Re: Futex Stack Test...

On 2/17/2025 11:01 PM, Paavo Helde wrote:

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.

[...]
Sorry about that. ;^o
Too used to C style casts. Thanks for giving it a go, Paavo. :^)
It's just an interesting way to use the std futex directly in the state of a lock-free stack. My code can be streamlined as well. For instance, the ct_stack::flush_wait function can be much better. Something like this, still not ideal, but a little better?
just typed this into the newsreader, but it should work fine:
_______________________
ct_node*
flush_wait()
{
     ct_node* 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);
         }
     }
     assert(head && head != CT_WAIT);
     return head;
}
_______________________
I think I could use a compare_exchange for the wait state in the slow-path, but trying to optimize a slow path is probably not even worth it all that much. Humm...