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authorStephen Boyd <sboyd@codeaurora.org>2014-02-17 10:45:36 -0800
committerThomas Gleixner <tglx@linutronix.de>2014-02-19 17:07:22 +0100
commit5ae8aabeaec3fe69c4fb21cbe5b17b72b35b5892 (patch)
tree5cd5d476049486e228d60e59de46811f4161f1dd /kernel
parent6d0abeca3242a88cab8232e4acd7e2bf088f3bc2 (diff)
downloadlinux-5ae8aabeaec3fe69c4fb21cbe5b17b72b35b5892.tar.gz
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>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/time/sched_clock.c46
1 files changed, 29 insertions, 17 deletions
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 0abb3646428..4d23dc4d813 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();