[RFC PATCH 1/2] errseq: add a new errseq_fetch helper

[Jeff Layton <jlayton@kernel.org>]

We need to be able to fetch the current value for nfsd's write
verifiers, but in the event that the value hasn't been SEEN, we don't
want to return a 0. Resurrect the old errseq_sample routine (before
Willy fixed it) and rechristen it as errseq_fetch.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 include/linux/errseq.h |  1 +
 lib/errseq.c           | 33 ++++++++++++++++++++++++++++++++-
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/linux/errseq.h b/include/linux/errseq.h
index fc2777770768..13a731236c9b 100644
--- a/include/linux/errseq.h
+++ b/include/linux/errseq.h
@@ -9,6 +9,7 @@ typedef u32	errseq_t;
 
 errseq_t errseq_set(errseq_t *eseq, int err);
 errseq_t errseq_sample(errseq_t *eseq);
+errseq_t errseq_fetch(errseq_t *eseq);
 int errseq_check(errseq_t *eseq, errseq_t since);
 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since);
 #endif
diff --git a/lib/errseq.c b/lib/errseq.c
index 93e9b94358dc..f243b7dc36f5 100644
--- a/lib/errseq.c
+++ b/lib/errseq.c
@@ -109,7 +109,7 @@ errseq_t errseq_set(errseq_t *eseq, int err)
 EXPORT_SYMBOL(errseq_set);
 
 /**
- * errseq_sample() - Grab current errseq_t value.
+ * errseq_sample() - Grab current errseq_t value (or 0 if it's unseen)
  * @eseq: Pointer to errseq_t to be sampled.
  *
  * This function allows callers to initialise their errseq_t variable.
@@ -131,6 +131,37 @@ errseq_t errseq_sample(errseq_t *eseq)
 }
 EXPORT_SYMBOL(errseq_sample);
 
+/**
+ * errseq_fetch() - Grab current errseq_t value
+ * @eseq: Pointer to errseq_t to be sampled.
+ *
+ * This function grabs the current errseq_t value, and returns it,
+ * and marks the value as SEEN. This differs from a "sample" in that we
+ * grab the actual value even if it has not been seen before (instead of
+ * returning 0 in that case).
+ *
+ * Context: Any context.
+ * Return: The current errseq value.
+ */
+errseq_t errseq_fetch(errseq_t *eseq)
+{
+	errseq_t old = READ_ONCE(*eseq);
+	errseq_t new = old;
+
+	/*
+	 * For the common case of no errors ever having been set, we can skip
+	 * marking the SEEN bit. Once an error has been set, the value will
+	 * never go back to zero.
+	 */
+	if (old != 0) {
+		new |= ERRSEQ_SEEN;
+		if (old != new)
+			cmpxchg(eseq, old, new);
+	}
+	return new;
+}
+EXPORT_SYMBOL(errseq_fetch);
+
 /**
  * errseq_check() - Has an error occurred since a particular sample point?
  * @eseq: Pointer to errseq_t value to be checked.
-- 
2.39.1