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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.arch
Subject: Re: Memory ordering
Date: Thu, 19 Dec 2024 13:19:24 -0800
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On 12/19/2024 10:33 AM, MitchAlsup1 wrote:
> On Thu, 5 Dec 2024 7:44:19 +0000, Chris M. Thomasson wrote:
> 
>> On 12/4/2024 8:13 AM, jseigh wrote:
>>> On 12/3/24 18:37, Stefan Monnier wrote:
>>>>>>                                            If there are places
>>>>>> in the code it doesn't know this can't happen it won't optimize
>>>>>> across it, more or less.
>>>>>
>>>>> The problem is HOW to TELL the COMPILER that these memory references
>>>>> are "more special" than normal--when languages give few mechanisms.
>>>>
>>>> We could start with something like
>>>>
>>>>      critical_region {
>>>>        ...
>>>>      }
>>>>
>>>> such that the compiler must refrain from any code motion within
>>>> those sections but is free to move things outside of those sections as
>>>> if
>>>> execution was singlethreaded.
>>>>
>>>
>>> C/C++11 already defines what lock acquire/release semantics are.
>>> Roughly you can move stuff outside of a critical section into it
>>> but not vice versa.
>>>
>>> Java uses synchronized blocks to denote the critical section.
>>> C++ (the society for using RAII for everything) has scoped_lock
>>> if you want to use RAII for your critical section.  It's not
>>> always obvious what the actual critical section is.  I usually
>>> use it inside its own bracket section to make it more obvious.
>>>    { std::scoped_lock m(mutex);
>>>      // .. critical section
>>>    }
>>>
>>> I'm not a big fan of c/c++ using acquire and release memory order
>>> directives on everything since apart from a few situations it's
>>> not intuitively obvious what they do in all cases.  You can
>>> look a compiler assembler output but you have to be real careful
>>> generalizing from what you see.
>>
>> The release on the unlock can allow some following stores and things to
>> sort of "bubble up before it?
>>
>> Acquire and release confines things to the "critical section", the
>> release can allow for some following things to go above it, so to speak.
>> This is making me think of Alex over on c.p.t. !
> 
> This sounds dangerous if the thing allowed to go above it is unCacheable
> while the lock:release is cacheable, the cacheable lock can arrive at
> another core before the unCacheable store arrives at its destination.

Humm... Need to ponder on that. Wrt the sparc:

membar #LoadStore | #StoreStore

can allow following stores to bubble up before it. If we want to block 
that then we would use a #StoreLoad. However, a #StoreLoad is not 
required for unlocking a mutex.




> 
>> :^)
>>
>> Did I miss anything? Sorry Joe.