Commit 13bb06f8 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

tick/common: Align tick period during sched_timer setup 



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

The next wake-up event is programmed based on the 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 device fires earlier than the exact time
frame. With a large enough offset the system programs the timer for the
next wake-up and the remaining time left is too small 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 high resolution clocksource is
available. At this point it is safe to align tick_next_period because
ktime_get() will now return accurate (not jiffies based) time.

[bigeasy: Patch description + testing].

Fixes: e9523a0d ("tick/common: Align tick period with the HZ tick.")
Reported-by: default avatarMathias Krause <minipli@grsecurity.net>
Reported-by: default avatar"Bhatnagar, Rishabh" <risbhat@amazon.com>
Suggested-by: default avatarMathias Krause <minipli@grsecurity.net>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarRichard W.M. Jones <rjones@redhat.com>
Tested-by: default avatarMathias Krause <minipli@grsecurity.net>
Acked-by: default avatarSeongJae Park <sj@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/5a56290d-806e-b9a5-f37c-f21958b5a8c0@grsecurity.net
Link: https://lore.kernel.org/12c6f9a3-d087-b824-0d05-0d18c9bc1bf3@amazon.com
Link: https://lore.kernel.org/r/20230615091830.RxMV2xf_@linutronix.de
parent 858fd168

kernel/time/tick-common.c

+1 −12
Original line number Diff line number Diff line
@@ -218,19 +218,8 @@ 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

kernel/time/tick-sched.c

+12 −1
Original line number Diff line number Diff line
@@ -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);