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authorRafael J. Wysocki <rafael.j.wysocki@intel.com>2015-02-13 14:49:02 +0100
committerRafael J. Wysocki <rjw@rjwysocki.net>2015-02-15 19:39:40 +0100
commit060407aed56c00960c9b5f70f5d19b2823adffd7 (patch)
tree1a3e285bb6af775bfd75e4b5143548687d773861 /kernel/time
parenttimekeeping: Pass readout base to update_fast_timekeeper() (diff)
downloadlinux-060407aed56c00960c9b5f70f5d19b2823adffd7.tar.xz
linux-060407aed56c00960c9b5f70f5d19b2823adffd7.zip
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>
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/timekeeping.c30
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);