Re: Userspace RCU+rtth hack (was Re: [patch 3/3] radix-tree: RCU lockless readside)

Paul E. McKenney
2006-06-22T20:25:21Z

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.

Thanx, Paul