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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

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
>> baremetal
>>   (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.

[...]
>> +spin:
>> +    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 */
>> +#define TICKET_LOCK_BATCH_MASK
>> (~(TICKET_BATCH<<TICKET_LOCK_INC_SHIFT) + \
>> +                  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
> (~((TICKET_BATCH<<TICKET_LOCK_INC_SHIFT) - 1)).

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