timers: Prevent base clock rewind when forwarding clock

Ashton and Michael reported, that kernel versions 4.8 and later suffer from
USB timeouts which are caused by the timer wheel rework.

This is caused by a bug in the base clock forwarding mechanism, which leads
to timers expiring early. The scenario which leads to this is:

run_timers()
  while (jiffies >= base->clk) {
    collect_expired_timers();
    base->clk++;
    expire_timers();
  }          

So base->clk = jiffies + 1. Now the cpu goes idle:

idle()
  get_next_timer_interrupt()
    nextevt = __next_time_interrupt();
    if (time_after(nextevt, base->clk))
       	base->clk = jiffies;

jiffies has not advanced since run_timers(), so this assignment effectively
decrements base->clk by one.

base->clk is the index into the timer wheel arrays. So let's assume the
following state after the base->clk increment in run_timers():

 jiffies = 0
 base->clk = 1

A timer gets enqueued with an expiry delta of 63 ticks (which is the case
with the USB timeout and HZ=250) so the resulting bucket index is:

  base->clk + delta = 1 + 63 = 64

The timer goes into the first wheel level. The array size is 64 so it ends
up in bucket 0, which is correct as it takes 63 ticks to advance base->clk
to index into bucket 0 again.

If the cpu goes idle before jiffies advance, then the bug in the forwarding
mechanism sets base->clk back to 0, so the next invocation of run_timers()
at the next tick will index into bucket 0 and therefore expire the timer 62
ticks too early.

Instead of blindly setting base->clk to jiffies we must make the forwarding
conditional on jiffies > base->clk, but we cannot use jiffies for this as
we might run into the following issue:

  if (time_after(jiffies, base->clk) {
    if (time_after(nextevt, base->clk))
       base->clk = jiffies;

jiffies can increment between the check and the assigment far enough to
advance beyond nextevt. So we need to use a stable value for checking.

get_next_timer_interrupt() has the basej argument which is the jiffies
value snapshot taken in the calling code. So we can just that.

Thanks to Ashton for bisecting and providing trace data!

Fixes: a683f390 ("timers: Forward the wheel clock whenever possible")
Reported-by: Ashton Holmes <scoopta@gmail.com>
Reported-by: Michael Thayer <michael.thayer@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Michal Necasek <michal.necasek@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: knut.osmundsen@oracle.com
Cc: stable@vger.kernel.org
Cc: stern@rowland.harvard.edu
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20161022110552.175308322@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>