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

Paul E. McKenney
2023-01-17T20:06:46Z

On Tue, Jan 17, 2023 at 07:27:29PM +0100, Jonas Oberhauser wrote:
> 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:

Ah, thank you for the pointer!

> 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.

Might it instead match the entire event?

> > > 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).

I am glad that I asked rather than kneejerk filing a bug report. ;-)

Other thoughts?

Thanx, Paul