention, it seems to me the ptrace task list traversal could be a great example of "mark obsolete objects" and is simple so I could just use that. Neil talks about it in his article here: https://lwn.net/Articles/610972/ . In "Group 3: Transform the way the list is walked" Cheers, - Joel
