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

Paul E. McKenney
2023-01-17T17:56:41Z

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?

> 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)

Given an Srcu-down and an Srcu-up:

let srcu-rscs = ( return_value(Srcu-lock) ; (dep | rfi)* ;
parameter(Srcu-unlock, 2) ) |
( return_value(Srcu-down) ; (dep | rf)* ;
parameter(Srcu-up, 2) )

Seem reasonable, or am I missing yet something else?

Thanx, Paul