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Commit d65670a7 authored by John Stultz's avatar John Stultz
Browse files

clocksource: Avoid selecting mult values that might overflow when adjusted 



For some frequencies, the clocks_calc_mult_shift() function will
unfortunately select mult values very close to 0xffffffff.  This
has the potential to overflow when NTP adjusts the clock, adding
to the mult value.

This patch adds a clocksource.maxadj value, which provides
an approximation of an 11% adjustment(NTP limits adjustments to
500ppm and the tick adjustment is limited to 10%), which could
be made to the clocksource.mult value. This is then used to both
check that the current mult value won't overflow/underflow, as
well as warning us if the timekeeping_adjust() code pushes over
that 11% boundary.

v2: Fix max_adjustment calculation, and improve WARN_ONCE
messages.

v3: Don't warn before maxadj has actually been set

CC: Yong Zhang <yong.zhang0@gmail.com>
CC: David Daney <ddaney.cavm@gmail.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Chen Jie <chenj@lemote.com>
CC: zhangfx <zhangfx@lemote.com>
CC: stable@kernel.org
Reported-by: default avatarChen Jie <chenj@lemote.com>
Reported-by: default avatarzhangfx <zhangfx@lemote.com>
Tested-by: default avatarYong Zhang <yong.zhang0@gmail.com>
Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.org>
parent e35f95b3

include/linux/clocksource.h

+2 −1
Original line number Diff line number Diff line
@@ -156,6 +156,7 @@ extern u64 timecounter_cyc2time(struct timecounter *tc, 
 * @mult:		cycle to nanosecond multiplier

 * @shift:		cycle to nanosecond divisor (power of two)

 * @max_idle_ns:	max idle time permitted by the clocksource (nsecs)

 * @maxadj		maximum adjustment value to mult (~11%)

 * @flags:		flags describing special properties

 * @archdata:		arch-specific data

 * @suspend:		suspend function for the clocksource, if necessary
@@ -172,7 +173,7 @@ struct clocksource { 
	u32 mult;

	u32 shift;

	u64 max_idle_ns;



	u32 maxadj;

#ifdef CONFIG_ARCH_CLOCKSOURCE_DATA

	struct arch_clocksource_data archdata;

#endif

kernel/time/clocksource.c

+48 −10
Original line number Diff line number Diff line
@@ -491,6 +491,22 @@ void clocksource_touch_watchdog(void) 
	clocksource_resume_watchdog();

}



/**

 * clocksource_max_adjustment- Returns max adjustment amount

 * @cs:         Pointer to clocksource

 *

 */

static u32 clocksource_max_adjustment(struct clocksource *cs)

{

	u64 ret;

	/*

	 * We won't try to correct for more then 11% adjustments (110,000 ppm),

	 */

	ret = (u64)cs->mult * 11;

	do_div(ret,100);

	return (u32)ret;

}



/**

 * clocksource_max_deferment - Returns max time the clocksource can be deferred

 * @cs:         Pointer to clocksource
@@ -503,25 +519,28 @@ static u64 clocksource_max_deferment(struct clocksource *cs) 
	/*

	 * Calculate the maximum number of cycles that we can pass to the

	 * cyc2ns function without overflowing a 64-bit signed result. The

	 * maximum number of cycles is equal to ULLONG_MAX/cs->mult which

	 * is equivalent to the below.

	 * max_cycles < (2^63)/cs->mult

	 * max_cycles < 2^(log2((2^63)/cs->mult))

	 * max_cycles < 2^(log2(2^63) - log2(cs->mult))

	 * max_cycles < 2^(63 - log2(cs->mult))

	 * max_cycles < 1 << (63 - log2(cs->mult))

	 * maximum number of cycles is equal to ULLONG_MAX/(cs->mult+cs->maxadj)

	 * which is equivalent to the below.

	 * max_cycles < (2^63)/(cs->mult + cs->maxadj)

	 * max_cycles < 2^(log2((2^63)/(cs->mult + cs->maxadj)))

	 * max_cycles < 2^(log2(2^63) - log2(cs->mult + cs->maxadj))

	 * max_cycles < 2^(63 - log2(cs->mult + cs->maxadj))

	 * max_cycles < 1 << (63 - log2(cs->mult + cs->maxadj))

	 * Please note that we add 1 to the result of the log2 to account for

	 * any rounding errors, ensure the above inequality is satisfied and

	 * no overflow will occur.

	 */

	max_cycles = 1ULL << (63 - (ilog2(cs->mult) + 1));

	max_cycles = 1ULL << (63 - (ilog2(cs->mult + cs->maxadj) + 1));



	/*

	 * The actual maximum number of cycles we can defer the clocksource is

	 * determined by the minimum of max_cycles and cs->mask.

	 * Note: Here we subtract the maxadj to make sure we don't sleep for

	 * too long if there's a large negative adjustment.

	 */

	max_cycles = min_t(u64, max_cycles, (u64) cs->mask);

	max_nsecs = clocksource_cyc2ns(max_cycles, cs->mult, cs->shift);

	max_nsecs = clocksource_cyc2ns(max_cycles, cs->mult - cs->maxadj,

					cs->shift);



	/*

	 * To ensure that the clocksource does not wrap whilst we are idle,
@@ -640,7 +659,6 @@ static void clocksource_enqueue(struct clocksource *cs) 
void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq)

{

	u64 sec;



	/*

	 * Calc the maximum number of seconds which we can run before

	 * wrapping around. For clocksources which have a mask > 32bit
@@ -661,6 +679,20 @@ void __clocksource_updatefreq_scale(struct clocksource *cs, u32 scale, u32 freq) 


	clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,

			       NSEC_PER_SEC / scale, sec * scale);



	/*

	 * for clocksources that have large mults, to avoid overflow.

	 * Since mult may be adjusted by ntp, add an safety extra margin

	 *

	 */

	cs->maxadj = clocksource_max_adjustment(cs);

	while ((cs->mult + cs->maxadj < cs->mult)

		|| (cs->mult - cs->maxadj > cs->mult)) {

		cs->mult >>= 1;

		cs->shift--;

		cs->maxadj = clocksource_max_adjustment(cs);

	}



	cs->max_idle_ns = clocksource_max_deferment(cs);

}

EXPORT_SYMBOL_GPL(__clocksource_updatefreq_scale);
@@ -701,6 +733,12 @@ EXPORT_SYMBOL_GPL(__clocksource_register_scale); 
 */

int clocksource_register(struct clocksource *cs)

{

	/* calculate max adjustment for given mult/shift */

	cs->maxadj = clocksource_max_adjustment(cs);

	WARN_ONCE(cs->mult + cs->maxadj < cs->mult,

		"Clocksource %s might overflow on 11%% adjustment\n",

		cs->name);



	/* calculate max idle time permitted for this clocksource */

	cs->max_idle_ns = clocksource_max_deferment(cs);

kernel/time/timekeeping.c

+7 −0
Original line number Diff line number Diff line
@@ -820,6 +820,13 @@ static void timekeeping_adjust(s64 offset) 
	} else

		return;



	WARN_ONCE(timekeeper.clock->maxadj &&

			(timekeeper.mult + adj > timekeeper.clock->mult +

						timekeeper.clock->maxadj),

			"Adjusting %s more then 11%% (%ld vs %ld)\n",

			timekeeper.clock->name, (long)timekeeper.mult + adj,

			(long)timekeeper.clock->mult +

				timekeeper.clock->maxadj);

	timekeeper.mult += adj;

	timekeeper.xtime_interval += interval;

	timekeeper.xtime_nsec -= offset;