[PATCH v2 2/3] errseq: Add mechanism to snapshot errseq_counter and check snapshot

[Sargun Dhillon <sargun@sargun.me>]

This adds the function errseq_counter_sample to allow for "subscribers"
to take point-in-time snapshots of the errseq_counter, and store the
counter + errseq_t.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
---
 include/linux/errseq.h |  4 ++++
 lib/errseq.c           | 51 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/include/linux/errseq.h b/include/linux/errseq.h
index 35818c484290..8998df499a3b 100644
--- a/include/linux/errseq.h
+++ b/include/linux/errseq.h
@@ -25,4 +25,8 @@ errseq_t errseq_set(errseq_t *eseq, int err);
 errseq_t errseq_sample(errseq_t *eseq);
 int errseq_check(errseq_t *eseq, errseq_t since);
 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since);
+void errseq_counter_sample(errseq_t *dst_errseq, int *dst_errors,
+			   struct errseq_counter *counter);
+int errseq_counter_check(struct errseq_counter *counter, errseq_t errseq_since,
+			 int errors_since);
 #endif
diff --git a/lib/errseq.c b/lib/errseq.c
index d555e7fc18d2..98fcfafa3d97 100644
--- a/lib/errseq.c
+++ b/lib/errseq.c
@@ -246,3 +246,54 @@ int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
 	return err;
 }
 EXPORT_SYMBOL(errseq_check_and_advance);
+
+/**
+ * errseq_counter_sample() - Grab the current errseq_counter value
+ * @dst_errseq: The errseq_t to copy to
+ * @dst_errors: The destination overflow to copy to
+ * @counter: The errseq_counter to copy from
+ *
+ * Grabs a point in time sample of the errseq_counter for latter comparison
+ */
+void errseq_counter_sample(errseq_t *dst_errseq, int *dst_errors,
+			   struct errseq_counter *counter)
+{
+	errseq_t cur;
+
+	do {
+		cur = READ_ONCE(counter->errseq);
+		*dst_errors = atomic_read(&counter->errors);
+	} while (cur != READ_ONCE(counter->errseq));
+
+	/* Clear the seen bit to make checking later easier */
+	*dst_errseq = cur & ~ERRSEQ_SEEN;
+}
+EXPORT_SYMBOL(errseq_counter_sample);
+
+/**
+ * errseq_counter_check() - Has an error occurred since the sample
+ * @counter: The errseq_counter from which to check.
+ * @errseq_since: The errseq_t sampled with errseq_counter_sample to check
+ * @errors_since: The errors sampled with errseq_counter_sample to check
+ *
+ * Returns: The latest error set in the errseq_t or 0 if there have been none.
+ */
+int errseq_counter_check(struct errseq_counter *counter, errseq_t errseq_since,
+			 int errors_since)
+{
+	errseq_t cur_errseq;
+	int cur_errors;
+
+	cur_errors = atomic_read(&counter->errors);
+	/* To match the barrier in errseq_counter_set */
+	smp_rmb();
+
+	/* Clear / ignore the seen bit as we do at sample time */
+	cur_errseq = READ_ONCE(counter->errseq) & ~ERRSEQ_SEEN;
+
+	if (cur_errseq == errseq_since && errors_since == cur_errors)
+		return 0;
+
+	return -(cur_errseq & MAX_ERRNO);
+}
+EXPORT_SYMBOL(errseq_counter_check);
-- 
2.25.1