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

[Peter Zijlstra]

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?

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.
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