linux-kernel - [PATCH 04/14] vdso/helpers: Add helpers for seqlocks of single vdso

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Message-Id: <20250701-vdso-auxclock-v1-4-df7d9f87b9b8@linutronix.de>
Date: Tue, 01 Jul 2025 10:57:58 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
 Vincenzo Frascino <vincenzo.frascino@....com>, 
 Shuah Khan <shuah@...nel.org>, 
 Anna-Maria Behnsen <anna-maria@...utronix.de>, 
 Frederic Weisbecker <frederic@...nel.org>, John Stultz <jstultz@...gle.com>, 
 Stephen Boyd <sboyd@...nel.org>, Catalin Marinas <catalin.marinas@....com>, 
 Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>
Cc: linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org, 
 Richard Cochran <richardcochran@...il.com>, 
 Christopher Hall <christopher.s.hall@...el.com>, 
 Frederic Weisbecker <frederic@...nel.org>, 
 Anna-Maria Behnsen <anna-maria@...utronix.de>, 
 Miroslav Lichvar <mlichvar@...hat.com>, 
 Werner Abt <werner.abt@...nberg-usa.com>, 
 David Woodhouse <dwmw2@...radead.org>, Stephen Boyd <sboyd@...nel.org>, 
 Kurt Kanzenbach <kurt@...utronix.de>, Nam Cao <namcao@...utronix.de>, 
 Antoine Tenart <atenart@...nel.org>, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH 04/14] vdso/helpers: Add helpers for seqlocks of single
 vdso_clock

Auxiliary clocks will have their vDSO data in a dedicated 'struct vdso_clock',
which needs to be synchronized independently.

Add a helper to synchronize a single vDSO clock.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 include/vdso/helpers.h | 40 +++++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/include/vdso/helpers.h b/include/vdso/helpers.h
index 0a98fed550ba66a84a620fbbd6aee3e3029b4772..a5679f5efdfdcaaf6efd5f4a317d1f132c3dc617 100644
--- a/include/vdso/helpers.h
+++ b/include/vdso/helpers.h
@@ -28,32 +28,46 @@ static __always_inline u32 vdso_read_retry(const struct vdso_clock *vc,
 	return seq != start;
 }
 
-static __always_inline void vdso_write_begin(struct vdso_time_data *vd)
+static __always_inline void vdso_write_begin_clock(struct vdso_clock *vc, bool last)
 {
-	struct vdso_clock *vc = vd->clock_data;
-
 	/*
 	 * WRITE_ONCE() is required otherwise the compiler can validly tear
-	 * updates to vd[x].seq and it is possible that the value seen by the
+	 * updates to vc->seq and it is possible that the value seen by the
 	 * reader is inconsistent.
 	 */
-	WRITE_ONCE(vc[CS_HRES_COARSE].seq, vc[CS_HRES_COARSE].seq + 1);
-	WRITE_ONCE(vc[CS_RAW].seq, vc[CS_RAW].seq + 1);
-	smp_wmb();
+	WRITE_ONCE(vc->seq, vc->seq + 1);
+
+	if (last)
+		smp_wmb();
 }
 
-static __always_inline void vdso_write_end(struct vdso_time_data *vd)
+static __always_inline void vdso_write_end_clock(struct vdso_clock *vc, bool first)
 {
-	struct vdso_clock *vc = vd->clock_data;
+	if (first)
+		smp_wmb();
 
-	smp_wmb();
 	/*
 	 * WRITE_ONCE() is required otherwise the compiler can validly tear
-	 * updates to vd[x].seq and it is possible that the value seen by the
+	 * updates to vc->seq and it is possible that the value seen by the
 	 * reader is inconsistent.
 	 */
-	WRITE_ONCE(vc[CS_HRES_COARSE].seq, vc[CS_HRES_COARSE].seq + 1);
-	WRITE_ONCE(vc[CS_RAW].seq, vc[CS_RAW].seq + 1);
+	WRITE_ONCE(vc->seq, vc->seq + 1);
+}
+
+static __always_inline void vdso_write_begin(struct vdso_time_data *vd)
+{
+	struct vdso_clock *vc = vd->clock_data;
+
+	vdso_write_begin_clock(&vc[CS_HRES_COARSE], false);
+	vdso_write_begin_clock(&vc[CS_RAW], true);
+}
+
+static __always_inline void vdso_write_end(struct vdso_time_data *vd)
+{
+	struct vdso_clock *vc = vd->clock_data;
+
+	vdso_write_end_clock(&vc[CS_HRES_COARSE], true);
+	vdso_write_end_clock(&vc[CS_RAW], false);
 }
 
 #endif /* !__ASSEMBLY__ */

-- 
2.50.0