Message-ID: <4ABAAB4F.2060905@nortel.com>
Date: Wed, 23 Sep 2009 17:12:15 -0600
From: "Chris Friesen" <cfriesen@nortel.com>
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To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [RFC] Userspace RCU: (ab)using futexes to save cpu cycles and
 energy
References: <20090923174820.GA12827@Krystal> <4ABA631A.8030306@nortel.com> <20090923190337.GA16983@Krystal> <20090923223204.GA2921@Krystal>
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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
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