sched/deadline: Implement cancel_dl_timer() to use in switched_from_dl()

sched/deadline: Implement cancel_dl_timer() to use in switched_from_dl()

Currently used hrtimer_try_to_cancel() is racy:

raw_spin_lock(&rq->lock)
...                            dl_task_timer                 raw_spin_lock(&rq->lock)
...                               raw_spin_lock(&rq->lock)   ...
   switched_from_dl()             ...                        ...
      hrtimer_try_to_cancel()     ...                        ...
   switched_to_fair()             ...                        ...
...                               ...                        ...
...                               ...                        ...
raw_spin_unlock(&rq->lock)        ...                        (asquired)
...                               ...                        ...
...                               ...                        ...
do_exit()                         ...                        ...
   schedule()                     ...                        ...
      raw_spin_lock(&rq->lock)    ...                        raw_spin_unlock(&rq->lock)
      ...                         ...                        ...
      raw_spin_unlock(&rq->lock)  ...                        raw_spin_lock(&rq->lock)
      ...                         ...                        (asquired)
      put_task_struct()           ...                        ...
          free_task_struct()      ...                        ...
      ...                         ...                        raw_spin_unlock(&rq->lock)
...                               (asquired)                 ...
...                               ...                        ...
...                               (use after free)           ...

So, let's implement 100% guaranteed way to cancel the timer and let's
be sure we are safe even in very unlikely situations.

rq unlocking does not limit the area of switched_from_dl() use, because
this has already been possible in pull_dl_task() below.

Let's consider the safety of of this unlocking. New code in the patch
is working when hrtimer_try_to_cancel() fails. This means the callback
is running. In this case hrtimer_cancel() is just waiting till the
callback is finished. Two

1) Since we are in switched_from_dl(), new class is not dl_sched_class and
new prio is not less MAX_DL_PRIO. So, the callback returns early; it's
right after !dl_task() check. After that hrtimer_cancel() returns back too.

The above is:

raw_spin_lock(rq->lock);                  ...
...                                       dl_task_timer()
...                                          raw_spin_lock(rq->lock);
   switched_from_dl()                        ...
       hrtimer_try_to_cancel()               ...
          raw_spin_unlock(rq->lock);         ...
          hrtimer_cancel()                   ...
          ...                                raw_spin_unlock(rq->lock);
          ...                                return HRTIMER_NORESTART;
          ...                             ...
          raw_spin_lock(rq->lock);        ...

2) But the below is also possible:
                                   dl_task_timer()
                                      raw_spin_lock(rq->lock);
                                      ...
                                      raw_spin_unlock(rq->lock);
raw_spin_lock(rq->lock);              ...
   switched_from_dl()                 ...
       hrtimer_try_to_cancel()        ...
       ...                            return HRTIMER_NORESTART;
       raw_spin_unlock(rq->lock);  ...
       hrtimer_cancel();           ...
       raw_spin_lock(rq->lock);    ...

In this case hrtimer_cancel() returns immediately. Very unlikely case,
just to mention.

Nobody can manipulate the task, because check_class_changed() is
always called with pi_lock locked. Nobody can force the task to
participate in (concurrent) priority inheritance schemes (the same reason).

All concurrent task operations require pi_lock, which is held by us.
No deadlocks with dl_task_timer() are possible, because it returns
right after !dl_task() check (it does nothing).

If we receive a new dl_task during the time of unlocked rq, we just
don't have to do pull_dl_task() in switched_from_dl() further.

Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
[ Added comments]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1414420852.19914.186.camel@tkhai
Signed-off-by: Ingo Molnar <mingo@kernel.org>