vdso/gettimeofday: Add a helper to read the sequence lock of a time namespace aware clock

Currently there are three different open-coded variants of a time
namespace aware variant of vdso_read_begin(). They make the code hard to
read and introduce an inconsistency, as only the first copy uses
unlikely().

Split the code into a shared helper function.
Move that next to the definition of the regular vdso_read_begin(), so
that any future changes can be kept in sync easily.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260227-vdso-cleanups-v1-2-c848b4bc4850@linutronix.de

1 files changed, 22 insertions, 0 deletions

diff --git a/include/vdso/helpers.h b/include/vdso/helpers.h
index 1a5ee9d9052c..9ccf6b53ef50 100644
--- a/include/vdso/helpers.h
+++ b/include/vdso/helpers.h
@@ -18,6 +18,28 @@ static __always_inline u32 vdso_read_begin(const struct vdso_clock *vc)
 	return seq;
 }
 
+/*
+ * Variant of vdso_read_begin() to handle VDSO_CLOCKMODE_TIMENS.
+ *
+ * Time namespace enabled tasks have a special VVAR page installed which has
+ * vc->seq set to 1 and vc->clock_mode set to VDSO_CLOCKMODE_TIMENS. For non
+ * time namespace affected tasks this does not affect performance because if
+ * vc->seq is odd, i.e. a concurrent update is in progress the extra check for
+ * vc->clock_mode is just a few extra instructions while spin waiting for
+ * vc->seq to become even again.
+ */
+static __always_inline bool vdso_read_begin_timens(const struct vdso_clock *vc, u32 *seq)
+{
+	while (unlikely((*seq = READ_ONCE(vc->seq)) & 1)) {
+		if (IS_ENABLED(CONFIG_TIME_NS) && vc->clock_mode == VDSO_CLOCKMODE_TIMENS)
+			return true;
+		cpu_relax();
+	}
+	smp_rmb();
+
+	return false;
+}
+
 static __always_inline u32 vdso_read_retry(const struct vdso_clock *vc,
 					   u32 start)
 {