Re: [RFC] Userspace RCU: (ab)using futexes to save cpu cycles and energy

["Chris Friesen" <cfriesen@nortel.com>]

On 09/23/2009 04:32 PM, Mathieu Desnoyers wrote:


> /*
>  * Defer thread waiting. Single thread.
>  */
> static void wait_defer(void)
> {
>         atomic_dec(&defer_thread_futex);
>         smp_mb();       /* Write futex before read queue */
>         if (rcu_defer_num_callbacks()) {
>                 smp_mb();       /* Read queue before write futex */
>                 /* Callbacks are queued, don't wait. */
>                 atomic_set(&defer_thread_futex, 0);
>         } else {
>                 smp_rmb();      /* Read queue before read futex */
>                 if (atomic_read(&defer_thread_futex) == -1)
>                         futex(&defer_thread_futex, FUTEX_WAIT, -1,
>                               NULL, NULL, 0);
>         }
> }

> The goal here is that if call_rcu() enqueues a callback (even if it
> races with defer thread going to sleep), there should not be a
> potentially infinite delay before it gets executed.

It doesn't seem like the test for the number of callbacks should be
necessary.  I don't see anything like that in the glibc code, nor do I
remember anything like that in the futex sample code.

I'm still not totally convinced that you can avoid race conditions
without using atomic test-and-set or compare-and-exchange.  I haven't
sat down and worked it out completely though.

Chris