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Re: fun with a mutex...

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From cross@spitfire.i.gajendra.net (Dan Cross)
Newsgroups comp.lang.c++
Subject Re: fun with a mutex...
Date Wed, 13 Aug 2025 11:41:35 -0000 (UTC)
Organization PANIX Public Access Internet and UNIX, NYC
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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's ok to be wrong.  It's less ok to keep trying to convince
others that you are right.

	- Dan C.

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Thread

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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