Messages in this thread | | | Date | Thu, 26 Jan 2023 15:21:48 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2 1/2] tools/memory-model: Unify UNLOCK+LOCK pairings to po-unlock-lock-po |
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On Thu, Jan 26, 2023 at 12:08:28PM -0800, Paul E. McKenney wrote: > On Thu, Jan 26, 2023 at 11:36:51AM -0500, Alan Stern wrote: > > On Thu, Jan 26, 2023 at 02:46:03PM +0100, Jonas Oberhauser wrote: > > > LKMM uses two relations for talking about UNLOCK+LOCK pairings: > > > > > > 1) po-unlock-lock-po, which handles UNLOCK+LOCK pairings > > > on the same CPU or immediate lock handovers on the same > > > lock variable > > > > > > 2) po;[UL];(co|po);[LKW];po, which handles UNLOCK+LOCK pairs > > > literally as described in rcupdate.h#L1002, i.e., even > > > after a sequence of handovers on the same lock variable. > > > > > > The latter relation is used only once, to provide the guarantee > > > defined in rcupdate.h#L1002 by smp_mb__after_unlock_lock(), which > > > makes any UNLOCK+LOCK pair followed by the fence behave like a full > > > barrier. > > > > > > This patch drops this use in favor of using po-unlock-lock-po > > > everywhere, which unifies the way the model talks about UNLOCK+LOCK > > > pairings. At first glance this seems to weaken the guarantee given > > > by LKMM: When considering a long sequence of lock handovers > > > such as below, where P0 hands the lock to P1, which hands it to P2, > > > which finally executes such an after_unlock_lock fence, the mb > > > relation currently links any stores in the critical section of P0 > > > to instructions P2 executes after its fence, but not so after the > > > patch. > > > > > > P0(int *x, int *y, spinlock_t *mylock) > > > { > > > spin_lock(mylock); > > > WRITE_ONCE(*x, 2); > > > spin_unlock(mylock); > > > WRITE_ONCE(*y, 1); > > > } > > > > > > P1(int *y, int *z, spinlock_t *mylock) > > > { > > > int r0 = READ_ONCE(*y); // reads 1 > > > spin_lock(mylock); > > > spin_unlock(mylock); > > > WRITE_ONCE(*z,1); > > > } > > > > > > P2(int *z, int *d, spinlock_t *mylock) > > > { > > > int r1 = READ_ONCE(*z); // reads 1 > > > spin_lock(mylock); > > > spin_unlock(mylock); > > > smp_mb__after_unlock_lock(); > > > WRITE_ONCE(*d,1); > > > } > > > > > > P3(int *x, int *d) > > > { > > > WRITE_ONCE(*d,2); > > > smp_mb(); > > > WRITE_ONCE(*x,1); > > > } > > > > > > exists (1:r0=1 /\ 2:r1=1 /\ x=2 /\ d=2) > > > > > > Nevertheless, the ordering guarantee given in rcupdate.h is actually > > > not weakened. This is because the unlock operations along the > > > sequence of handovers are A-cumulative fences. They ensure that any > > > stores that propagate to the CPU performing the first unlock > > > operation in the sequence must also propagate to every CPU that > > > performs a subsequent lock operation in the sequence. Therefore any > > > such stores will also be ordered correctly by the fence even if only > > > the final handover is considered a full barrier. > > > > > > Indeed this patch does not affect the behaviors allowed by LKMM at > > > all. The mb relation is used to define ordering through: > > > 1) mb/.../ppo/hb, where the ordering is subsumed by hb+ where the > > > lock-release, rfe, and unlock-acquire orderings each provide hb > > > 2) mb/strong-fence/cumul-fence/prop, where the rfe and A-cumulative > > > lock-release orderings simply add more fine-grained cumul-fence > > > edges to substitute a single strong-fence edge provided by a long > > > lock handover sequence > > > 3) mb/strong-fence/pb and various similar uses in the definition of > > > data races, where as discussed above any long handover sequence > > > can be turned into a sequence of cumul-fence edges that provide > > > the same ordering. > > > > > > Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com> > > > --- > > > > Reviewed-by: Alan Stern <stern@rowland.harvard.edu> > > A quick spot check showed no change in performance, so thank you both! > > Queued for review and further testing.
And testing on https://github.com/paulmckrcu/litmus for litmus tests up to ten processes and allowing 10 minutes per litmus test got this:
Exact output matches: 5208 !!! Timed out: 38 !!! Unknown primitive: 7
This test compared output with and without your patch.
For the tests with a Results clause, these failed:
manual/kernel/C-srcu-nest-7.litmus manual/kernel/C-srcu-nest-5.litmus manual/kernel/C-srcu-nest-6.litmus manual/kernel/C-srcu-nest-8.litmus
But all of these will continue to fail until we get Alan's new-age SRCU patch applied.
Therefore, this constitutes success, so good show thus far on testing! ;-)
Also, I am going to be pushing the scripts I use to mainline. They might not be perfect, but they will be quite useful for this sort of change to the memory model.
Thanx, Paul
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