On Tue, 8 Sep 2026 16:58:31 -0700 "Paul E. McKenney" <[email protected]> wrote:
> On Mon, Sep 07, 2026 at 03:58:17PM +0800, Kunwu Chan wrote: > > From: Kunwu Chan <[email protected]> > > > > synchronize_srcu_atomic() may end its grace period immediately when > > its scan of the per-CPU lock counters finds no readers. Correctness > > requires the grace-period anchor written by srcu_gp_start() to precede > > the smp_mb() ordering the lock scan. This ordering ensures that any > > reader whose lock increment is missed by the scan cannot have > > incremented its lock counter before the grace-period anchor, and > > therefore cannot be a pre-existing reader of this grace period. > > > > This litmus test models the key ordering between the grace-period > > anchor and the lock counter scan, where "seq" models the > > grace-period anchor in ->srcu_gp_seq and "ctr" models the per-CPU > > ->srcu_ctrs[].srcu_locks counter. P0 writes the anchor before the > > smp_mb() and the lock scan. P1 models the reader-side counter > > increment, with the smp_mb() of __srcu_read_lock() following the > > increment. P2 models an observer that sees the reader's increment > > before seeing the anchor. > > > > The outcome is forbidden by LKMM, and herd7 reports "Never". See > > SRCU-fastpath-scan-before-anchor.litmus for the reversed ordering, > > which permits this outcome. > > > > Tested with herd7 7.58 using linux-kernel.cfg. > > > > Signed-off-by: Kunwu Chan <[email protected]> > > Litmus tests! Very nice!!! > > Could you please put both of these in Documentation/litmus-tests, in a > new "srcu" subdirectory? > > One thing for your consideration is use of the "filter" clause for the > first term of your "exists" clause. Not a big deal at all for this small > of a litmus test, but the idea is that this litmus test only cares about > the 0:r2=0 case: If that condition does not hold, then P0() and P1() > aren't the beginning and end of a valid SRCU read-side critical section. > > Use of the "filter" allows herd7 to abandon a given execution early, > so it is a big deal for larger litmus tests. > > Again, what you have is fine (or will be when moved to the other > directory), just pointing out the additional feature. > > If you would like to see a use case, please see: > > Documentation/litmus-tests/locking/RM-fixed.litmus Thanks, Paul. I’ll move both tests to Documentation/litmus-tests/srcu/ and use a "filter" clause for the first test as suggested. I’ll send the two litmus tests as a separate follow-up series, so this won’t hold up the current atomic SRCU series. I’d also like to continue maintaining the SRCU litmus tests as they evolve. If you think a MAINTAINERS entry for the SRCU litmus tests would be appropriate, I’d be happy to prepare that as well. Thanks, KunWu > > Thanx, Paul > > > --- > > .../SRCU-fastpath-anchor-before-scan.litmus | 56 +++++++++++++++++++ > > 1 file changed, 56 insertions(+) > > create mode 100644 > > tools/memory-model/litmus-tests/SRCU-fastpath-anchor-before-scan.litmus > > > > diff --git > > a/tools/memory-model/litmus-tests/SRCU-fastpath-anchor-before-scan.litmus > > b/tools/memory-model/litmus-tests/SRCU-fastpath-anchor-before-scan.litmus > > new file mode 100644 > > index 000000000000..8200a75e15ef > > --- /dev/null > > +++ > > b/tools/memory-model/litmus-tests/SRCU-fastpath-anchor-before-scan.litmus > > @@ -0,0 +1,56 @@ > > +C SRCU-fastpath-anchor-before-scan > > + > > +(* > > + * Result: Never > > + * > > + * The synchronize_srcu_atomic() fastpath may end its grace period > > + * immediately when its scan of the per-CPU lock counters finds no > > + * readers. Correctness requires the grace-period anchor written by > > + * srcu_gp_start() to precede the smp_mb() ordering the lock scan. > > + * This ordering ensures that any reader whose lock increment is missed > > + * by the scan cannot have incremented its lock counter before the > > + * grace-period anchor, and therefore cannot be a pre-existing reader > > + * of this grace period. > > + * > > + * This litmus test models the key ordering between the grace-period > > + * anchor and the lock counter scan, where "seq" models the > > + * grace-period anchor in ->srcu_gp_seq and "ctr" models the per-CPU > > + * ->srcu_ctrs[].srcu_locks counter. P0 writes the anchor before the > > + * smp_mb() and the lock scan. P1 models the reader-side counter > > + * increment, with the smp_mb() of __srcu_read_lock() following the > > + * increment. P2 models an observer that sees the reader's increment > > + * before seeing the anchor. > > + * > > + * The outcome is forbidden by LKMM, and herd7 reports "Never". See > > + * SRCU-fastpath-scan-before-anchor.litmus for the reversed ordering, > > + * which permits this outcome. > > + *) > > + > > +{} > > + > > +P0(int *seq, int *ctr) > > +{ > > + int r2; > > + > > + WRITE_ONCE(*seq, 1); > > + smp_mb(); > > + r2 = READ_ONCE(*ctr); > > +} > > + > > +P1(int *ctr) > > +{ > > + WRITE_ONCE(*ctr, 1); > > + smp_mb(); > > +} > > + > > +P2(int *seq, int *ctr) > > +{ > > + int r3; > > + int r4; > > + > > + r3 = READ_ONCE(*ctr); > > + smp_mb(); > > + r4 = READ_ONCE(*seq); > > +} > > + > > +exists (0:r2 = 0 /\ 2:r3 = 1 /\ 2:r4 = 0) > > -- > > 2.43.0 > > > Sent using hkml (https://github.com/sjp38/hackermail)
