Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost)

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

On Mon, Feb 09, 2009 at 02:03:17AM -0500, Mathieu Desnoyers wrote:

[ . . . ]

> I just added modified rcutorture.h and api.h from your git tree
> specifically for an urcutorture program to the repository. Some results :
> 
> 8-way x86_64
> E5405 @2 GHZ
> 
> ./urcutorture 8 perf
> n_reads: 1937650000  n_updates: 3  nreaders: 8  nupdaters: 1 duration: 1
> ns/read: 4.12871  ns/update: 3.33333e+08
> 
> ./urcutorture 8 uperf
> n_reads: 0  n_updates: 4413892  nreaders: 0  nupdaters: 8 duration: 1
> ns/read: nan  ns/update: 1812.46
> 
> n_reads: 98844204  n_updates: 10  n_mberror: 0
> rcu_stress_count: 98844171 33 0 0 0 0 0 0 0 0 0
> 
> However, I've tried removing the second switch_qparity() call, and the
> rcutorture test did not detect anything wrong. I also did a variation
> which calls the "sched_yield" version of the urcu, "urcutorture-yield".

My confusion -- I was testing my old approach where the memory barriers
are in rcu_read_lock() and rcu_read_unlock().  To force the failures in
your signal-handler-memory-barrier approach, I suspect that you are
going to need a bigger hammer.  In this case, one such bigger hammer
would be:

o	Just before exit from the signal handler, do a
	pthread_cond_wait() under a pthread_mutex().

o	In force_mb_all_threads(), refrain from sending a signal to self.

	Then it should be safe in force_mb_all_threads() to do a
	pthread_cond_broadcast() under the same pthread_mutex().

This should raise the probability of seeing the failure in the case
where there is a single switch_qparity().

							Thanx, Paul