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 4396e058c52e("timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC").
Suggested-by: Stephen Boyd sboyd@codeaurora.org Signed-off-by: Daniel Thompson daniel.thompson@linaro.org Cc: Russell King linux@arm.linux.org.uk Cc: Will Deacon will.deacon@arm.com Cc: Catalin Marinas catalin.marinas@arm.com --- kernel/time/sched_clock.c | 103 ++++++++++++++++++++++++++++++---------------- 1 file changed, 68 insertions(+), 35 deletions(-)
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c index 9280327676dc..a23d98c33dab 100644 --- a/kernel/time/sched_clock.c +++ b/kernel/time/sched_clock.c @@ -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_sched_clock = jiffy_sched_clock_read, }, + .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; @@ -109,26 +110,50 @@ unsigned long long notrace sched_clock(void) }
/* + * 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); cd.wrap_kt = ns_to_ktime(wrap - (wrap >> 3));
+ 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);