Messages in this thread | | | Date | Sat, 24 Jun 2006 12:20:24 +0200 | From | Nick Piggin <> | Subject | Re: Userspace RCU+rtth hack (was Re: [patch 3/3] radix-tree: RCU lockless readside) |
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On Thu, Jun 22, 2006 at 01:25:53PM -0700, Paul E. McKenney wrote: > On Thu, Jun 22, 2006 at 08:23:43PM +0200, Nick Piggin wrote: > > > > Just out of interest, attached is my userspace RCU implementation > > and RCU radix-tree concurrent tests for Andrew Morton's radix-tree > > test harness. > > > > The RCU implementation is only 100 lines. Awful performance, of > > course, but I've stretched the rcu_read_lock/unlock over large > > periods so that we can get full concurrency at the cost of a > > bit of memory build up. And it still seems to catch use-after > > RCU-freed errors pretty easily. > > Interesting approach! One caution -- this approach can result in > RCU callbacks being invoked in the context of either call_rcu() or > rcu_read_unlock(). In some legitimate uses of RCU, this can result > in deadlock. See Documentation/RCU/UP.txt for more info. > > One solution is to have some other context (perhaps just a separate > pthread, given that performance is not critical) to invoke the callbacks.
Ah that's true. And I knew that, but it didn't occur to me ;)
> > Another user-level RCU implementation is available here: > > http://www.cs.toronto.edu/~tomhart/perflab/ipdps06.tgz
Interesting, thanks.
> I have a few user-mode implementations myself, but the lawyers won't > let me release them. :-(
I imagine they're quite a bit faster than my quick hack, too ;)
> > > Question - our kernel's call_rcu implies a smp_wmb, right? Because > > that did catch me out initially, because I initially had no barrier > > to prevent the freeing of the object becoming visible before > > removal of its last reference becoming visible (fixed by adding > > smp_wmb() in my call_rcu). > > No and yes... The kernel's call_rcu() itself does not have an smp_wmb(), > but the Classic RCU grace-period mechanism forces a memory barrier on each > CPU as part of grace-period detection -- which is why rcu_read_lock() > and rcu_read_unlock() don't need memory barriers. Looks like your need > for an smp_wmb() in call_rcu() itself is due to the fact that you can > execute callbacks in the context of the call_rcu() itself.
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