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@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-v2-16-554456a44a15@linutronix.de

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ed0e328fedd0232eb74b02e35a39358dd6acee90..1b8db11916148b3a6283db32ba576f76674981b8 100644 (file)
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1510,45 +1510,35 @@ 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 *tks = &tk_core.shadow_timekeeper;
 
-       if (timespec64_compare(&tk->wall_to_monotonic, &ts_delta) > 0) {
-               ret = -EINVAL;
-               goto out;
-       }
+               timekeeping_forward_now(tks);
 
-       tk_set_wall_to_mono(tk, timespec64_sub(tk->wall_to_monotonic, ts_delta));
+               xt = tk_xtime(tks);
+               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(&tks->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(tks, timespec64_sub(tks->wall_to_monotonic, ts_delta));
+               tk_set_xtime(tks, ts);
+               timekeeping_update_from_shadow(&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);