Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlockimplementation

[Waiman Long]

On 01/30/2014 02:28 PM, Peter Zijlstra wrote:
> On Thu, Jan 30, 2014 at 11:00:30AM -0800, Tim Chen wrote:
> > On Tue, 2014-01-28 at 13:19 -0500, Waiman Long wrote:
> >
> > > +/**
> > > + * queue_spin_lock_slowpath - acquire the queue spinlock
> > > + * @lock: Pointer to queue spinlock structure
> > > + */
> > > +void queue_spin_lock_slowpath(struct qspinlock *lock)
> > > +{
> > > +	unsigned int cpu_nr, qn_idx;
> > > +	struct qnode *node, *next = NULL;
> > > +	u32 prev_qcode, my_qcode;
> > > +
> > > +	/*
> > > +	 * Get the queue node
> > > +	 */
> > > +	cpu_nr = smp_processor_id();
> > > +	node   = this_cpu_ptr(&qnodes[0]);
> > > +	qn_idx = 0;
> > > +
> > > +	if (unlikely(node->used)) {
> > > +		/*
> > > +		 * This node has been used, try to find an empty queue
> > > +		 * node entry.
> > > +		 */
> > > +		for (qn_idx = 1; qn_idx<  MAX_QNODES; qn_idx++)
> > > +			if (!node[qn_idx].used)
> > > +				break;
> > > +		if (unlikely(qn_idx == MAX_QNODES)) {
> > > +			/*
> > > +			 * This shouldn't happen, print a warning message
> > > +			 *&  busy spinning on the lock.
> > > +			 */
> > > +			printk_sched(
> > > +			  "qspinlock: queue node table exhausted at cpu %d!\n",
> > > +			  cpu_nr);
> > > +			while (!unfair_trylock(lock))
> > > +				arch_mutex_cpu_relax();
> > > +			return;
> > > +		}
> > > +		/* Adjust node pointer */
> > > +		node += qn_idx;
> > > +	}
> > > +
> > > +	/*
> > > +	 * Set up the new cpu code to be exchanged
> > > +	 */
> > > +	my_qcode = SET_QCODE(cpu_nr, qn_idx);
> > > +
> > If we get interrupted here before we have a chance to set the used flag,
> > the interrupt handler could pick up the same qnode if it tries to
> > acquire queued spin lock.  Then we could overwrite the qcode we have set
> > here.
> >
> > Perhaps an exchange operation for the used flag to prevent this race
> > condition?
> I don't get why we need the used thing at all; something like:
>
> struct qna {
> 	int cnt;
> 	struct qnode nodes[4];
> };
>
> DEFINE_PER_CPU(struct qna, qna);
>
> struct qnode *get_qnode(void)
> {
> 	struct qna *qna = this_cpu_ptr(&qna);
>
> 	return qna->nodes[qna->cnt++]; /* RMW */
> }
>
> void put_qnode(struct qnode *qnode)
> {
> 	struct qna *qna = this_cpu_ptr(&qna);
> 	qna->cnt--;
> }
>
> Should do fine, right?

Yes, we can do something like that. However I think put_qnode() needs to 
use atomic dec as well. As a result, we will need 2 additional atomic 
operations per slowpath invocation. The code may look simpler, but I 
don't think it will be faster than what I am currently doing as the 
cases where the used flag is set will be relatively rare.

> If we interrupt the RMW above the interrupted context hasn't yet used
> the queue and once we return its free again, so all should be well even
> on load-store archs.
>
> The nodes array might as well be 3, because NMIs should never contend on
> a spinlock, so all we're left with is task, softirq and hardirq context.

I am not so sure about NMI not taking a spinlock. I seem to remember 
seeing code that did that. Actually, I think the NMI code is trying to 
printk something which, in turn, need to acquire a spinlock.

-Longman
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