Re: [PATCH v9 04/19] qspinlock: Extract out the exchange of tail code word

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

On 04/18/2014 04:15 AM, Peter Zijlstra wrote:
> On Thu, Apr 17, 2014 at 05:28:17PM -0400, Waiman Long wrote:
>> On 04/17/2014 11:49 AM, Peter Zijlstra wrote:
>>> On Thu, Apr 17, 2014 at 11:03:56AM -0400, Waiman Long wrote:
>>>> @@ -192,36 +220,25 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>>>>   	node->next = NULL;
>>>>
>>>>   	/*
>>>> +	 * We touched a (possibly) cold cacheline; attempt the trylock once
>>>> +	 * more in the hope someone let go while we weren't watching as long
>>>> +	 * as no one was queuing.
>>>>   	 */
>>>> +	if (!(val&   _Q_TAIL_MASK)&&   queue_spin_trylock(lock))
>>>> +		goto release;
>>> But you just did a potentially very expensive op; @val isn't
>>> representative anymore!
>> That is not true. I pass in a pointer to val to trylock_pending() (the
>> pointer thing) so that it will store the latest value that it reads from the
>> lock back into val. I did miss one in the PV qspinlock exit loop. I will add
>> it back when I do the next version.
> But you did that read _before_ you touched a cold cacheline, that's 100s
> of cycles. Whatever value you read back then is now complete nonsense.

For spin_lock(), the lock cacheline is touched by a cmpxchg(). It can 
takes 100s of cycles whether it is hot or cold.

I will take the precheck out, it is not such a big deal anyway.

-Longman