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>

diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 0abb364..4d23dc4 100644 (file)
--- 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();