[PATCH] tick/common: Align tick period during sched_timer setup.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

From: Thomas Gleixner <tglx@linutronix.de>

The tick period is aligned very early while the first clock_event_device
is registered. The system runs in periodic mode and switches later to
one-shot mode if possible.

The next wake-up event is programmed based on aligned value
(tick_next_period) but the delta value, that is used to program the
clock_event_device, is computed based on ktime_get().

With the subtracted offset, the devices fires in less than the exacted
time frame. With a large enough offset the system programs the timer for
the next wake-up and the remaining time left is too little to make any
boot progress. The system hangs.

Move the alignment later to the setup of tick_sched timer. At this point
the system switches to oneshot mode and a highres clocksource is
available. It safe to update tick_next_period ktime_get() will now
return accurate (not jiffies based) time.

[bigeasy: Patch description + testing].

Reported-by: Mathias Krause <minipli@grsecurity.net>
Reported-by: "Bhatnagar, Rishabh" <risbhat@amazon.com>
Fixes: e9523a0d81899 ("tick/common: Align tick period with the HZ tick.")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/5a56290d-806e-b9a5-f37c-f21958b5a8c0@grsecurity.net
Link: https://lore.kernel.org/12c6f9a3-d087-b824-0d05-0d18c9bc1bf3@amazon.com
---
 kernel/time/tick-common.c | 11 +----------
 kernel/time/tick-sched.c  | 13 ++++++++++++-
 2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 65b8658da829e..b85f2f9c32426 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -218,19 +218,10 @@ static void tick_setup_device(struct tick_device *td,
 		 * this cpu:
 		 */
 		if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
-			ktime_t next_p;
-			u32 rem;
 
 			tick_do_timer_cpu = cpu;
 
-			next_p = ktime_get();
-			div_u64_rem(next_p, TICK_NSEC, &rem);
-			if (rem) {
-				next_p -= rem;
-				next_p += TICK_NSEC;
-			}
-
-			tick_next_period = next_p;
+			tick_next_period = ktime_get();
 #ifdef CONFIG_NO_HZ_FULL
 			/*
 			 * The boot CPU may be nohz_full, in which case set
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 52254679ec489..42c0be3080bde 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -161,8 +161,19 @@ static ktime_t tick_init_jiffy_update(void)
 	raw_spin_lock(&jiffies_lock);
 	write_seqcount_begin(&jiffies_seq);
 	/* Did we start the jiffies update yet ? */
-	if (last_jiffies_update == 0)
+	if (last_jiffies_update == 0) {
+		u32 rem;
+
+		/*
+		 * Ensure that the tick is aligned to a multiple of
+		 * TICK_NSEC.
+		 */
+		div_u64_rem(tick_next_period, TICK_NSEC, &rem);
+		if (rem)
+			tick_next_period += TICK_NSEC - rem;
+
 		last_jiffies_update = tick_next_period;
+	}
 	period = last_jiffies_update;
 	write_seqcount_end(&jiffies_seq);
 	raw_spin_unlock(&jiffies_lock);
-- 
2.40.1