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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4ABAAB4F.2060905@nortel.com>
Date:	Wed, 23 Sep 2009 17:12:15 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] Userspace RCU: (ab)using futexes to save cpu cycles and
 energy

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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/