Commit 5ae8aabe authored by Stephen Boyd's avatar Stephen Boyd Committed by Thomas Gleixner
Browse files

sched_clock: Prevent callers from seeing half-updated data 



The generic sched_clock registration function was previously
done lockless, due to the fact that it was expected to be called
only once. However, now there are systems that may register
multiple sched_clock sources, for which the lack of locking has
casued problems:

If two sched_clock sources are registered we may end up in a
situation where a call to sched_clock() may be accessing the
epoch cycle count for the old counter and the cycle count for the
new counter. This can lead to confusing results where
sched_clock() values jump and then are reset to 0 (due to the way
the registration function forces the epoch_ns to be 0).

Fix this by reorganizing the registration function to hold the
seqlock for as short a time as possible while we update the
clock_data structure for a new counter. We also put any
accumulated time into epoch_ns instead of resetting the time to
0 so that the clock doesn't reset after each successful
registration.

[jstultz: Added extra context to the commit message]

Reported-by: default avatarWill Deacon <will.deacon@arm.com>
Signed-off-by: default avatarStephen Boyd <sboyd@codeaurora.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Josh Cartwright <joshc@codeaurora.org>
Link: http://lkml.kernel.org/r/1392662736-7803-2-git-send-email-john.stultz@linaro.org


Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 6d0abeca

kernel/time/sched_clock.c

+29 −17
Original line number Diff line number Diff line
@@ -116,20 +116,42 @@ static enum hrtimer_restart sched_clock_poll(struct hrtimer *hrt) 
void __init sched_clock_register(u64 (*read)(void), int bits,

				 unsigned long rate)

{

	u64 res, wrap, new_mask, new_epoch, cyc, ns;

	u32 new_mult, new_shift;

	ktime_t new_wrap_kt;

	unsigned long r;

	u64 res, wrap;

	char r_unit;



	if (cd.rate > rate)

		return;



	WARN_ON(!irqs_disabled());

	read_sched_clock = read;

	sched_clock_mask = CLOCKSOURCE_MASK(bits);

	cd.rate = rate;



	/* calculate the mult/shift to convert counter ticks to ns. */

	clocks_calc_mult_shift(&cd.mult, &cd.shift, rate, NSEC_PER_SEC, 3600);

	clocks_calc_mult_shift(&new_mult, &new_shift, rate, NSEC_PER_SEC, 3600);



	new_mask = CLOCKSOURCE_MASK(bits);



	/* calculate how many ns until we wrap */

	wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask);

	new_wrap_kt = ns_to_ktime(wrap - (wrap >> 3));



	/* update epoch for new counter and update epoch_ns from old counter*/

	new_epoch = read();

	cyc = read_sched_clock();

	ns = cd.epoch_ns + cyc_to_ns((cyc - cd.epoch_cyc) & sched_clock_mask,

			  cd.mult, cd.shift);



	raw_write_seqcount_begin(&cd.seq);

	read_sched_clock = read;

	sched_clock_mask = new_mask;

	cd.rate = rate;

	cd.wrap_kt = new_wrap_kt;

	cd.mult = new_mult;

	cd.shift = new_shift;

	cd.epoch_cyc = new_epoch;

	cd.epoch_ns = ns;

	raw_write_seqcount_end(&cd.seq);



	r = rate;

	if (r >= 4000000) {
@@ -141,22 +163,12 @@ void __init sched_clock_register(u64 (*read)(void), int bits, 
	} else

		r_unit = ' ';



	/* calculate how many ns until we wrap */

	wrap = clocks_calc_max_nsecs(cd.mult, cd.shift, 0, sched_clock_mask);

	cd.wrap_kt = ns_to_ktime(wrap - (wrap >> 3));



	/* calculate the ns resolution of this counter */

	res = cyc_to_ns(1ULL, cd.mult, cd.shift);

	res = cyc_to_ns(1ULL, new_mult, new_shift);



	pr_info("sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lluns\n",

		bits, r, r_unit, res, wrap);



	update_sched_clock();



	/*

	 * Ensure that sched_clock() starts off at 0ns

	 */

	cd.epoch_ns = 0;



	/* Enable IRQ time accounting if we have a fast enough sched_clock */

	if (irqtime > 0 || (irqtime == -1 && rate >= 1000000))

		enable_sched_clock_irqtime();

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