Re: [tip:locking/core] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Fri, Sep 10, 2021 at 12:36:32PM -0400, Alan Stern wrote:
> --- usb-devel.orig/tools/memory-model/Documentation/explanation.txt
> +++ usb-devel/tools/memory-model/Documentation/explanation.txt
> @@ -1813,15 +1813,16 @@ spin_trylock() -- we can call these thin
>  lock-acquires -- have two properties beyond those of ordinary releases
>  and acquires.
>  
> +First, when a lock-acquire reads from or is po-after a lock-release,
> +the LKMM requires that every instruction po-before the lock-release
> +must execute before any instruction po-after the lock-acquire.  This
> +would naturally hold if the release and acquire operations were on
> +different CPUs and accessed the same lock variable, but the LKMM says
> +it also holds when they are on the same CPU, even if they access
> +different lock variables.  For example:

Could be I don't understand this right, but the way I'm reading it, it
seems to imply RCsc. Which I don't think we're actually asking at this
time.