timer: fix race condition

timer: fix race condition

rte_timer_manage() adds expired timers to a "run list", and walks the
list, transitioning each timer from the PENDING to the RUNNING state.
If another lcore resets or stops the timer at precisely this
moment, the timer state would instead be set to CONFIG by that other
lcore, which would cause timer_manage() to skip over it. This is
expected behavior.

However, if a timer expires quickly enough, there exists the
following race condition that causes the timer_manage() routine to
misinterpret a timer in CONFIG state, resulting in lost timers:

- Thread A:
  - starts a timer with rte_timer_reset()
  - the timer is moved to CONFIG state
  - the spinlock associated with the appropriate skiplist is acquired
  - timer is inserted into the skiplist
  - the spinlock is released
- Thread B:
  - executes rte_timer_manage()
  - find above timer as expired, add it to run list
  - walk run list, see above timer still in CONFIG state, unlink it from
    run list and continue on
- Thread A:
  - move timer to PENDING state
  - return from rte_timer_reset()
  - timer is now in PENDING state, but not actually linked into a
    pending list or a run list and will never get processed further
    by rte_timer_manage()

This commit fixes this race condition by only releasing the spinlock
after the timer state has been transitioned from CONFIG to PENDING,
which prevents rte_timer_manage() from seeing an incorrect state.

Fixes: 9b15ba895b9f ("timer: use a skip list")
Cc: stable@dpdk.org
Signed-off-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>