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

Alan Stern
2023-01-17T15:56:43Z

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?

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.)

Alan