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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.arch
Subject: Re: arm ldxr/stxr vs cas
Date: Mon, 9 Sep 2024 14:04:21 -0700
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On 9/9/2024 12:14 AM, Terje Mathisen wrote:
> jseigh wrote:
>>
>> I'm not so sure about making the memory lock granularity same as
>> cache line size but that's an implementation decision I guess.
> 
> Just make sure you never have multiple locks residing inside the same 
> cache line!

Yeah. That would be interestingly bad...

> 
>>
>> I do like the idea of detecting potential contention at the
>> start of LL/SC so you can do back off.  Right now the only way I
>> can detect contention is after the fact when the CAS fails and
>> I probably have the cache line exclusive at that point.  It's
>> pretty problematic.
> 
> I do prefer LOCK XADD instead of CAS (CmpXchg*), because the return 
> value will also tell you which queue entry to pick/work on.
> 
> It will not be optimal when really contended, but at least one 
> participant will make forward progress, and typically several of them.

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