On 02/18/2014 04:37 PM, Peter Zijlstra wrote:
On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote:
+ /*
+ * At the head of the wait queue now
+ */
+ while (true) {
+ u32 qcode;
+ int retval;
+
+ retval = queue_get_lock_qcode(lock,&qcode, my_qcode);
+ if (retval> 0)
+ ; /* Lock not available yet */
+ else if (retval< 0)
+ /* Lock taken, can release the node& return */
+ goto release_node;
+ else if (qcode != my_qcode) {
+ /*
+ * Just get the lock with other spinners waiting
+ * in the queue.
+ */
+ if (queue_spin_trylock_unfair(lock))
+ goto notify_next;
Why is this an option at all?
Are you referring to the case (qcode != my_qcode)? This condition will be
true if more than one tasks have queued up.
But in no case should we revert to unfair spinning or stealing. We
should always respect the queueing order.
If the lock tail no longer points to us, then there's further waiters
and we should wait for ->next and unlock it -- after we've taken the
lock.
A task will be in this loop when it is already the head of a queue and
is entitled to take the lock. The condition (qcode != my_qcode) is to
decide whether it should just take the lock or take the lock & clear the
code simultaneously. I am a bit cautious to use
queue_spin_trylock_unfair() as there is a possibility that a CPU may run
out of the queue node and need to do unfair busy spinning.
-Longman
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