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Re: fun with a mutex...

From "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups comp.lang.c++
Subject Re: fun with a mutex...
Date 2025-08-18 16:11 -0700
Organization A noiseless patient Spider
Message-ID <1080bvs$3e5uq$1@dont-email.me> (permalink)
References <106cc2f$31ti3$1@dont-email.me> <107ojfk$1e84t$2@dont-email.me> <1080292$h40$2@reader1.panix.com> <108031c$3bdl8$2@dont-email.me> <10803bq$h40$4@reader1.panix.com>

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On 8/18/2025 1:44 PM, Dan Cross wrote:
> In article <108031c$3bdl8$2@dont-email.me>,
> Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
>> On 8/18/2025 1:26 PM, Dan Cross wrote:
>>> In article <107ojfk$1e84t$2@dont-email.me>,
>>> Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
>>>> On 8/13/2025 4:41 AM, Dan Cross wrote:
>>>>> In article <107dkai$2rg26$2@dont-email.me>,
>>>>> Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
>>>>>> On 8/9/2025 8:08 PM, Dan Cross wrote:
>>>>>>> In article <10781l0$1eamj$2@dont-email.me>,
>>>>>>> Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
>>>>>>>> On 8/9/2025 9:08 AM, Dan Cross wrote:
>>>>>>>>> In article <10775om$17ms4$1@raubtier-asyl.eternal-september.org>,
>>>>>>>>> Bonita Montero  <Bonita.Montero@gmail.com> wrote:
>>>>>>>>>> Am 09.08.2025 um 11:17 schrieb Dan Cross:
>>>>>>>>>>
>>>>>>>>>>> People have written code that tries to take a lock, and if that
>>>>>>>>>>> fails, does something else, for decades.  The idea is neither
>>>>>>>>>>> new, nor terribly interesting.  Sorry.
>>>>>>>>>>
>>>>>>>>>> I think this pattern is extremely rare because the task that needs to
>>>>>>>>>> be done elsewhere must be able to be omitted completely if necessary,
>>>>>>>>>> and if not, it must be short enough that it doesn't interfere with
>>>>>>>>>> waiting for the mutex.
>>>>>>>>>
>>>>>>>>> Yeah.  That's why I mentioned concentrating on the specific
>>>>>>>>> critical section the mutex is protecting, not the locking
>>>>>>>>> protocol.  Where this has any utility at all, it's in places
>>>>>>>>> where the "something else" is well-known, which implies it being
>>>>>>>>> a function of the critsec.  It's going to be things like, "if I
>>>>>>>>> fail to acquire the scheduler lock, then increment this per-core
>>>>>>>>> atomic counter that's measuring contention...." sort of thing.
>>>>>>>>
>>>>>>>> More grasping at straws and/or trying to breath underwater without scuba
>>>>>>>> gear... The other work is very specialized. It can be as simple as
>>>>>>>> processing a per-thread local queue or something. Just something that
>>>>>>>> can act like a backoff that does real work in a case of contention. Sigh.
>>>>>>>
>>>>>>> That's the sort of thing where you cannot know what's on that
>>>>>>> "per-thread local queue" that makes your scheme fail.
>>>>>>
>>>>>> Well, that per-thread local queue can be for organization, maintenance,
>>>>>> sending the results to a "logic" thread, ect. Think of a server io
>>>>>> thread that takes the io completions and organizes them into local
>>>>>> queues. These io threads do not call into user logic, or processing wrt
>>>>>> say, parsing HTTP, or some protocol. So, some work that can be used
>>>>>> after a try_lock fails can be to push the local queues to the logic threads.
>>>>>
>>>>> This, yet again, seems to be where you don't understand the
>>>>> issue at stake here.
>>>>>
>>>>> You can _probably_ find _any number of things_ that you can do
>>>>> while looping over `try_lock`.  I don't care about the specific
>>>>> examples: they obviously exist.
>>>>>
>>>>> But what you _cannot_ do, and what makes this sort of scheme
>>>>> fail, is know that _all_ such work is safe.  Unless you can wave
>>>>> a magic wand and solve the halting problem, you won't be able to
>>>>> know that some jerk won't send you "work" that amounts to a
>>>>> non-terminating infinite loop.  You won't be able to know that
>>>>> some other jerk hasn't sent you "work" that causes you to
>>>>> deadlock trying to take a mutex you already hold.  Etc.
>>>>>
>>>>> If you _ever_ open yourself up to accepting "work" on some kind
>>>>> of queue, where the types of all work that can possibly be sent
>>>>> to you is not known in advance _in some way that can be enforced
>>>>> in code_, then your scheme fails.  And if you do have some way
>>>>> to enforce those constraints, then the scheme is neither novel
>>>>> nor particularly interesting.
>>>>>
>>>>>>> If your lock is contended, you could just sleep on it, and let
>>>>>>> the kernel schedule something else instead.  There's no value to
>>>>>>> keeping your thread "hot" unless you've got a profile showing
>>>>>>> that it's an issue.
>>>>>>
>>>>>> But why sleep when the thread can do some real work?
>>>>>
>>>>> Because the OS (or even a user-level thread scheduler, or
>>>>> language runtime, or whatever) can find work to usefully do on
>>>>> that CPU, _without_ the downsides mentioned above.  So you lose
>>>>> nothing but gain correctness and stability.
>>>>>
>>>>>> Argh. Grasping
>>>>>> again. Feels like I am drowning. Shit! ;^o
>>>>>
>>>>> Perhaps stop doing that, then.  It's clear you are not going to
>>>>> find a way to make whatever partially developed idea you have
>>>>> work.
>>>>
>>>> It should be okay if the other work is okay with it?
>>>
>>> Again: you have no way of knowing that "the other work is okay
>>> with it".
>>
>> Sure you can in highly controlled scenarios? A programmer can say, this
>> type of work can be done in this condition because the programmer knows
>> what the work consists of... I listed a few of them before. I am not
>> going for some sort of general case type of thing.
>>
>>
>>>>> It's ok to be wrong.  It's less ok to keep trying to convince
>>>>> others that you are right.
>>>>
>>>> It just seems to be okay, well, after a try_lock fails try to do
>>>> something that is _compatible_ instead of something like an adaptive
>>>> mutex? Shit. Yikes!
>>>>
>>>> There I go again. Sorry.
>>>>
>>>> [...]
>>>
>>> Please stop.
>>
>> It sure seems like it cannot be 100% wrong all the time. Having trouble
>> seeing that.
> 
> It's already in the above quoted text.  The relevant section is
> here:
> 
> |You can _probably_ find _any number of things_ that you can do
> |while looping over `try_lock`.  I don't care about the specific
> |examples: they obviously exist.
> |
> |But what you _cannot_ do, and what makes this sort of scheme
> |fail, is know that _all_ such work is safe.  Unless you can wave
> |a magic wand and solve the halting problem, you won't be able to
> |know that some jerk won't send you "work" that amounts to a
> |non-terminating infinite loop.  You won't be able to know that
> |some other jerk hasn't sent you "work" that causes you to
> |deadlock trying to take a mutex you already hold.  Etc.
> |
> |If you _ever_ open yourself up to accepting "work" on some kind
> |of queue, where the types of all work that can possibly be sent
> |to you is not known in advance _in some way that can be enforced
> |in code_, then your scheme fails.  And if you do have some way
> |to enforce those constraints, then the scheme is neither novel
> |nor particularly interesting.
> 
> Just saying, "yay! I can do something while spinning on a lock!"
> is so vaccuously obvious that it's simply uninteresting; people
> have done such things for many, many years.  Your error is in
> trying to extrapolate that to "other" work.
> 
> 	- Dan C.
> 

It was fun when Sun gave me a T2000 SunFire server wrt to their 
CoolThreads contest.

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fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-07-29 22:54 -0700
  Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-02 12:58 -0700
    Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-02 13:00 -0700
    Re: fun with a mutex... Muttley@DastardlyHQ.org - 2025-08-04 08:48 +0000
      Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-04 12:34 -0700
        Re: fun with a mutex... Muttley@DastardlyHQ.org - 2025-08-05 07:27 +0000
          Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-05 16:10 -0700
            Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-06 00:38 +0000
              Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-05 19:14 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-06 11:29 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-06 15:43 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-08 01:41 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-07 20:52 -0700
              Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-05 19:16 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-06 12:04 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-06 15:47 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-08 01:49 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-07 20:59 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-09 09:17 +0000
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-09 11:52 +0200
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-09 16:08 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-09 10:48 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-10 03:08 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-11 13:37 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-13 11:41 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-15 17:28 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-18 20:25 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 13:34 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-18 20:39 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 14:06 -0700
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 14:28 -0700
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 14:33 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-18 22:56 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 16:09 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-18 23:44 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 16:55 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-18 23:57 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-19 13:25 -0700
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-15 17:30 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-18 20:26 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 13:39 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-18 20:44 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 14:08 -0700
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 16:11 -0700
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-18 16:20 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-11 11:04 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-11 13:30 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-12 10:23 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-12 01:31 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-12 10:39 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-12 01:45 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-12 10:56 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-12 02:14 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-12 15:08 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-15 21:36 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-16 10:13 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-16 01:56 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-16 11:05 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-16 13:31 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-17 14:50 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-17 13:43 -0700
                Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-13 11:45 +0000
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-13 19:07 +0200
                Re: fun with a mutex... scott@slp53.sl.home (Scott Lurndal) - 2025-08-13 17:31 +0000
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-15 10:00 -0700
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-13 12:41 -0700
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-09 10:44 -0700
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-09 11:00 -0700
        Re: fun with a mutex... "Fred. Zwarts" <F.Zwarts@HetNet.nl> - 2025-08-05 10:27 +0200
          Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-05 16:07 -0700
            Re: fun with a mutex... "Fred. Zwarts" <F.Zwarts@HetNet.nl> - 2025-08-06 10:56 +0200
              Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-06 15:36 -0700
      Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-04 12:38 -0700
        Re: fun with a mutex... Muttley@DastardlyHQ.org - 2025-08-05 07:29 +0000
          Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-05 16:08 -0700
  Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-06 15:51 +0200
    Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-06 15:34 -0700
      Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-07 06:44 +0200
        Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-07 12:17 -0700
          Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-07 21:51 +0200
            Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-07 12:52 -0700
              Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-16 11:21 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-16 13:22 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-17 03:59 +0200
                Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-16 23:53 -0700
                Re: fun with a mutex... Bonita Montero <Bonita.Montero@gmail.com> - 2025-08-17 10:37 +0200
        Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-07 12:32 -0700
      Re: fun with a mutex... cross@spitfire.i.gajendra.net (Dan Cross) - 2025-08-08 01:51 +0000
        Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-07 21:01 -0700
          Re: fun with a mutex... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2025-08-07 21:08 -0700

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