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

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 7e865f05793596b467f4cea59a963e936ca938a9..f77782f557ce9e5decfa4a0828c3d8c0d43f1d6d 100644 (file)
--- 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)