Re: Problem when function alarmtimer_suspend returns 0 if time delta is zero

[Thomas Gleixner]

Michael,

On Mon, 2 Sep 2019, Alexandre Belloni wrote:
> On 31/08/2019 20:32:06+0200, Michael wrote:
> > currently I have a problem with the alarmtimer i'm using to cyclically wake
> > up my i.MX6 ULL board from suspend to RAM.
> > 
> > The problem is that in principle the timer wake ups work fine but seem to be
> > not 100% stable. In about 1 percent the wake up alarm from suspend is
> > missing.

> > In my error case the alarm wake up always fails if the path "if(min==0)" is
> > entered. If I understand this code correctly that means that
> > when ever one of the timers in the list has a remaining tick time of zero,
> > the function just returns 0 and continues the suspend process until
> > it reaches suspend mode.

No. That code is simply broken because it tries to handle the case where a
alarmtimer nanosleep got woken up by the freezer. That's broken because it
makes the delta = 0 assumption which leads to the issue you discovered.

That whole cruft can be removed by switching alarmtimer nanosleep to use
freezable_schedule(). That keeps the timer queued and avoids all the issues.

Completely untested patch below.

Thanks,

        tglx

8<----------------------

kernel/time/alarmtimer.c |   57 +++--------------------------------------------
 1 file changed, 4 insertions(+), 53 deletions(-)

--- a/kernel/time/alarmtimer.c
+++ b/kernel/time/alarmtimer.c
@@ -46,14 +46,6 @@ static struct alarm_base {
        clockid_t               base_clockid;
 } alarm_bases[ALARM_NUMTYPE];
 
-#if defined(CONFIG_POSIX_TIMERS) || defined(CONFIG_RTC_CLASS)
-/* freezer information to handle clock_nanosleep triggered wakeups */
-static enum alarmtimer_type freezer_alarmtype;
-static ktime_t freezer_expires;
-static ktime_t freezer_delta;
-static DEFINE_SPINLOCK(freezer_delta_lock);
-#endif
-
 #ifdef CONFIG_RTC_CLASS
 static struct wakeup_source *ws;
 
@@ -241,19 +233,12 @@ EXPORT_SYMBOL_GPL(alarm_expires_remainin
  */
 static int alarmtimer_suspend(struct device *dev)
 {
-       ktime_t min, now, expires;
+       ktime_t now, expires, min = KTIME_MAX;
        int i, ret, type;
        struct rtc_device *rtc;
        unsigned long flags;
        struct rtc_time tm;
 
-       spin_lock_irqsave(&freezer_delta_lock, flags);
-       min = freezer_delta;
-       expires = freezer_expires;
-       type = freezer_alarmtype;
-       freezer_delta = 0;
-       spin_unlock_irqrestore(&freezer_delta_lock, flags);
-
        rtc = alarmtimer_get_rtcdev();
        /* If we have no rtcdev, just return */
        if (!rtc)
@@ -271,13 +256,13 @@ static int alarmtimer_suspend(struct dev
                if (!next)
                        continue;
                delta = ktime_sub(next->expires, base->gettime());
-               if (!min || (delta < min)) {
+               if (delta < min) {
                        expires = next->expires;
                        min = delta;
                        type = i;
                }
        }
-       if (min == 0)
+       if (min == KTIME_MAX)
                return 0;
 
        if (ktime_to_ns(min) < 2 * NSEC_PER_SEC) {
@@ -479,38 +464,6 @@ u64 alarm_forward_now(struct alarm *alar
 EXPORT_SYMBOL_GPL(alarm_forward_now);
 
 #ifdef CONFIG_POSIX_TIMERS
-
-static void alarmtimer_freezerset(ktime_t absexp, enum alarmtimer_type type)
-{
-       struct alarm_base *base;
-       unsigned long flags;
-       ktime_t delta;
-
-       switch(type) {
-       case ALARM_REALTIME:
-               base = &alarm_bases[ALARM_REALTIME];
-               type = ALARM_REALTIME_FREEZER;
-               break;
-       case ALARM_BOOTTIME:
-               base = &alarm_bases[ALARM_BOOTTIME];
-               type = ALARM_BOOTTIME_FREEZER;
-               break;
-       default:
-               WARN_ONCE(1, "Invalid alarm type: %d\n", type);
-               return;
-       }
-
-       delta = ktime_sub(absexp, base->gettime());
-
-       spin_lock_irqsave(&freezer_delta_lock, flags);
-       if (!freezer_delta || (delta < freezer_delta)) {
-               freezer_delta = delta;
-               freezer_expires = absexp;
-               freezer_alarmtype = type;
-       }
-       spin_unlock_irqrestore(&freezer_delta_lock, flags);
-}
-
 /**
  * clock2alarm - helper that converts from clockid to alarmtypes
  * @clockid: clockid.
@@ -715,7 +668,7 @@ static int alarmtimer_do_nsleep(struct a
                set_current_state(TASK_INTERRUPTIBLE);
                alarm_start(alarm, absexp);
                if (likely(alarm->data))
-                       schedule();
+                       freezable_schedule();
 
                alarm_cancel(alarm);
        } while (alarm->data && !signal_pending(current));
@@ -727,8 +680,6 @@ static int alarmtimer_do_nsleep(struct a
        if (!alarm->data)
                return 0;
 
-       if (freezing(current))
-               alarmtimer_freezerset(absexp, type);
        restart = &current->restart_block;
        if (restart->nanosleep.type != TT_NONE) {
                struct timespec64 rmt;