Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

[Alan Stern <stern@rowland.harvard.edu>]

On Mon, Jan 23, 2023 at 12:48:42PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/21/2023 6:36 PM, Alan Stern wrote:
> > On Fri, Jan 20, 2023 at 10:41:14PM +0100, Jonas Oberhauser wrote:
> > > 
> > > On 1/20/2023 5:18 PM, Alan Stern wrote:
> > > > On Fri, Jan 20, 2023 at 11:13:00AM +0100, Jonas Oberhauser wrote:
> > > > > Perhaps we could say that reading an index without using it later is
> > > > > forbidden?
> > > > > 
> > > > > flag ~empty [Srcu-lock];data;rf;[~ domain(data;[Srcu-unlock])] as
> > > > > thrown-srcu-cookie-on-floor
> > > > We already flag locks that don't have a matching unlock.
> > > Of course, but as you know this is completely orthogonal.
> > Yeah, okay.  It doesn't hurt to add this check, but the check isn't
> > complete.  For example, it won't catch the invalid usage here:
> > 
> > P0(srcu_struct *ss)
> > {
> > 	int r1, r2;
> > 
> > 	r1 = srcu_read_lock(ss);
> > 	srcu_read_unlock(&ss, r1);
> > 	r2 = srcu_read_lock(ss);
> > 	srcu_read_unlock(&ss, r2);
> > }
> > 
> > exists (~0:r1=0:r2)
> > 
> > On the other hand, how often will people make this sort of mistake in
> > their litmus tests?  My guess is not very.
> I currently don't care too much about the incorrect usage of herd (by
> inspecting some final state incorrectly), only incorrect usage in the code.

I'm inclined to add this check to the memory model.  Would you prefer to 
submit it yourself as a separate patch?  Or are you happy to have it 
merged with my patch, and if so, do you have a final, preferred form for 
the check?

Alan