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

[Jonas Oberhauser]

On 1/17/2023 6:43 PM, Paul E. McKenney wrote:
> On Tue, Jan 17, 2023 at 10:56:34AM -0500, Alan Stern wrote:
> > On Tue, Jan 17, 2023 at 07:14:16AM -0800, Paul E. McKenney wrote:
> > > On Tue, Jan 17, 2023 at 12:46:28PM +0100, Andrea Parri wrote:
> > > > This was reminiscent of old discussions, in fact, we do have:
> > > >
> > > > [tools/memory-model/Documentation/litmus-tests.txt]
> > > >
> > > > e.	Although sleepable RCU (SRCU) is now modeled, there
> > > > 	are some subtle differences between its semantics and
> > > > 	those in the Linux kernel.  For example, the kernel
> > > > 	might interpret the following sequence as two partially
> > > > 	overlapping SRCU read-side critical sections:
> > > >
> > > > 		 1  r1 = srcu_read_lock(&my_srcu);
> > > > 		 2  do_something_1();
> > > > 		 3  r2 = srcu_read_lock(&my_srcu);
> > > > 		 4  do_something_2();
> > > > 		 5  srcu_read_unlock(&my_srcu, r1);
> > > > 		 6  do_something_3();
> > > > 		 7  srcu_read_unlock(&my_srcu, r2);
> > > >
> > > > 	In contrast, LKMM will interpret this as a nested pair of
> > > > 	SRCU read-side critical sections, with the outer critical
> > > > 	section spanning lines 1-7 and the inner critical section
> > > > 	spanning lines 3-5.
> > > >
> > > > 	This difference would be more of a concern had anyone
> > > > 	identified a reasonable use case for partially overlapping
> > > > 	SRCU read-side critical sections.  For more information
> > > > 	on the trickiness of such overlapping, please see:
> > > > 	https://paulmck.livejournal.com/40593.html
> > > Good point, if we do change the definition, we also need to update
> > > this documentation.
> > >
> > > > More recently/related,
> > > >
> > > >    https://lore.kernel.org/lkml/20220421230848.GA194034@paulmck-ThinkPad-P17-Gen-1/T/#m2a8701c7c377ccb27190a6679e58b0929b0b0ad9
> > > It would not be a bad thing for LKMM to be able to show people the
> > > error of their ways when they try non-nested partially overlapping SRCU
> > > read-side critical sections.  Or, should they find some valid use case,
> > > to help them prove their point.  ;-)
> > Isn't it true that the current code will flag srcu-bad-nesting if a
> > litmus test has non-nested overlapping SRCU read-side critical sections?
> Now that you mention it, it does indeed, flagging srcu-bad-nesting.
>
> Just to see if I understand, different-values yields true if the set
> contains multiple elements with the same value mapping to different
> values.  Or, to put it another way, if the relation does not correspond
> to a function.
>
> Or am I still missing something?

based on https://lkml.org/lkml/2019/1/10/155:
I think different-values(r) is the same as r \ same-values, where 
same-values links all reads and writes that have the same value (e.g., 
"write 5 to x" and "read 5 from y").

With this in mind, I think the idea is to 1) forbid partial overlap, and 
using the different-values to 2) force them to provide the appropriate 
value.
This works because apparently srcu-lock is a read and srcu-unlock is a 
write, so in case of
int r1 = srcu-lock(&ss);   ==>  Read(&ss, x), r1 := x
...
srcu-unlock(&ss, r1);  ==> Write(&ss, r1), which is Write(&ss, x)

This guarantees that the read and write have the same value, hence 
different-values(...) will be the empty relation, and so no flag.

> > And if it is true, is there any need to change the memory model at this
> > point?
> >
> > (And if it's not true, that's most likely due to a bug in herd7.)
> Agreed, changes must wait for SRCU support in herd7.
>
> At which point something roughly similar to this might work?
>
> let srcu-rscs = return_value(Srcu-lock) ; (dep | rfi)* ;
> 		parameter(Srcu-unlock, 2)

I would like instead to be able to give names to the arguments of events 
that become dependency relations, like
   event srcu_unlock(struct srcu_struct *srcu_addr, struct srcu_token 
*srcu_data)
and then
    let srcu-rscs = [Srcu-lock] ; srcu_data ; (data; rfi)*

Personally I would also like to not have Linux-specific primitives in 
herd7/cat, that means that to understand LKMM you also need to 
understand the herd7 tool, and sounds quite brittle.

I would prefer if herd7 had some means to define custom 
events/instructions and uninterpreted relations between them, like

relation rf : [write] x [read]
[read] <= range(rf)
empty rf ;rf^-1 \ id

and some way to say
[read] ; .return <= rf^-1 ; .data
(where .return is a functional relation relating every event to the 
value it returns, and .xyz is the functional relation relating every 
event to the value of its argument xyz).