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 timekeeping_inject_offset() 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>
---
 kernel/time/timekeeping.c | 41 ++++++++++++++++-------------------------
 1 file changed, 16 insertions(+), 25 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index efcfb0da351e..8b8d77463f3e 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1519,40 +1519,31 @@ EXPORT_SYMBOL(do_settimeofday64);
  */
 static int timekeeping_inject_offset(const struct timespec64 *ts)
 {
-	struct timekeeper *tk = &tk_core.timekeeper;
-	unsigned long flags;
-	struct timespec64 tmp;
-	int ret = 0;
-
 	if (ts->tv_nsec < 0 || ts->tv_nsec >= NSEC_PER_SEC)
 		return -EINVAL;
 
-	raw_spin_lock_irqsave(&tk_core.lock, flags);
-	write_seqcount_begin(&tk_core.seq);
-
-	timekeeping_forward_now(tk);
-
-	/* Make sure the proposed value is valid */
-	tmp = timespec64_add(tk_xtime(tk), *ts);
-	if (timespec64_compare(&tk->wall_to_monotonic, ts) > 0 ||
-	    !timespec64_valid_settod(&tmp)) {
-		ret = -EINVAL;
-		goto error;
-	}
+	scoped_guard (raw_spinlock_irqsave, &tk_core.lock) {
+		struct timekeeper *tk = &tk_core.shadow_timekeeper;
+		struct timespec64 tmp;
 
-	tk_xtime_add(tk, ts);
-	tk_set_wall_to_mono(tk, timespec64_sub(tk->wall_to_monotonic, *ts));
+		timekeeping_forward_now(tk);
 
-error: /* even if we error out, we forwarded the time, so call update */
-	timekeeping_update(&tk_core, tk, TK_UPDATE_ALL | TK_MIRROR);
+		/* Make sure the proposed value is valid */
+		tmp = timespec64_add(tk_xtime(tk), *ts);
+		if (timespec64_compare(&tk->wall_to_monotonic, ts) > 0 ||
+		    !timespec64_valid_settod(&tmp)) {
+			timekeeping_restore_shadow(&tk_core);
+			return -EINVAL;
+		}
 
-	write_seqcount_end(&tk_core.seq);
-	raw_spin_unlock_irqrestore(&tk_core.lock, flags);
+		tk_xtime_add(tk, ts);
+		tk_set_wall_to_mono(tk, timespec64_sub(tk->wall_to_monotonic, *ts));
+		timekeeping_update_staged(&tk_core, TK_UPDATE_ALL);
+	}
 
 	/* Signal hrtimers about time change */
 	clock_was_set(CLOCK_SET_WALL);
-
-	return ret;
+	return 0;
 }
 
 /*

-- 
2.39.2