From: Anna-Maria Behnsen <anna-maria@linutronix.de>

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>
---
 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 051041e92e54..6d21e9bf5c35 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1596,9 +1596,7 @@ 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.
@@ -1614,16 +1612,14 @@ 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 *tk = &tk_core.shadow_timekeeper;
 
-	write_seqcount_end(&tk_core.seq);
-	raw_spin_unlock_irqrestore(&tk_core.lock, flags);
+		timekeeping_forward_now(tk);
+		old = tk->tkr_mono.clock;
+		tk_setup_internals(tk, new);
+		timekeeping_update_staged(&tk_core, TK_UPDATE_ALL);
+	}
 
 	if (old) {
 		if (old->disable)

-- 
2.39.5

On Wed, Oct 9, 2024 at 1:29 AM Anna-Maria Behnsen
<anna-maria@linutronix.de> wrote:
>
> From: Anna-Maria Behnsen <anna-maria@linutronix.de>
>
> 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>
> ---
>  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 051041e92e54..6d21e9bf5c35 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1596,9 +1596,7 @@ 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.
> @@ -1614,16 +1612,14 @@ 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 *tk = &tk_core.shadow_timekeeper;

Same nit: tk_shadow maybe?

Acked-by: John Stultz <jstultz@google.com>