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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.arch
Subject: Re: Memory ordering
Date: Mon, 18 Nov 2024 19:38:41 -0800
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On 11/17/2024 1:30 PM, Chris M. Thomasson wrote:
> On 11/17/2024 6:03 AM, jseigh wrote:
>> On 11/16/24 16:21, Chris M. Thomasson wrote:
>>
>>> Fwiw, in C++ std::memory_order_consume is useful for traversing a 
>>> node based stack of something in RCU. In most systems it only acts 
>>> like a compiler barrier. On the Alpha, it must emit a membar 
>>> instruction. Iirc, mb for alpha? Cannot remember that one right now.
>>
>> That got deprecated.  Too hard for compilers to deal with.  It's now
>> same as memory_order_acquire.
> 
> Strange! C++ has:
> 
> https://en.cppreference.com/w/cpp/atomic/atomic_signal_fence
> 
> That only deals with compilers, not the arch memory order... Humm...
> 
> Interesting Joe!
> 
> 
>> Which brings up an interesting point.  Even if the hardware memory
>> memory model is strongly ordered, compilers can reorder stuff,
>> so you still have to program as if a weak memory model was in
>> effect.   Or maybe disable reordering or optimization altogether
>> for those target architectures.
> 
> Indeed. The compiler needs to know about these things. Iirc, there was 
> an old post over c.p.t that deals with a compiler (think it was GCC) 
> that messed up a pthread try lock for a mutex. It's a very old post. But 
> I remember it for sure.
> 

a song for contention:

https://youtu.be/Sdq4T3iRV80?list=RDMMy3hf0T4qpYg

;^D