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Commit 1809bfa4 authored by Daniel Thompson's avatar Daniel Thompson Committed by Ingo Molnar
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timers, sched/clock: Avoid deadlock during read from NMI 



Currently it is possible for an NMI (or FIQ on ARM) to come in
and read sched_clock() whilst update_sched_clock() has locked
the seqcount for writing. This results in the NMI handler
locking up when it calls raw_read_seqcount_begin().

This patch fixes the NMI safety issues by providing banked clock
data. This is a similar approach to the one used in Thomas
Gleixner's 4396e058("timekeeping: Provide fast and NMI safe
access to CLOCK_MONOTONIC").

Suggested-by: default avatarStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: default avatarDaniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: default avatarJohn Stultz <john.stultz@linaro.org>
Reviewed-by: default avatarStephen Boyd <sboyd@codeaurora.org>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1427397806-20889-6-git-send-email-john.stultz@linaro.org


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 9fee69a8

kernel/time/sched_clock.c

+68 −35
Original line number Diff line number Diff line
@@ -47,19 +47,20 @@ struct clock_read_data { 
 * struct clock_data - all data needed for sched_clock (including

 *                     registration of a new clock source)

 *

 * @seq:		Sequence counter for protecting updates.

 * @seq:		Sequence counter for protecting updates. The lowest

 *			bit is the index for @read_data.

 * @read_data:		Data required to read from sched_clock.

 * @wrap_kt:		Duration for which clock can run before wrapping

 * @rate:		Tick rate of the registered clock

 * @actual_read_sched_clock: Registered clock read function

 *

 * The ordering of this structure has been chosen to optimize cache

 * performance. In particular seq and read_data (combined) should fit

 * performance. In particular seq and read_data[0] (combined) should fit

 * into a single 64 byte cache line.

 */

struct clock_data {

	seqcount_t seq;

	struct clock_read_data read_data;

	struct clock_read_data read_data[2];

	ktime_t wrap_kt;

	unsigned long rate;

	u64 (*actual_read_sched_clock)(void);
@@ -80,10 +81,9 @@ static u64 notrace jiffy_sched_clock_read(void) 
}



static struct clock_data cd ____cacheline_aligned = {

	.read_data = { .mult = NSEC_PER_SEC / HZ,

	.read_data[0] = { .mult = NSEC_PER_SEC / HZ,

			  .read_sched_clock = jiffy_sched_clock_read, },

	.actual_read_sched_clock = jiffy_sched_clock_read,



};



static inline u64 notrace cyc_to_ns(u64 cyc, u32 mult, u32 shift)
@@ -95,10 +95,11 @@ unsigned long long notrace sched_clock(void) 
{

	u64 cyc, res;

	unsigned long seq;

	struct clock_read_data *rd = &cd.read_data;

	struct clock_read_data *rd;



	do {

		seq = raw_read_seqcount_begin(&cd.seq);

		seq = raw_read_seqcount(&cd.seq);

		rd = cd.read_data + (seq & 1);



		cyc = (rd->read_sched_clock() - rd->epoch_cyc) &

		      rd->sched_clock_mask;
@@ -108,27 +109,51 @@ unsigned long long notrace sched_clock(void) 
	return res;

}



/*

 * Updating the data required to read the clock.

 *

 * sched_clock will never observe mis-matched data even if called from

 * an NMI. We do this by maintaining an odd/even copy of the data and

 * steering sched_clock to one or the other using a sequence counter.

 * In order to preserve the data cache profile of sched_clock as much

 * as possible the system reverts back to the even copy when the update

 * completes; the odd copy is used *only* during an update.

 */

static void update_clock_read_data(struct clock_read_data *rd)

{

	/* update the backup (odd) copy with the new data */

	cd.read_data[1] = *rd;



	/* steer readers towards the odd copy */

	raw_write_seqcount_latch(&cd.seq);



	/* now its safe for us to update the normal (even) copy */

	cd.read_data[0] = *rd;



	/* switch readers back to the even copy */

	raw_write_seqcount_latch(&cd.seq);

}



/*

 * Atomically update the sched_clock epoch.

 */

static void update_sched_clock(void)

{

	unsigned long flags;

	u64 cyc;

	u64 ns;

	struct clock_read_data *rd = &cd.read_data;

	struct clock_read_data rd;



	rd = cd.read_data[0];



	cyc = cd.actual_read_sched_clock();

	ns = rd->epoch_ns +

	     cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,

		       rd->mult, rd->shift);



	raw_local_irq_save(flags);

	raw_write_seqcount_begin(&cd.seq);

	rd->epoch_ns = ns;

	rd->epoch_cyc = cyc;

	raw_write_seqcount_end(&cd.seq);

	raw_local_irq_restore(flags);

	ns = rd.epoch_ns +

	     cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask,

		       rd.mult, rd.shift);



	rd.epoch_ns = ns;

	rd.epoch_cyc = cyc;



	update_clock_read_data(&rd);

}



static enum hrtimer_restart sched_clock_poll(struct hrtimer *hrt)
@@ -145,7 +170,7 @@ void __init sched_clock_register(u64 (*read)(void), int bits, 
	u32 new_mult, new_shift;

	unsigned long r;

	char r_unit;

	struct clock_read_data *rd = &cd.read_data;

	struct clock_read_data rd;



	if (cd.rate > rate)

		return;
@@ -162,22 +187,23 @@ void __init sched_clock_register(u64 (*read)(void), int bits, 
	wrap = clocks_calc_max_nsecs(new_mult, new_shift, 0, new_mask, NULL);

	cd.wrap_kt = ns_to_ktime(wrap);



	rd = cd.read_data[0];



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

	new_epoch = read();

	cyc = cd.actual_read_sched_clock();

	ns = rd->epoch_ns +

	     cyc_to_ns((cyc - rd->epoch_cyc) & rd->sched_clock_mask,

		       rd->mult, rd->shift);

	ns = rd.epoch_ns +

	     cyc_to_ns((cyc - rd.epoch_cyc) & rd.sched_clock_mask,

		       rd.mult, rd.shift);

	cd.actual_read_sched_clock = read;



	raw_write_seqcount_begin(&cd.seq);

	rd->read_sched_clock = read;

	rd->sched_clock_mask = new_mask;

	rd->mult = new_mult;

	rd->shift = new_shift;

	rd->epoch_cyc = new_epoch;

	rd->epoch_ns = ns;

	raw_write_seqcount_end(&cd.seq);

	rd.read_sched_clock = read;

	rd.sched_clock_mask = new_mask;

	rd.mult = new_mult;

	rd.shift = new_shift;

	rd.epoch_cyc = new_epoch;

	rd.epoch_ns = ns;

	update_clock_read_data(&rd);



	r = rate;

	if (r >= 4000000) {
@@ -227,15 +253,22 @@ void __init sched_clock_postinit(void) 
 *

 * This function makes it appear to sched_clock() as if the clock

 * stopped counting at its last update.

 *

 * This function must only be called from the critical

 * section in sched_clock(). It relies on the read_seqcount_retry()

 * at the end of the critical section to be sure we observe the

 * correct copy of epoch_cyc.

 */

static u64 notrace suspended_sched_clock_read(void)

{

	return cd.read_data.epoch_cyc;

	unsigned long seq = raw_read_seqcount(&cd.seq);



	return cd.read_data[seq & 1].epoch_cyc;

}



static int sched_clock_suspend(void)

{

	struct clock_read_data *rd = &cd.read_data;

	struct clock_read_data *rd = &cd.read_data[0];



	update_sched_clock();

	hrtimer_cancel(&sched_clock_timer);
@@ -245,7 +278,7 @@ static int sched_clock_suspend(void) 


static void sched_clock_resume(void)

{

	struct clock_read_data *rd = &cd.read_data;

	struct clock_read_data *rd = &cd.read_data[0];



	rd->epoch_cyc = cd.actual_read_sched_clock();

	hrtimer_start(&sched_clock_timer, cd.wrap_kt, HRTIMER_MODE_REL);