We need to ensure that only one cpu can work on dequeueing of the reorder
queue the time. Calculating in which percpu reorder queue the next object
will arrive takes some time. A spinlock would be highly contended. Also
it is not clear in which order the objects arrive to the reorder queues.
So a cpu could wait to get the lock just to notice that there is nothing to
do at the moment. Therefore we use a trylock and let the holder of the
lock care for all the objects enqueued during the holdtime of the lock.
The timer is to handle a race that appears with the trylock. If cpu1 queues
an object to the reorder queue while cpu2 holds the pd->lock but left the
while loop in padata_reorder already, cpu2 can't care for this object but cpu1
exits because it can't get the lock. Usually the next cpu that takes the
lock cares for this object too. We need the timer just if this object was the
last one that arrives to the reorder queues. The timer function sends it out
We could hit the race that the timer handles, if we move this into the lock.
cpu1 cpu2
spin_trylock_bh()
|
|
|
test pd->reorder_objects == 0
delete timer
|
hardinterrupt
| set pd->reorder_objects == 1
| enqueue object
| spin_trylock_bh() busy
| exit
|
spin_unlock_bh()
--
