From: "Peter Wächtler" <pwaechtler@loewe-komp.de>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)
Date: Mon, 25 Mar 2002 10:47:46 +0100 [thread overview]
Message-ID: <3C9EF242.6080106@loewe-komp.de> (raw)
In-Reply-To: <E16pKE0-0004Rb-00@wagner.rustcorp.com.au>
Rusty Russell wrote:
> In message <3C9E1A10.F7AA6D6E@loewe-komp.de> you write:
>
>>I can't see a reason why the ack-futex is needed. I think we can simply
>>delete it.
>>When deleted, the broadcast wouldn't block on ack (also preventing
>>schedule ping-pong). With the cond->lock it's save to have several
>>broadcasters. That's fine.
>>
>
> No, you might end up waking someone who did the pthread_cond_wait()
> after you did the pthread_cond_broadcast in place of one of the
> existing pthread_cond_wait() threads.
>
> I don't believe this is allowed.
>
Indeed, I suspect that this isn't wanted.
With the cond->lock you almost prevent this: an ongoing broadcast
can't be intermixed with newly incoming waiters (they will block
on futex_down(&cond->lock))
But there is the window between a->b...
>
>>But:
>>static int __pthread_cond_wait(pthread_cond_t *cond,
>> pthread_mutex_t *mutex,
>> const struct timespec *reltime)
>>{
>> int ret;
>>
>> /* Increment first so broadcaster knows we are waiting. */
>> futex_down(&cond->lock);
>> atomic_inc(cond->num_waiting);
>>(*) futex_up(&mutex, 1);
>>a) futex_up(&cond->lock, 1); [move into syscall]
>> do {
>>b) ret = futex_down_time(&cond, ABSTIME); [cond_timed_wait]
>> } while (ret < 0 && errno == EINTR);
>> [futex_up(&cond->lock, 1); /* release condvar */]
>>
>> futex_down(&mutex->futex);
>> return ret;
>>}
>>
>>With the original code, we have a "signal/broadcast lost window (a->b)"
>>that shouldn't be there:
>>
>
> Where? Having done the inc, the futex_up at (a) will fall through,
> giving the "thread behaves as if it [signal or broadcast] were issued
> after the about-to-block thread has blocked."
>
Right after (a) another thread gets scheduled, issueing a signal/broadcast.
Ah, and then the futex_down_timed() wouldn't block, OK ;-)
But this way you have to use the ack->lock
I strongly believe, that the implementation of a condvar needs a lock
to prevent intermixed calls. You will see my comment on your implementation
with uwaitq. ;-)
next prev parent reply other threads:[~2002-03-25 9:48 UTC|newest]
Thread overview: 84+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-03-13 9:12 [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores) Martin Wirth
2002-03-13 19:41 ` Bill Davidsen
2002-03-13 19:52 ` Dave McCracken
2002-03-13 22:17 ` Bill Davidsen
2002-03-13 20:06 ` Alan Cox
2002-03-15 7:31 ` Rusty Russell
2002-03-15 8:41 ` Martin Wirth
2002-03-15 15:29 ` Hubertus Franke
2002-03-15 16:23 ` Peter Wächtler
2002-03-16 0:12 ` Rusty Russell
2002-03-16 11:23 ` Martin Wirth
2002-03-18 0:52 ` Ulrich Drepper
2002-03-19 3:28 ` Rusty Russell
2002-03-19 4:05 ` Ulrich Drepper
2002-03-20 6:20 ` Rusty Russell
2002-03-20 10:42 ` Peter Wächtler
2002-03-20 17:20 ` Ulrich Drepper
2002-03-19 8:34 ` Martin Wirth
2002-03-20 6:45 ` Rusty Russell
2002-03-21 6:48 ` Martin Wirth
2002-03-24 18:25 ` Peter Wächtler
2002-03-25 2:28 ` Rusty Russell
2002-03-25 4:46 ` Rusty Russell
2002-03-25 11:56 ` Peter Wächtler
2002-03-26 1:02 ` Rusty Russell
2002-03-26 8:17 ` Martin Wirth
2002-03-26 23:10 ` Rusty Russell
2002-03-27 21:05 ` Hubertus Franke
2002-03-27 23:53 ` Rusty Russell
2002-03-25 9:47 ` Peter Wächtler [this message]
2002-03-16 19:48 ` Peter Wächtler
2002-03-17 6:50 ` Rusty Russell
-- strict thread matches above, loose matches on Subject: below --
2002-03-05 7:01 Rusty Russell
2002-03-05 22:39 ` Davide Libenzi
2002-03-05 23:16 ` Hubertus Franke
2002-03-05 23:26 ` Davide Libenzi
2002-03-05 23:37 ` Peter Svensson
2002-03-05 23:50 ` Davide Libenzi
2002-03-08 0:07 ` Richard Henderson
2002-03-06 1:46 ` Rusty Russell
2002-03-06 2:03 ` Davide Libenzi
2002-03-08 18:07 ` Linus Torvalds
2002-03-08 19:03 ` Hubertus Franke
2002-03-08 19:22 ` Linus Torvalds
2002-03-08 20:29 ` Hubertus Franke
2002-03-08 20:48 ` Matthew Kirkwood
2002-03-08 21:02 ` Linus Torvalds
2002-03-08 23:15 ` Hubertus Franke
2002-03-08 23:36 ` Alan Cox
2002-03-08 23:41 ` Linus Torvalds
2002-03-08 23:56 ` Hubertus Franke
2002-03-09 2:12 ` Linus Torvalds
2002-03-11 14:14 ` Hubertus Franke
2002-03-09 0:03 ` H. Peter Anvin
2002-03-09 1:15 ` Alan Cox
2002-03-10 19:41 ` Linus Torvalds
2002-03-11 20:49 ` Pavel Machek
2002-03-10 19:58 ` Martin J. Bligh
2002-03-10 20:40 ` Alan Cox
2002-03-10 20:28 ` Martin J. Bligh
2002-03-10 21:05 ` Alan Cox
2002-03-13 7:40 ` Rusty Russell
2002-03-13 16:37 ` Alan Cox
2002-03-12 9:35 ` Helge Hafting
2002-03-08 20:40 ` Alan Cox
2002-03-08 20:57 ` Linus Torvalds
2002-03-08 23:43 ` H. Peter Anvin
2002-03-08 22:55 ` Hubertus Franke
2002-03-08 23:38 ` Alan Cox
2002-03-08 23:44 ` H. Peter Anvin
2002-03-08 20:47 ` george anzinger
2002-03-08 23:02 ` Hubertus Franke
2002-03-08 23:47 ` george anzinger
2002-03-09 1:11 ` Alan Cox
2002-03-09 1:20 ` Linus Torvalds
2002-03-09 4:49 ` Rusty Russell
2002-03-11 22:45 ` Linus Torvalds
2002-03-11 23:12 ` Hubertus Franke
2002-03-12 7:20 ` Rusty Russell
2002-03-12 14:56 ` Hubertus Franke
2002-03-13 4:02 ` Rusty Russell
2002-03-12 17:17 ` Linus Torvalds
2002-03-13 2:57 ` Rusty Russell
2002-03-09 4:51 ` Rusty Russell
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