Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation

[Waiman Long <waiman.long@hp.com>]

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