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From: cross@spitfire.i.gajendra.net (Dan Cross)
Newsgroups: comp.arch
Subject: Re: MSI interrupts
Date: Tue, 25 Mar 2025 23:33:43 -0000 (UTC)
Organization: PANIX Public Access Internet and UNIX, NYC
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Originator: cross@spitfire.i.gajendra.net (Dan Cross)

In article <jwvjz8coho7.fsf-monnier+comp.arch@gnu.org>,
Stefan Monnier  <monnier@iro.umontreal.ca> wrote:
>Dan Cross [2025-03-25 22:19:07] wrote:
>> From what I gather, you've said to just eschew the atomic event
>> stuff and build a traditional lock using those primitives.  But
>> you have also said that a traditional lock can be silently
>> unlocked if the thread holding it (which, apparently, is an
>> abstraction known to hardware) is preempted.  It will be like
>> the lock had never been acquired.
>
>Here's the misunderstanding: the lock that can be silently taken away
>is the lock implemented in the CPU to run an ATOMIC sequence, not the
>lock implemented as a data-structure on top of it: once you've run
>(atomically) your `lock_acquire`, of course that lock is yours.  The CPU
>doesn't even know that this data-structure is a lock and has no
>way to steal it.

I hope you are right, and I'd like Mitch to confirm.

Even so, I don't see the point of the proposed atomic framework.
It seems to have no significant advantage over a more
traditional scheme; CAS, LL-SC, whatever seem more general and
less intertwined with primitives that the hardware has no real
business injecting itself into (like threads).

	- Dan C.