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

[Peter Zijlstra]

On Fri, Jan 31, 2014 at 01:26:29PM -0500, Waiman Long wrote:
> >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.

No, put doesn't need an atomic; nor is it as well; because the inc
doesn't need an atomic either.

> >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.

Yeah I know, terribly broken that, I've been waiting for that to explode
:-)
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