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Re: [Xen-devel] [RFC] Implement Batched (group) ticket lock

On 05/30/2014 04:15 AM, Waiman Long wrote:
On 05/28/2014 08:16 AM, Raghavendra K T wrote:
- we need an intelligent way to nullify the effect of batching for
  (because extra cmpxchg is not required).

To do this, you will need to have 2 slightly different algorithms
depending on the paravirt_ticketlocks_enabled jump label.

Thanks for the hint Waiman.

+    for (;;) {
+        inc.head = ACCESS_ONCE(lock->tickets.head);
+        if (!(inc.head&  TICKET_LOCK_HEAD_INC)) {
+            new.head = inc.head | TICKET_LOCK_HEAD_INC;
+            if (cmpxchg(&lock->tickets.head, inc.head, new.head)
+                    == inc.head)
+                goto out;
+        }
+        cpu_relax();
+    }

It had taken me some time to figure out the the LSB of inc.head is used
as a bit lock for the contending tasks in the spin loop. I would suggest
adding some comment here to make it easier to look at.

Agree. 'll add a comment.

+#define TICKET_BATCH    0x4 /* 4 waiters can contend simultaneously */
+                  TICKET_LOCK_TAIL_INC - 1)

I don't think TAIL_INC has anything to do with setting the BATCH_MASK.
It works here because TAIL_INC is 2. I think it is clearer to define it
as either "(~(TICKET_BATCH<<TICKET_LOCK_INC_SHIFT) + 1)" or

You are right.
Thanks for pointing out. Your expression is simple and clearer. 'll use
one of them.

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