linux-kernel - [PATCH 15/24] timekeeping: Rework do_settimeofday64() to use shadow

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Message-Id: <20240911-devel-anna-maria-b4-timers-ptp-timekeeping-v1-15-f7cae09e25d6@linutronix.de>
Date: Wed, 11 Sep 2024 15:29:59 +0200
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: John Stultz <jstultz@...gle.com>, 
 Frederic Weisbecker <frederic@...nel.org>, 
 Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
 Miroslav Lichvar <mlichvar@...hat.com>, 
 Richard Cochran <richardcochran@...il.com>, 
 Christopher S Hall <christopher.s.hall@...el.com>, 
 Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: [PATCH 15/24] timekeeping: Rework do_settimeofday64() to use
 shadow_timekeeper

Updates of the timekeeper can be done by operating on the shadow timekeeper
and afterwards copying the result into the real timekeeper. This has the
advantage, that the sequence count write protected region is kept as small
as possible.

Convert do_settimeofday64() to use this scheme.

That allows to use a scoped_guard() for locking the timekeeper lock as the
usage of the shadow timekeeper allows a rollback in the error case instead
of the full timekeeper update of the original code.

Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
---
 kernel/time/timekeeping.c | 42 ++++++++++++++++--------------------------
 1 file changed, 16 insertions(+), 26 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 345117ff665d..efcfb0da351e 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1479,45 +1479,35 @@ EXPORT_SYMBOL_GPL(timekeeping_clocksource_has_base);
  */
 int do_settimeofday64(const struct timespec64 *ts)
 {
-	struct timekeeper *tk = &tk_core.timekeeper;
 	struct timespec64 ts_delta, xt;
-	unsigned long flags;
-	int ret = 0;
 
 	if (!timespec64_valid_settod(ts))
 		return -EINVAL;
 
-	raw_spin_lock_irqsave(&tk_core.lock, flags);
-	write_seqcount_begin(&tk_core.seq);
-
-	timekeeping_forward_now(tk);
-
-	xt = tk_xtime(tk);
-	ts_delta = timespec64_sub(*ts, xt);
+	scoped_guard (raw_spinlock_irqsave, &tk_core.lock) {
+		struct timekeeper *tk = &tk_core.shadow_timekeeper;
 
-	if (timespec64_compare(&tk->wall_to_monotonic, &ts_delta) > 0) {
-		ret = -EINVAL;
-		goto out;
-	}
+		timekeeping_forward_now(tk);
 
-	tk_set_wall_to_mono(tk, timespec64_sub(tk->wall_to_monotonic, ts_delta));
+		xt = tk_xtime(tk);
+		ts_delta = timespec64_sub(*ts, xt);
 
-	tk_set_xtime(tk, ts);
-out:
-	timekeeping_update(&tk_core, tk, TK_UPDATE_ALL | TK_MIRROR);
+		if (timespec64_compare(&tk->wall_to_monotonic, &ts_delta) > 0) {
+			timekeeping_restore_shadow(&tk_core);
+			return -EINVAL;
+		}
 
-	write_seqcount_end(&tk_core.seq);
-	raw_spin_unlock_irqrestore(&tk_core.lock, flags);
+		tk_set_wall_to_mono(tk, timespec64_sub(tk->wall_to_monotonic, ts_delta));
+		tk_set_xtime(tk, ts);
+		timekeeping_update_staged(&tk_core, TK_UPDATE_ALL);
+	}
 
 	/* Signal hrtimers about time change */
 	clock_was_set(CLOCK_SET_WALL);
 
-	if (!ret) {
-		audit_tk_injoffset(ts_delta);
-		add_device_randomness(ts, sizeof(*ts));
-	}
-
-	return ret;
+	audit_tk_injoffset(ts_delta);
+	add_device_randomness(ts, sizeof(*ts));
+	return 0;
 }
 EXPORT_SYMBOL(do_settimeofday64);
 

-- 
2.39.2