Commit d0304569 authored by Adrian Hunter's avatar Adrian Hunter Committed by Thomas Gleixner
Browse files

clocksource: Make watchdog and suspend-timing multiplication overflow safe 



Kernel timekeeping is designed to keep the change in cycles (since the last
timer interrupt) below max_cycles, which prevents multiplication overflow
when converting cycles to nanoseconds. However, if timer interrupts stop,
the clocksource_cyc2ns() calculation will eventually overflow.

Add protection against that. Simplify by folding together
clocksource_delta() and clocksource_cyc2ns() into cycles_to_nsec_safe().
Check against max_cycles, falling back to a slower higher precision
calculation.

Suggested-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarAdrian Hunter <adrian.hunter@intel.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240325064023.2997-20-adrian.hunter@intel.com
parent 135225a3

kernel/time/clocksource.c

+20 −22
Original line number Diff line number Diff line
@@ -20,6 +20,16 @@ 
#include "tick-internal.h"

#include "timekeeping_internal.h"



static noinline u64 cycles_to_nsec_safe(struct clocksource *cs, u64 start, u64 end)

{

	u64 delta = clocksource_delta(end, start, cs->mask);



	if (likely(delta < cs->max_cycles))

		return clocksource_cyc2ns(delta, cs->mult, cs->shift);



	return mul_u64_u32_shr(delta, cs->mult, cs->shift);

}



/**

 * clocks_calc_mult_shift - calculate mult/shift factors for scaled math of clocks

 * @mult:	pointer to mult variable
@@ -222,8 +232,8 @@ enum wd_read_status { 
static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, u64 *wdnow)

{

	unsigned int nretries, max_retries;

	u64 wd_end, wd_end2, wd_delta;

	int64_t wd_delay, wd_seq_delay;

	u64 wd_end, wd_end2;



	max_retries = clocksource_get_max_watchdog_retry();

	for (nretries = 0; nretries <= max_retries; nretries++) {
@@ -234,9 +244,7 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, 
		wd_end2 = watchdog->read(watchdog);

		local_irq_enable();



		wd_delta = clocksource_delta(wd_end, *wdnow, watchdog->mask);

		wd_delay = clocksource_cyc2ns(wd_delta, watchdog->mult,

					      watchdog->shift);

		wd_delay = cycles_to_nsec_safe(watchdog, *wdnow, wd_end);

		if (wd_delay <= WATCHDOG_MAX_SKEW) {

			if (nretries > 1 || nretries >= max_retries) {

				pr_warn("timekeeping watchdog on CPU%d: %s retried %d times before success\n",
@@ -254,8 +262,7 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow, 
		 * report system busy, reinit the watchdog and skip the current

		 * watchdog test.

		 */

		wd_delta = clocksource_delta(wd_end2, wd_end, watchdog->mask);

		wd_seq_delay = clocksource_cyc2ns(wd_delta, watchdog->mult, watchdog->shift);

		wd_seq_delay = cycles_to_nsec_safe(watchdog, wd_end, wd_end2);

		if (wd_seq_delay > WATCHDOG_MAX_SKEW/2)

			goto skip_test;

	}
@@ -366,8 +373,7 @@ void clocksource_verify_percpu(struct clocksource *cs) 
		delta = (csnow_end - csnow_mid) & cs->mask;

		if (delta < 0)

			cpumask_set_cpu(cpu, &cpus_ahead);

		delta = clocksource_delta(csnow_end, csnow_begin, cs->mask);

		cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);

		cs_nsec = cycles_to_nsec_safe(cs, csnow_begin, csnow_end);

		if (cs_nsec > cs_nsec_max)

			cs_nsec_max = cs_nsec;

		if (cs_nsec < cs_nsec_min)
@@ -398,8 +404,8 @@ static inline void clocksource_reset_watchdog(void) 


static void clocksource_watchdog(struct timer_list *unused)

{

	u64 csnow, wdnow, cslast, wdlast, delta;

	int64_t wd_nsec, cs_nsec, interval;

	u64 csnow, wdnow, cslast, wdlast;

	int next_cpu, reset_pending;

	struct clocksource *cs;

	enum wd_read_status read_ret;
@@ -456,12 +462,8 @@ static void clocksource_watchdog(struct timer_list *unused) 
			continue;

		}



		delta = clocksource_delta(wdnow, cs->wd_last, watchdog->mask);

		wd_nsec = clocksource_cyc2ns(delta, watchdog->mult,

					     watchdog->shift);



		delta = clocksource_delta(csnow, cs->cs_last, cs->mask);

		cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift);

		wd_nsec = cycles_to_nsec_safe(watchdog, cs->wd_last, wdnow);

		cs_nsec = cycles_to_nsec_safe(cs, cs->cs_last, csnow);

		wdlast = cs->wd_last; /* save these in case we print them */

		cslast = cs->cs_last;

		cs->cs_last = csnow;
@@ -832,7 +834,7 @@ void clocksource_start_suspend_timing(struct clocksource *cs, u64 start_cycles) 
 */

u64 clocksource_stop_suspend_timing(struct clocksource *cs, u64 cycle_now)

{

	u64 now, delta, nsec = 0;

	u64 now, nsec = 0;



	if (!suspend_clocksource)

		return 0;
@@ -847,12 +849,8 @@ u64 clocksource_stop_suspend_timing(struct clocksource *cs, u64 cycle_now) 
	else

		now = suspend_clocksource->read(suspend_clocksource);



	if (now > suspend_start) {

		delta = clocksource_delta(now, suspend_start,

					  suspend_clocksource->mask);

		nsec = mul_u64_u32_shr(delta, suspend_clocksource->mult,

				       suspend_clocksource->shift);

	}

	if (now > suspend_start)

		nsec = cycles_to_nsec_safe(suspend_clocksource, suspend_start, now);



	/*

	 * Disable the suspend timer to save power if current clocksource is