timekeeping: Make it safe to use the fast timekeeper while suspended

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>
Acked-by: John Stultz <john.stultz@linaro.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

1 files changed, 30 insertions, 0 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index abf08f4366c1..aef5dc722abf 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -332,6 +332,35 @@ u64 notrace ktime_get_mono_fast_ns(void)
 }
 EXPORT_SYMBOL_GPL(ktime_get_mono_fast_ns);
 
+/* Suspend-time cycles value for halted fast timekeeper. */
+static cycle_t cycles_at_suspend;
+
+static cycle_t dummy_clock_read(struct clocksource *cs)
+{
+        return cycles_at_suspend;
+}
+
+/**
+ * halt_fast_timekeeper - Prevent fast timekeeper from accessing clocksource.
+ * @tk: Timekeeper to snapshot.
+ *
+ * It generally is unsafe to access the clocksource after timekeeping has been
+ * suspended, so take a snapshot of the readout base of @tk and use it as the
+ * fast timekeeper's readout base while suspended.  It will return the same
+ * number of cycles every time until timekeeping is resumed at which time the
+ * proper readout base for the fast timekeeper will be restored automatically.
+ */
+static void halt_fast_timekeeper(struct timekeeper *tk)
+{
+        static struct tk_read_base tkr_dummy;
+        struct tk_read_base *tkr = &tk->tkr;
+
+        memcpy(&tkr_dummy, tkr, sizeof(tkr_dummy));
+        cycles_at_suspend = tkr->read(tkr->clock);
+        tkr_dummy.read = dummy_clock_read;
+        update_fast_timekeeper(&tkr_dummy);
+}
+
 #ifdef CONFIG_GENERIC_TIME_VSYSCALL_OLD
 
 static inline void update_vsyscall(struct timekeeper *tk)
@@ -1294,6 +1323,7 @@ static int timekeeping_suspend(void)
         }
 
         timekeeping_update(tk, TK_MIRROR);
+        halt_fast_timekeeper(tk);
         write_seqcount_end(&tk_core.seq);
         raw_spin_unlock_irqrestore(&timekeeper_lock, flags);