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Subject[PATCH v4 5/5] sched_clock: Avoid deadlock during read from NMI
Date
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);
--
2.1.0


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