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Re: fun with a mutex...

From cross@spitfire.i.gajendra.net (Dan Cross)
Newsgroups comp.lang.c++
Subject Re: fun with a mutex...
Date 2025-08-08 01:41 +0000
Organization PANIX Public Access Internet and UNIX, NYC
Message-ID <1073kkb$mpg$1@reader1.panix.com> (permalink)
References <106cc2f$31ti3$1@dont-email.me> <106udpt$3400b$1@dont-email.me> <106veb1$eha$1@reader1.panix.com> <1070lq1$3lg2n$3@dont-email.me>

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In article <1070lq1$3lg2n$3@dont-email.me>,
Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
>On 8/6/2025 4:29 AM, Dan Cross wrote:
>> In article <106udpt$3400b$1@dont-email.me>,
>> Chris M. Thomasson <chris.m.thomasson.1@gmail.com> wrote:
>>> [snip]
>>> No. If there is no other work to do, it falls back to lock(). If if did
>>> too much work, it falls back to lock.
>> 
>> You're missing the point: there's no good way to determine
>> whether _any_ amount of work is "too much".
>
>Fair enough. But, there is a chance to hit the fast path, so to speak. 

You literally have no way of knowing that beforehand.

>try_lock fails, then the thread does a call to say, 
>GetQueuedCompletionStatusEx with a timeout of zero, (aka try), if it 
>succeeds, it can push the batch into thread local queues, then try the 
>try_lock again and succeeds.

You are, again, missing the point.  Not only does making a call
to `GetQueuedCompletionStatusEx` involve a round trip through
the kernel, in which case you may as well just block in the
kernel and let another thread run, you seem to be missing the
specific that IOCP completion is irrelevant, because it is just
one example.

>So, instead of waiting in the kernel for 
>the mutex it did actual work, and god knows how many other threads got 
>the mutex in the mean time. So, forward progress is guaranteed. The 
>nature of the "tried" work must be known, and if its compatible, so be 
>it. If not, so be it. Fair enough?

No.  You keep saying that "forward progress is guaranteed"
because you bound the total number of times that your locking
thread can do "something else."  But again, the point that you
are missing is that any given "something else" can itself be
completely unbounded, possibly in a way that is totally
unknowable at the time you go to do it; see the lock ordering
constraint violation I gave earlier as an example: there's no
way you can know whether that will be problematic until runtime,
and there's no way you can meaningfully assess "compatibility"
in general.

So even if you bound the number of times you'll do "something
else", since you have no guarantee that ANY instance of other
work is itself bounded, you can't make any guarantees about
whether the scheme as a whole is bounded.

And if it's really just, "for this specific critical section, if
you fail to immediately acquire the lock, go try this other,
very specific thing known to be safe in this specific context,
and then try again...." then as I've said, this is well-known,
but is also wholly unoriginal and thus uninteresting.

	- 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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