Re: MSI interrupts

On Tue, 25 Mar 2025 22:49:29 +0000, Stefan Monnier 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.

You are getting at the "why". In Compare-Swap atomic sequences with
your typical ISA, the HW does not see the initial LD (of the memory
participating in the compare-swap instruction) as locked. In the LL-
SC version, HW does have that tidbit of information.
With that information, the event is secured both by data and by
address, preventing the ABA problem {At least in ESM}.

>
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        Stefan