timer: fix race condition

timer: fix race condition

Eliminate problematic race condition in rte_timer_manage() that can
lead to corruption of per-lcore pending-lists (implemented as
skip-lists). The race condition occurs when rte_timer_manage() expires
multiple timers on lcore A, while lcore B simultaneously invokes
rte_timer_reset() for one of the expiring timers (other than the first
one).

Lcore A splits its pending-list, creating a local list of expired timers
linked through their sl_next[0] pointers, and sets the first expired
timer to the RUNNING state, all during one list-lock round trip.
Lcore A then unlocks the list-lock to run the first callback, and that
is when A and B can have different interpretations of the subsequent
expired timers' true state. Lcore B sees an expired timer still in the
PENDING state, atomically changes the timer to the CONFIG state, locks
lcore A's list-lock, and reinserts the timer into A's pending-list.
The two lcores try to use the same next-pointers to maintain both lists!

Our solution is to remove expired timers from the pending-list and try
to set them all to the RUNNING state in one atomic step, i.e.,
rte_timer_manage() should perform these two actions within one
ownership of the list-lock.

After splitting the pending-list at the current point in time and trying
to set all expired timers to the RUNNING state, we must put back into
the pending-list any timers that we failed to set to the RUNNING state,
all while still holding the list-lock. It is then safe to release the
lock and run the callback functions for all expired timers that remain
on our local run-list.

Signed-off-by: Robert Sanford <rsanford@akamai.com>