[PATCH 3/6] timekeeping: Make it safe to use the fast timekeeper while suspended

From: Rafael J. Wysocki
Date: Tue Feb 10 2015 - 22:43:10 EST

From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>

Theoretically, ktime_get_mono_fast_ns() may be executed after
timekeeping has been suspended (or before it is resumed) which
in turn may lead to undefined behavior, for example, when the
clocksource read from timekeeping_get_ns() called by it is
not accessible at that time.

Prevent that from happening by setting up a dummy readout base for
the fast timekeeper during timekeeping_suspend() such that it will
always return the same number of cycles.

After the last timekeeping_update() in timekeeping_suspend() the
clocksource is read and the result is stored as cycles_at_suspend.
The readout base from the current timekeeper is copied onto the
dummy and the ->read pointer of the dummy is set to a routine
unconditionally returning cycles_at_suspend. Next, the dummy is
passed to update_fast_timekeeper().

Then, ktime_get_mono_fast_ns() will work until the subsequent
timekeeping_resume() and the proper readout base for the fast
timekeeper will be restored by the timekeeping_update() called
right after clearing timekeeping_suspended.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
kernel/time/timekeeping.c | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)

Index: linux-pm/kernel/time/timekeeping.c
--- linux-pm.orig/kernel/time/timekeeping.c
+++ linux-pm/kernel/time/timekeeping.c
@@ -1249,9 +1249,23 @@ static void timekeeping_resume(void)

+ * Dummy readout base and suspend-time cycles value for the fast timekeeper to
+ * work in a consistent way after timekeeping has been suspended if the core
+ * timekeeper clocksource is not suspend-nonstop.
+ */
+static struct tk_read_base tkr_dummy;
+static cycle_t cycles_at_suspend;
+static cycle_t dummy_clock_read(struct clocksource *cs)
+ return cycles_at_suspend;
static int timekeeping_suspend(void)
struct timekeeper *tk = &tk_core.timekeeper;
+ struct clocksource *clock = tk->tkr.clock;
unsigned long flags;
struct timespec64 delta, delta_delta;
static struct timespec64 old_delta;
@@ -1294,6 +1308,14 @@ static int timekeeping_suspend(void)

timekeeping_update(tk, TK_MIRROR);
+ if (!(clock->flags & CLOCK_SOURCE_SUSPEND_NONSTOP)) {
+ memcpy(&tkr_dummy, &tk->tkr, sizeof(tkr_dummy));
+ cycles_at_suspend = tk->tkr.read(clock);
+ tkr_dummy.read = dummy_clock_read;
+ update_fast_timekeeper(&tkr_dummy);
+ }
raw_spin_unlock_irqrestore(&timekeeper_lock, flags);

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