[PATCH v2 0/2] Documentation/litmus-tests: Add SRCU fastpath litmus tests

[PATCH v2 0/2] Documentation/litmus-tests: Add SRCU fastpath litmus tests

[Kunwu Chan]

This series moves the SRCU fastpath litmus tests from
tools/memory-model/litmus-tests/ to Documentation/litmus-tests/srcu/,
as suggested by Paul McKenney.

This series splits the two litmus-test patches from the earlier
[PATCH 00/13] srcu: Round out atomic SRCU support series:
https://lore.kernel.org/rcu/20260907075829.2073224-1-kunwu.chan@linux.dev/

The two tests verify the memory ordering required by the reader-free
synchronize_srcu_atomic() fastpath: the grace-period anchor must be
ordered before the per-CPU lock-counter scan.

Patch 1 verifies that the anchor-before-scan ordering forbids a reader
from being missed by the scan, while patch 2 shows that reversing the
ordering permits the problematic outcome.

Changes in v2:
- Use a filter for the P0 scan result, as suggested by Paul McKenney.

Tested with herd7 7.58 using linux-kernel.cfg.

Kunwu Chan (2):
  Documentation/litmus-tests: Add SRCU fastpath anchor-before-scan test
  Documentation/litmus-tests: Add SRCU fastpath scan-before-anchor test

 Documentation/litmus-tests/README             | 21 +++++++
 .../SRCU-fastpath-anchor-before-scan.litmus   | 57 +++++++++++++++++++
 .../SRCU-fastpath-scan-before-anchor.litmus   | 54 ++++++++++++++++++
 3 files changed, 132 insertions(+)
 create mode 100644 Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus
 create mode 100644 Documentation/litmus-tests/srcu/SRCU-fastpath-scan-before-anchor.litmus

-- 
2.43.0

[PATCH v2 1/2] Documentation/litmus-tests: Add SRCU fastpath anchor-before-scan test

[Kunwu Chan]

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 <kunwu.chan@gmail.com>
---
 .../SRCU-fastpath-anchor-before-scan.litmus   | 57 +++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus

diff --git a/Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus b/Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus
new file mode 100644
index 000000000000..efa9c0c4e047
--- /dev/null
+++ b/Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus
@@ -0,0 +1,57 @@
+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);
+}
+
+filter (0:r2 = 0)
+exists (2:r3 = 1 /\ 2:r4 = 0)
-- 
2.43.0

Re: [PATCH v2 1/2] Documentation/litmus-tests: Add SRCU fastpath anchor-before-scan test

[Akira Yokosawa]

Hi,

This is more of a knee-jerk reaction, but

On Sat, 12 Sep 2026 10:42:24 +0800, Kunwu Chan wrote:
> 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 <kunwu.chan@gmail.com>
> ---
>  .../SRCU-fastpath-anchor-before-scan.litmus   | 57 +++++++++++++++++++
>  1 file changed, 57 insertions(+)
>  create mode 100644 Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus
> 
> diff --git a/Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus b/Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus
> new file mode 100644
> index 000000000000..efa9c0c4e047
> --- /dev/null
> +++ b/Documentation/litmus-tests/srcu/SRCU-fastpath-anchor-before-scan.litmus
> @@ -0,0 +1,57 @@
> +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();
> +}

As P1() has a single memory access, this smp_mb() can't have any
effect on the outcome.

The one you see in "__srcu_read_lock() following the increment" is
there for other ordering requirements, I guess.

Ditto smp_mb() in P1() of 2/2.

Thanks, Akira

> +
> +P2(int *seq, int *ctr)
> +{
> +	int r3;
> +	int r4;
> +
> +	r3 = READ_ONCE(*ctr);
> +	smp_mb();
> +	r4 = READ_ONCE(*seq);
> +}
> +
> +filter (0:r2 = 0)
> +exists (2:r3 = 1 /\ 2:r4 = 0)
> -- 
> 2.43.0

[PATCH v2 2/2] Documentation/litmus-tests: Add SRCU fastpath scan-before-anchor test

[Kunwu Chan]

If the synchronize_srcu_atomic() fastpath instead places its lock scan
before the grace-period anchor, the scan can miss a reader whose
increment was already visible before the anchor.  That reader already
existed when the grace period started, so completing the grace period
without waiting for it would violate the SRCU grace-period guarantee.

This litmus test models the reversed ordering, with the lock scan placed
before the grace-period anchor.  "seq" models the grace-period anchor in
->srcu_gp_seq and "ctr" models the per-CPU ->srcu_ctrs[].srcu_locks
counter.  P0 scans the lock counter before writing the anchor, with an
smp_mb() between them.  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 same outcome is allowed with this ordering, and herd7 reports
"Sometimes".  The litmus-tests README is also updated to describe both
SRCU fastpath tests.

Tested with herd7 7.58 using linux-kernel.cfg.

Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
---
 Documentation/litmus-tests/README             | 21 ++++++++
 .../SRCU-fastpath-scan-before-anchor.litmus   | 54 +++++++++++++++++++
 2 files changed, 75 insertions(+)
 create mode 100644 Documentation/litmus-tests/srcu/SRCU-fastpath-scan-before-anchor.litmus

diff --git a/Documentation/litmus-tests/README b/Documentation/litmus-tests/README
index 6c666f3422ea..076a6edd70ff 100644
--- a/Documentation/litmus-tests/README
+++ b/Documentation/litmus-tests/README
@@ -78,3 +78,24 @@ RCU+sync+read.litmus
 RCU+sync+free.litmus
     Both the above litmus tests demonstrate the RCU grace period guarantee
     that an RCU read-side critical section can never span a grace period.
+
+
+SRCU (/srcu directory)
+--------------------
+
+SRCU-fastpath-anchor-before-scan.litmus
+        This models the synchronize_srcu_atomic() fastpath with the
+        grace-period anchor ordered before the lock-counter scan.  This
+        ordering prevents readers that existed before the grace period
+        from being missed by the scan.  See
+        SRCU-fastpath-scan-before-anchor.litmus for the reversed
+        ordering.
+
+SRCU-fastpath-scan-before-anchor.litmus
+        This models the synchronize_srcu_atomic() fastpath with the
+        lock-counter scan ordered before the grace-period anchor.  This
+        permits the scan to miss readers that existed before the grace
+        period, violating the SRCU grace-period guarantee.  See
+        SRCU-fastpath-anchor-before-scan.litmus for the opposite
+        ordering.
+
diff --git a/Documentation/litmus-tests/srcu/SRCU-fastpath-scan-before-anchor.litmus b/Documentation/litmus-tests/srcu/SRCU-fastpath-scan-before-anchor.litmus
new file mode 100644
index 000000000000..4d3bdb14d888
--- /dev/null
+++ b/Documentation/litmus-tests/srcu/SRCU-fastpath-scan-before-anchor.litmus
@@ -0,0 +1,54 @@
+C SRCU-fastpath-scan-before-anchor
+
+(*
+ * Result: Sometimes
+ *
+ * If the synchronize_srcu_atomic() fastpath instead places its lock
+ * scan before the grace-period anchor, the scan can miss a reader whose
+ * increment was already visible before the anchor.  That reader already
+ * existed when the grace period started, so completing the grace period
+ * without waiting for it would violate the SRCU grace-period guarantee.
+ *
+ * This litmus test models the reversed ordering, with the lock scan
+ * placed before the grace-period anchor.  "seq" models the grace-period
+ * anchor in ->srcu_gp_seq and "ctr" models the per-CPU
+ * ->srcu_ctrs[].srcu_locks counter.  P0 scans the lock counter before
+ * writing the anchor, with an smp_mb() between them.  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 same outcome is allowed with this ordering, and herd7 reports
+ * "Sometimes".  See SRCU-fastpath-anchor-before-scan.litmus for the
+ * opposite ordering, which forbids this outcome.
+ *)
+
+{}
+
+P0(int *seq, int *ctr)
+{
+	int r2;
+
+	r2 = READ_ONCE(*ctr);
+	smp_mb();
+	WRITE_ONCE(*seq, 1);
+}
+
+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);
+}
+
+filter (0:r2 = 0)
+exists (2:r3 = 1 /\ 2:r4 = 0)
-- 
2.43.0