Messages in this thread | | | Date | Tue, 17 Jan 2023 07:14:16 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
| |
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)
| |