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]

Date:	Thu, 1 Oct 2009 07:40:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] Userspace RCU: (ab)using futexes to save cpu cycles and
	energy

On Wed, Sep 23, 2009 at 01:48:20PM -0400, Mathieu Desnoyers wrote:
> Hi,
> 
> When implementing the call_rcu() "worker thread" in userspace, I ran
> into the problem that it had to be woken up periodically to check if
> there are any callbacks to execute. However, I easily imagine that this
> does not fit well with the "green computing" definition.
> 
> Therefore, I've looked at ways to have the call_rcu() callers waking up
> this worker thread when callbacks are enqueued. However, I don't want to
> take any lock and the fast path (when no wake up is required) should not
> cause any cache-line exchange.
> 
> Here are the primitives I've created. I'd like to have feedback on my
> futex use, just to make sure I did not do any incorrect assumptions.
> 
> This could also be eventually used in the QSBR Userspace RCU quiescent
> state and in mb/signal userspace RCU when exiting RCU read-side C.S. to
> ensure synchronize_rcu() does not busy-wait for too long.
> 
> /*
>  * Wake-up any waiting defer thread. Called from many concurrent threads.
>  */
> static void wake_up_defer(void)
> {
>         if (unlikely(atomic_read(&defer_thread_futex) == -1))
>                 atomic_set(&defer_thread_futex, 0);
>                 futex(&defer_thread_futex, FUTEX_WAKE,
>                       0, NULL, NULL, 0);
> }
> 
> /*
>  * Defer thread waiting. Single thread.
>  */
> static void wait_defer(void)
> {
>         atomic_dec(&defer_thread_futex);
>         if (atomic_read(&defer_thread_futex) == -1)
>                 futex(&defer_thread_futex, FUTEX_WAIT, -1,
>                       NULL, NULL, 0);
> }

The standard approach would be to use pthread_cond_wait() and
pthread_cond_broadcast().  Unfortunately, this would require holding a
pthread_mutex_lock across both operations, which would not necessarily
be so good for wake-up-side scalability.

That said, without this sort of heavy-locking approach, wakeup races
are quite difficult to avoid.

							Thanx, Paul
--
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/