Messages in this thread | | | Date | Thu, 22 Jun 2006 13:25:53 -0700 | From | "Paul E. McKenney" <> | 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 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.
Another user-level RCU implementation is available here:
http://www.cs.toronto.edu/~tomhart/perflab/ipdps06.tgz
Tom and his major prof unfortunately felt the need to rename everything. Here is a decoder ring:
Linux Name perflab name
rcupdate.c qsbr.c rcupdate.h qsbr.h call_rcu() free_node_later() rcu_read_lock() N/A rcu_read_unlock() N/A
The perflab package invokes callbacks from the quiescent state (called quiescent_state(), appropriately enough).
FWIW, "QSBR" stands for quiescent-state-based reclamation.
I have a few user-mode implementations myself, but the lawyers won't let me release them. :-(
> 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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