Problem with hrtimer_cancel() does not cancel timer

[Alexander Gerasiov <gq@cs.msu.su>]

Hello folks,

I met some problem with preempt version of kernel 4.4.

I have hrtimer which runs callback, it forwards time and returns
HRTIMER_RESTART. Very common scheme. When I stop the routine I call
hrtimer_cancel(). It should wait for callback completion if any and
unqueue any future timers.

But suddenly I noticed, that sometimes hrtimer's routine continue to
work and work after hrtimer_cancel(). It could be reproduced with simple
test:

Start hrtimer.
In callback increase counter and restart.
In the other thread cancel timer, save counter, sleep, check that
counter is not changed, restart timer, repeat.
Here is sample module:
https://gist.github.com/gerasiov/010e6c2cb96fd891c1802facf816fe84

It works OK on v3.10.74-rt79 with PREEMPL FULL.
It works OK on v4.4.47-rt59 with preempt patch but PREEMPT NONE.
But on 4.4 (tested on v4.4.47-rt59 and v4.4.32-rt43) with PREEMPT FULL
it fails: counter continue to increase (BUG_ON() line 45 hits), that
means that hrtimer continue to work after cancel(). And I must say,
that this is not single iteration, it continues to work in cycle.

It was reproduced on i386 SMP box.
kernel config:
https://github.com/redlab-i/linux/blob/v4.4.47-rt59-stand2/.config

Do anyone have any ideas where could be the problem? Or may be I missed
something in my research?

-- 
Best regards,
 Alexander Gerasiov

 Contacts:
 e-mail: gq@cs.msu.su  Homepage: http://gerasiov.net  Skype: gerasiov
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