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: Will Deacon <will.deacon@arm.com>
Signed-off-by: Stephen 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: John Stultz <john.stultz@linaro.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 29 insertions, 17 deletions

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 0abb36464281..4d23dc4d8139 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -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();