The following commit has been merged into the timers/core branch of tip:

Commit-ID:     d1189b155ecedbc136b1ed505f300d1184f9784c
Gitweb:        https://git.kernel.org/tip/d1189b155ecedbc136b1ed505f300d1184f9784c
Author:        Anna-Maria Behnsen <anna-maria@linutronix.de>
AuthorDate:    Wed, 09 Oct 2024 10:29:11 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 25 Oct 2024 16:41:13 +02:00

timekeeping: Rework change_clocksource() 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 change_clocksource() to use this scheme.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20241009-devel-anna-maria-b4-timers-ptp-timekeeping-v2-18-554456a44a15@linutronix.de

---
 kernel/time/timekeeping.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 7e865f0..f77782f 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1627,9 +1627,7 @@ static void __timekeeping_set_tai_offset(struct timekeeper *tk, s32 tai_offset)
  */
 static int change_clocksource(void *data)
 {
-	struct timekeeper *tk = &tk_core.timekeeper;
 	struct clocksource *new = data, *old = NULL;
-	unsigned long flags;
 
 	/*
 	 * If the clocksource is in a module, get a module reference.
@@ -1645,16 +1643,14 @@ static int change_clocksource(void *data)
 		return 0;
 	}
 
-	raw_spin_lock_irqsave(&tk_core.lock, flags);
-	write_seqcount_begin(&tk_core.seq);
-
-	timekeeping_forward_now(tk);
-	old = tk->tkr_mono.clock;
-	tk_setup_internals(tk, new);
-	timekeeping_update(&tk_core, tk, TK_UPDATE_ALL | TK_MIRROR);
+	scoped_guard (raw_spinlock_irqsave, &tk_core.lock) {
+		struct timekeeper *tks = &tk_core.shadow_timekeeper;
 
-	write_seqcount_end(&tk_core.seq);
-	raw_spin_unlock_irqrestore(&tk_core.lock, flags);
+		timekeeping_forward_now(tks);
+		old = tks->tkr_mono.clock;
+		tk_setup_internals(tks, new);
+		timekeeping_update_from_shadow(&tk_core, TK_UPDATE_ALL);
+	}
 
 	if (old) {
 		if (old->disable)