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Commit 060407ae authored by Rafael J. Wysocki's avatar Rafael J. Wysocki Committed by Rafael J. Wysocki
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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: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: default avatarJohn Stultz <john.stultz@linaro.org>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
parent affe3e85

kernel/time/timekeeping.c

+30 −0
Original line number Diff line number Diff line
@@ -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);