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

Paul E. McKenney
2023-01-17T15:14:28Z

On Tue, Jan 17, 2023 at 12:46:28PM +0100, Andrea Parri wrote:
> On Mon, Jan 16, 2023 at 02:13:57PM -0800, Paul E. McKenney wrote:
> > On Mon, Jan 16, 2023 at 02:20:57PM -0500, Alan Stern wrote:
> > > On Mon, Jan 16, 2023 at 11:06:52AM -0800, Paul E. McKenney wrote:
> > > > On Mon, Jan 16, 2023 at 01:11:41PM -0500, Alan Stern wrote:
> > >
> > > > > Why do you want to prohibit nesting? Why would that be a better
> > > > > approximation?
> > > >
> > > > Because the current LKMM gives wrong answers for nested critical
> > > > sections.
> > >
> > > I don't agree. Or at least, it depends on whose definition of "nested
> > > critical sections" you adopt.
> >
> > Fair point, and I have therefore updated the test's header comment
> > to read as follows:
> >
> > (*
> > * Result: Sometimes
> > *
> > * This demonstrates non-nested overlapping of SRCU read-side critical
> > * sections. Unlike RCU, SRCU critical sections do not unconditionally
> > * nest.
> > *)
> >
> > > > For example, for the litmus test shown below, mainline
> > > > LKMM will incorrectly report "Never". The two SRCU read-side critical
> > > > sections are independent, so the fact that P1()'s synchronize_srcu() is
> > > > guaranteed to wait for the first on to complete says nothing about the
> > > > second having completed. Therefore, in Linux-kernel SRCU, the "exists"
> > > > clause could be satisfied.
> > > >
> > > > In contrast, the proposed change flags this as having nesting.
> > >
> > > In fact, this litmus test has overlapping critical sections, not nested
> > > ones. But the current LKML incorrectly _thinks_ they are nested,
> > > because it matches each lock with the first unmatched unlock.
> > >
> > > If you write a litmus test that has properly nested (not overlapping!)
> > > read-side critical sections, the current LKMM will match the locks and
> > > unlocks correctly and will give the right answer.
> > >
> > > So what you really want to do is rule out overlapping, not nesting. But
> > > I guess there's no way to do one without the other.
> >
> > None that I could see!
>
> 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. ;-)

Thanx, Paul

> Thanks,
> Andrea
>
>
> >
> > Thanx, Paul
> >
> > > Alan
> > >
> > > > Thaxn, Paul
> > > >
> > > > ------------------------------------------------------------------------
> > > >
> > > > C C-srcu-nest-5
> > > >
> > > > (*
> > > > * Result: Sometimes
> > > > *
> > > > * This demonstrates non-nesting of SRCU read-side critical sections.
> > > > * Unlike RCU, SRCU critical sections do not nest.
> > > > *)
> > > >
> > > > {}
> > > >
> > > > P0(int *x, int *y, struct srcu_struct *s1)
> > > > {
> > > > int r1;
> > > > int r2;
> > > > int r3;
> > > > int r4;
> > > >
> > > > r3 = srcu_read_lock(s1);
> > > > r2 = READ_ONCE(*y);
> > > > r4 = srcu_read_lock(s1);
> > > > srcu_read_unlock(s1, r3);
> > > > r1 = READ_ONCE(*x);
> > > > srcu_read_unlock(s1, r4);
> > > > }
> > > >
> > > > P1(int *x, int *y, struct srcu_struct *s1)
> > > > {
> > > > WRITE_ONCE(*y, 1);
> > > > synchronize_srcu(s1);
> > > > WRITE_ONCE(*x, 1);
> > > > }
> > > >
> > > > locations [0:r1]
> > > > exists (0:r1=1 /\ 0:r2=0)