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

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Date:	Tue, 18 Feb 2014 19:58:49 -0500
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
Subject: Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock
 implementation

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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