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 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>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Peter 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: Ingo Molnar <mingo@kernel.org>

1 files changed, 68 insertions, 35 deletions

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 8adb9d0c969a..eeea1e950b72 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, 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);