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


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