From: Rusty Russell <rusty@rustcorp.com.au>
To: frankeh@watson.ibm.com, Linus Torvalds <torvalds@transmeta.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores)
Date: Wed, 13 Mar 2002 15:02:10 +1100 [thread overview]
Message-ID: <E16kzxq-0004HJ-00@wagner.rustcorp.com.au> (raw)
In-Reply-To: Your message of "Tue, 12 Mar 2002 09:56:42 CDT." <20020312145542.2C8613FE06@smtp.linux.ibm.com>
In message <20020312145542.2C8613FE06@smtp.linux.ibm.com> you write:
> > If we basically export "add_to_waitqueue", "del_from_waitqueue",
> > "wait_for_waitqueue" and "wakeup_waitqueue" syscalls, we have a more
> > powerful interface: the kernel need not touch userspace addresses at
> > all (no kmap/kunmap, no worried about spinlocks vs. rwlocks).
>
> Rusty, aren't you now going back to the design that I implemented after Ben's
> comments.
> From the get-go, I never touched the user address in the kernel, as
> I thought it would require detailed knowledge of the user level
> locking strategy.
Yes, as with my initial patch (like you, I had a semaphore in the
kernel). However, with two separate syscalls, you don't need to
allocate anything in the kernel, and still know nothing about the
userspace locking.
> Could you explain, why you need add_to_waitqueue and wait_for_waitqueue as
> separate calls ? Is it for resolving a race conditions ?
Yep. Exactly analogous to the kernel idiom (add to queue, check, sleep).
(Except the "waker dequeues us" microoptimization).
> One comment with respect to multiple wait queues and rwsems:
> Again it will allow you to do reader-pref and/or writer-pref, but not
> something like FIFO, i.e. wake up a writer if first waiter or wake up all
> readers if first ..... and so on.
> I don't know whether the latter is terrible important ...
(Aside: I'm still reluctant to implement strict FIFO locks until
someone shows a starvation case in the current locks).
I thought about this a little. If we add a flags arg to the
sys_uwaitq_wake() and make sys_uwaitq_wait() return the flags used by
the waker, and sys_uwaitq_wake return 1 if it woke someone...
#define UWAITQ_PASSING 1
up:
ret = sys_uwaitq_wake(cookie, UWAITQ_PASSING);
if (ret < 0) return -1;
/* No waiters actually waiting? */
if (ret == 0) {
lock->counter = 1;
sys_uwaitq_wake(cookie, 0);
}
down:
sys_uwaitq_queue(cookie);
if (atomic dec counter to 0) {
sys_uwaitq_queue(0); /* unqueue */
return 0;
}
ret = sys_uwaitq_wait(cookie);
if (ret < 0)
return -1;
if (ret == UWAITQ_PASSING)
return 0;
goto down; /* spin again */
Rusty.
--
Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
next prev parent reply other threads:[~2002-03-13 4:02 UTC|newest]
Thread overview: 95+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-03-05 7:01 [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores) Rusty Russell
2002-03-05 21:23 ` Futexes III : performance numbers Hubertus Franke
2002-03-06 2:08 ` Rusty Russell
2002-03-06 14:28 ` Hubertus Franke
2002-03-06 17:23 ` [Lse-tech] " george anzinger
2002-03-07 0:25 ` Hubertus Franke
2002-03-07 0:35 ` Hubertus Franke
2002-03-05 22:39 ` [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores) 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
2002-03-06 7:54 ` Futexes III : performance numbers Rusty Russell
2002-03-06 14:46 ` Hubertus Franke
2002-03-06 16:13 ` Hubertus Franke
2002-03-06 20:36 ` Futexes V : Hubertus Franke
2002-03-07 4:21 ` Rusty Russell
2002-03-08 18:07 ` [PATCH] Futexes IV (Fast Lightweight Userspace Semaphores) 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 [this message]
2002-03-12 17:17 ` Linus Torvalds
2002-03-13 2:57 ` Rusty Russell
2002-03-09 4:51 ` Rusty Russell
2002-03-13 9:12 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
2002-03-16 19:48 ` Peter Wächtler
2002-03-17 6:50 ` Rusty Russell
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