[PATCH 2/2] lib/tests: errseq: add a concurrent check_and_advance test

[Shashank Mohan Jain <jain.sm@gmail.com>]

Add a test case that races errseq_check_and_advance() on one CPU
against errseq_set() of a different error on another CPU, then records
the first error again and lets a second cursor consume it. The second
-EIO is recorded after the first cursor's check returned, so that
cursor's next check must report an error, whichever way the race with
-ENOSPC went.

Without the previous commit 1-6% of 2 million rounds lose the error
on UML with 4 CPUs (seccomp=on ncpus=4); with it none do. The case is
skipped on a single CPU and stops after 5 seconds.

Assisted-by: LLM
Signed-off-by: Shashank Mohan Jain <jain.sm@gmail.com>
---
Needs the errseq KUnit suite from commit b52f5c1605f2 ("lib/tests: add
KUnit tests for errseq"), which is in mm-nonmm-unstable. The case is
skipped unless the kernel has at least two CPUs (on UML:
--kernel_args seccomp=on --kernel_args ncpus=4).

 lib/tests/errseq_kunit.c | 110 +++++++++++++++++++++++++++++++++++++--
 1 file changed, 105 insertions(+), 5 deletions(-)

diff --git a/lib/tests/errseq_kunit.c b/lib/tests/errseq_kunit.c
index 8f39ebc4a248..5ebc411ea749 100644
--- a/lib/tests/errseq_kunit.c
+++ b/lib/tests/errseq_kunit.c
@@ -2,21 +2,26 @@
 /*
  * KUnit tests for the errseq_t error-tracking infrastructure.
  *
- * These exercise the documented single-threaded semantics of the errseq
- * API (see Documentation/core-api/errseq.rst and lib/errseq.c): error
+ * Most cases exercise the documented single-threaded semantics of the
+ * errseq API (see Documentation/core-api/errseq.rst and lib/errseq.c): error
  * recording and overwriting, the "seen" handoff between errseq_sample()
  * and errseq_check_and_advance(), and the re-reporting of an error that
  * is recorded again after it has been seen.
  *
- * The lockless properties of errseq_t under concurrent updates are
- * outside the scope of these deterministic tests, as is the WARN path
- * for invalid error values.
+ * The WARN path for invalid error values is not tested.  The last case
+ * races errseq_set() against errseq_check_and_advance() on another CPU.
  */
 #include <kunit/test.h>
 
+#include <linux/atomic.h>
+#include <linux/cpumask.h>
 #include <linux/err.h>
 #include <linux/errno.h>
 #include <linux/errseq.h>
+#include <linux/jiffies.h>
+#include <linux/kthread.h>
+#include <linux/random.h>
+#include <linux/sched.h>
 
 /*
  * A zeroed errseq_t is the "no error has ever occurred" epoch: it
@@ -209,6 +214,100 @@ static void errseq_test_two_subscribers_independent(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, errseq_check_and_advance(&eseq, &cursor_b), 0);
 }
 
+/*
+ * errseq_check_and_advance() racing with errseq_set() on another CPU.
+ *
+ * Each round, cursor @f checks an unseen -EIO while another CPU records
+ * -ENOSPC.  Afterwards -EIO is recorded again and cursor @g checks.  The
+ * second -EIO is recorded after @f's check returned, so @f's next check
+ * must report an error, whichever way the race with -ENOSPC went.
+ */
+struct errseq_race {
+	errseq_t eseq;
+	atomic_t go;		/* round the writer should run */
+	atomic_t done;		/* last round the writer completed */
+	unsigned int wdelay;
+};
+
+static void errseq_race_spin(unsigned int n)
+{
+	while (n--)
+		cpu_relax();
+}
+
+static int errseq_race_writer(void *data)
+{
+	struct errseq_race *r = data;
+	int seen = 0;
+
+	while (!kthread_should_stop()) {
+		int round = atomic_read_acquire(&r->go);
+
+		if (round == seen) {
+			cond_resched();
+			continue;
+		}
+		seen = round;
+		errseq_race_spin(READ_ONCE(r->wdelay));
+		errseq_set(&r->eseq, -ENOSPC);
+		atomic_set_release(&r->done, round);
+	}
+	return 0;
+}
+
+#define ERRSEQ_RACE_ROUNDS	2000000
+#define ERRSEQ_RACE_SECONDS	5
+
+static void errseq_test_check_and_advance_race(struct kunit *test)
+{
+	struct errseq_race *r;
+	struct task_struct *writer;
+	unsigned long deadline;
+	int round, unreported = 0, missed = 0;
+
+	if (num_online_cpus() < 2)
+		kunit_skip(test, "needs at least two CPUs");
+
+	r = kunit_kzalloc(test, sizeof(*r), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_NULL(test, r);
+
+	writer = kthread_run(errseq_race_writer, r, "errseq_race");
+	KUNIT_ASSERT_FALSE(test, IS_ERR(writer));
+
+	deadline = jiffies + ERRSEQ_RACE_SECONDS * HZ;
+	for (round = 1; round <= ERRSEQ_RACE_ROUNDS; round++) {
+		errseq_t f = 0, g = 0;
+
+		WRITE_ONCE(r->eseq, 0);
+		errseq_set(&r->eseq, -EIO);
+		WRITE_ONCE(r->wdelay, get_random_u32_below(64));
+		atomic_set_release(&r->go, round);
+
+		errseq_race_spin(get_random_u32_below(64));
+		if (!errseq_check_and_advance(&r->eseq, &f))
+			unreported++;
+
+		while (atomic_read_acquire(&r->done) != round)
+			cond_resched();
+
+		/* Recorded after f's check returned: f must hear about it. */
+		errseq_set(&r->eseq, -EIO);
+		errseq_check_and_advance(&r->eseq, &g);
+		if (!errseq_check_and_advance(&r->eseq, &f))
+			missed++;
+
+		if (time_after(jiffies, deadline))
+			break;
+		if (!(round & 1023))
+			cond_resched();
+	}
+	kthread_stop(writer);
+
+	kunit_info(test, "%d rounds, %d missed errors\n", round - 1, missed);
+	KUNIT_EXPECT_EQ(test, unreported, 0);
+	KUNIT_EXPECT_EQ(test, missed, 0);
+}
+
 static struct kunit_case errseq_test_cases[] = {
 	KUNIT_CASE(errseq_test_zero_epoch_reports_no_error),
 	KUNIT_CASE(errseq_test_set_records_error),
@@ -223,6 +322,7 @@ static struct kunit_case errseq_test_cases[] = {
 	KUNIT_CASE(errseq_test_repeat_error_visible_to_all_cursors),
 	KUNIT_CASE(errseq_test_advance_stable_when_unchanged),
 	KUNIT_CASE(errseq_test_two_subscribers_independent),
+	KUNIT_CASE_SLOW(errseq_test_check_and_advance_race),
 	{}
 };
 
-- 
2.43.0