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Groups > comp.lang.c++ > #122565
| From | cross@spitfire.i.gajendra.net (Dan Cross) |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: fun with a mutex... |
| Date | 2025-08-18 20:26 +0000 |
| Organization | PANIX Public Access Internet and UNIX, NYC |
| Message-ID | <1080292$h40$2@reader1.panix.com> (permalink) |
| References | <106cc2f$31ti3$1@dont-email.me> <107dkai$2rg26$2@dont-email.me> <107htlf$bq7$1@reader1.panix.com> <107ojfk$1e84t$2@dont-email.me> |
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". >> 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. - Dan C.
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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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