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

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

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

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@intel.com>
---
 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)
 	hrtimers_resume();
 }
 
+/*
+ * 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);
+	}
+
 	write_seqcount_end(&tk_core.seq);
 	raw_spin_unlock_irqrestore(&timekeeper_lock, flags);