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From: Paavo Helde <eesnimi@osa.pri.ee>
Newsgroups: comp.lang.c++
Subject: Re: Futex Stack Test...
Date: Tue, 18 Feb 2025 09:01:55 +0200
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On 18.02.2025 01:17, Chris M. Thomasson wrote:
> This is a little C++20 test using a futex to allow one to wait on a 
> lock-free stack. The main stack logic is in struct ct_stack. Well, can 
> you get to compile and run? Thanks...

Seamed to work fine, but compilation produced some warnings about the 
CT_WAIT macro.


Build started at 08:55...
1>------ Build started: Project: ConsoleTest2022, Configuration: Release 
x64 ------
1>futex-stack-test.cpp
1>C:\Test\ConsoleTestVS2022\ConsoleTest2022\futex-stack-test.cpp(55,25): 
warning C4312: 'type cast': conversion from 'unsigned int' to 'ct_node 
*' of greater size
1>C:\Test\ConsoleTestVS2022\ConsoleTest2022\futex-stack-test.cpp(71,21): 
warning C4312: 'type cast': conversion from 'unsigned int' to 'ct_node 
*' of greater size
1>C:\Test\ConsoleTestVS2022\ConsoleTest2022\futex-stack-test.cpp(87,38): 
warning C4312: 'type cast': conversion from 'unsigned int' to 'ct_node 
*' of greater size
1>C:\Test\ConsoleTestVS2022\ConsoleTest2022\futex-stack-test.cpp(90,40): 
warning C4312: 'type cast': conversion from 'unsigned int' to 'ct_node 
*' of greater size
1>C:\Test\ConsoleTestVS2022\ConsoleTest2022\futex-stack-test.cpp(92,39): 
warning C4312: 'type cast': conversion from 'unsigned int' to 'ct_node 
*' of greater size
1>C:\Test\ConsoleTestVS2022\ConsoleTest2022\futex-stack-test.cpp(94,33): 
warning C4312: 'type cast': conversion from 'unsigned int' to 'ct_node 
*' of greater size
1>C:\Test\ConsoleTestVS2022\ConsoleTest2022\futex-stack-test.cpp(185,38): 
warning C4312: 'type cast': conversion from 'unsigned int' to 'ct_node 
*' of greater size
1>Generating code
1>77 of 78 functions (98.7%) were compiled, the rest were copied from 
previous compilation.
1>  66 functions were new in current compilation
1>  0 functions had inline decision re-evaluated but remain unchanged
1>Finished generating code
1>ConsoleTest2022.vcxproj -> 
C:\Test\ConsoleTestVS2022\ConsoleTest2022\x64\Release\ConsoleTest2022.exe
1>Done building project "ConsoleTest2022.vcxproj".
========== Build: 1 succeeded, 0 failed, 0 up-to-date, 0 skipped ==========


Futex Stack Test
by: Chris M. Thomasson
version: (0.0.1)
_________________________________

CT_THREAD_N = 42
CT_WORK_N = 10000000
CT_WAIT = 00000000DEADBEEF
ct_shared::ct_shared()


Launching threads...


Generate work...


Completed all work!

Sending shutdown state...


Threads joined...

Dump shutdown state...


Shutdown complete!

ct_shared::~ct_shared()

g_ct_work_alloations = 10000042
g_ct_work_dealloations = 10000042

Fin!



> I still need to check my algorithm out in Relacy. I think it might be 
> able to model futex.
> 
> 
> My code:
> ______________________________________
> #include <iostream>
> #include <thread>
> #include <atomic>
> #include <algorithm>
> #include <cassert>
> 
> 
> #define CT_THREAD_N (42)
> #define CT_WORK_N (10000000)
> #define CT_WAIT ((ct_node*)0xDEADBEEF)
> 
> 
> struct ct_node
> {
>      ct_node* m_next = { nullptr };
> 
>      ct_node()
>      {
>          //std::cout << "(" << this << ")::ct_node::ct_node()\n";
>      }
> 
>      ~ct_node()
>      {
>          //std::cout << "(" << this << ")::ct_node::~ct_node()\n";
>      }
> };
> 
> 
> struct ct_stack
> {
>      std::atomic<ct_node*> m_head = { nullptr };
> 
>      void
>      push(
>          ct_node* node
>      ) {
>          ct_node* head = m_head.load(std::memory_order_relaxed);
> 
>          do
>          {
>              if (head == CT_WAIT)
>              {
>                  node->m_next = nullptr;
>              }
> 
>              else
>              {
>                  node->m_next = head;
>              }
> 
>          } while (! m_head.compare_exchange_weak(
>              head,
>              node,
>              std::memory_order_release)
>            );
> 
>          if (head == CT_WAIT)
>          {
>              // slow path...
>              m_head.notify_one();
>          }
>      }
> 
>      ct_node*
>      flush_wait()
>      {
>          ct_node* head = nullptr;
> 
>          for (;;)
>          {
>              head = m_head.exchange(nullptr, std::memory_order_acquire);
> 
>              while (! head || head == CT_WAIT)
>              {
>                  // slow path...
>                  head = m_head.exchange(CT_WAIT, 
> std::memory_order_acquire);
> 
>                  if (! head || head == CT_WAIT)
>                  {
>                      m_head.wait(CT_WAIT, std::memory_order_relaxed);
>                      continue;
>                  }
>              }
> 
>              break;
>          }
> 
>          assert(head && head != CT_WAIT);
========== REMAINDER OF ARTICLE TRUNCATED ==========