Re: [PATCH 1/2] rcu: introduce list_last_or_null_rcu

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, May 28, 2015 at 05:12:00PM -0400, Dan Streetman wrote:
> On Thu, May 28, 2015 at 5:05 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
> > On Thu, 28 May 2015 16:35:27 -0400
> > Dan Streetman <ddstreet@ieee.org> wrote:
> >
> >> Add list_last_or_null_rcu(), to simplify getting the last entry from a
> >> rcu-protected list.  The standard list_last_entry() can't be used as it
> >> is not rcu-protected; the list may be modified concurrently.  And the
> >> ->prev pointer can't be used, as only the ->next pointers are protected
> >> by rcu.
> >>
> >> This simply iterates forward through the entire list, to get to the last
> >> entry.  If the list is empty, it returns NULL.
> >
> > May I asked what this would be used for? It seems awfully inefficient
> > in its implementation. What use cases would this be for? I hate to add
> > something like this as a generic function which would encourage people
> > to use it. Iterating over an entire list to find the last element is
> > just nasty.
> 
> i have a patch series that will update zswap to be able to change its
> parameters at runtime, instead of only at boot time.  To do that, it
> creates new "pools" dynamically, and keeps them all in a list, with
> only the 1st pool being actively used; any following pools still have
> everything that was stored in them, but they aren't added to.  When
> zswap has to "shrink" - by telling one of the pools to get rid of 1 or
> more items - it picks the last on the list.  Once a pool is empty,
> it's removed/freed.
> 
> So zswap *could* just manually iterate the list to the last element,
> instead of using this new function.  But if rcu lists are ever
> improved later on, e.g. if ->prev is somehow rcu-protected as well as
> ->next, then this function should be faster than manually iterating.
> 
> if there's a better rcu-way to get to the last list entry, then we
> should definitely use it, although based on my understanding of the
> rcu list implementation, you can only iterate forwards, safely
> (without locking).

The usual approach would be to maintain a tail pointer.  How big are
these lists likely to get?

							Thanx, Paul