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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.arch
Subject: Re: Memory ordering
Date: Sat, 16 Nov 2024 13:23:41 -0800
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On 11/16/2024 1:21 PM, Chris M. Thomasson wrote:
> On 11/15/2024 12:42 PM, Chris M. Thomasson wrote:
>> On 11/15/2024 5:24 AM, Michael S wrote:
>>> On Fri, 15 Nov 2024 03:17:22 -0800
>>> "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> wrote:
>>>
>>>> On 11/14/2024 11:25 PM, Anton Ertl wrote:
>>>>> aph@littlepinkcloud.invalid writes:
>>>>>> Yes. That Alpha behaviour was a historic error. No one wants to do
>>>>>> that again.
>>>>>
>>>>> Was it an actual behaviour of any Alpha for public sale, or was it
>>>>> just the Alpha specification?  I certainly think that Alpha's lack
>>>>> of guarantees in memory ordering is a bad idea, and so is ARM's:
>>>>> "It's only 32 pages" <YfxXO.384093$EEm7.56154@fx16.iad>.  Seriously?
>>>>> Sequential consistency can be specified in one sentence: "The result
>>>>> of any execution is the same as if the operations of all the
>>>>> processors were executed in some sequential order, and the
>>>>> operations of each individual processor appear in this sequence in
>>>>> the order specified by its program."
>>>> [...]
>>>>
>>>>
>>>> Well, iirc, the Alpha is the only system that requires an explicit
>>>> membar for a RCU based algorithm. Even SPARC in RMO mode does not
>>>> need this. Iirc, akin to memory_order_consume in C++:
>>>>
>>>> https://en.cppreference.com/w/cpp/atomic/memory_order
>>>>
>>>> data dependent loads
>>>>
>>>
>>> You response does not answer Anton's question.
>>>
>>
>> I guess not. Shit happens. ;^o
> 
> 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.

I think, iirc, there is a way to use an acquire membar on the loading of 
the initial node of a collection to iterate it without using 
memory_order_consume for every node. I might be wrong on that. It's been 
a while!