Re: [PATCH] Futex Asynchronous Interface

From: Peter Wächtler (
Date: Wed Jun 12 2002 - 11:50:03 EST

Hubertus Franke wrote:
> On Wednesday 12 June 2002 05:16 am, Peter Wächtler wrote:
>>Rusty Russell wrote:
>>>In message <>
>>>you wri
>>>>this makes no sense:
>>>>D: This changes the implementation so that the waker actually unpins
>>>>D: the page. This is preparation for the async interface, where the
>>>>D: process which registered interest is not in the kernel.
>>>>Whazzup? The closing of the fd will unpin the page, the waker has no
>>>>reason to do so. It is very much against the linux philosophy (and a
>>>>design disaster anyway) to have the waker muck with the data structures
>>>>of anything waiting.
>>>Good catch: now the fd is a "one-shot" thing anyway, making close
>>>unpin the page makes more sense. Tested patch below (against 2.5.21).
>>>FYI: I already violate this philosophy as I remove the waiter from the
>>>queue when I wake them: this allows them to tell that they were woken
>>>(waker does a list_del_init() on the waiting entry, so waiting knows
>>>if (list_empty()) I was woken).
>>>It would be more natural for the waiter to examine the futex value,
>>>and if it's still unchanged go back to sleep. But this makes
>>>assumptions about what they're using the futex value for. For
>>>example, we "PASS_THIS_DIRECTLY" value into the futex. This requires
>>>that one (and ONLY one) process waiting actually wakes up.
>>>This is why coming up with a primitive which allowed us to build posix
>>>threads and fair queueing as well as "normal" unfair semantics took so
>>>damn long.
>>What are the plans on how to deal with a waiter when the lock holder
>>dies abnormally?
>>What about sending a signal (SIGTRAP or SIGLOST), returning -1 and
>>setting errno to a reasonable value (EIO?)
>>I couldn't find anything in susv3
> I thing this was decided some time ago that we won't deal with this situation.
> If we need to, the following needs to happen.
> A) we need to follow a protocol to register the PID/TID id within the lock.
> Example of this is described in
> "Recoverable User-Level Mutual Exclusion" by Phiilip Bohannon
> Proceedings of the 7th IEEE Symposium on Parallel and Distributed
> Systems, 1995.
> B) this again requires that some entity (kernel ?) knows about all locks,
> whether waited on in the kernel or not.
> C) I believe we need a deamon that occasinally identifies dead locks.
> Is it worth all this trouble? Or do we need two versions of the ?
> The paper describes that for a Sun SS20/61 the safe spin locks obtained
> only 25% of the performance of spinlocks.

Oops, I see it already myself. We lack a way for saying who is/was owning
the futex and so we can hardly tell who is waiting on this "unknown" lock.

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