On Wed, 2017-05-03 at 17:19 +0200, Marta Rybczynska wrote:
> > > +static inline bool nvme_rdma_queue_sig_limit(struct nvme_rdma_queue *queue)
> > > +{
> > > + int v, old;
> > > +
> > > + v = atomic_read(&queue->sig_count);
> > > + while (1) {
> > > + if (v > 1) {
> > > + old = atomic_cmpxchg(&queue->sig_count, v, v - 1);
> > > + if (old == v)
> > > + return false;
> > > + } else {
> > > + int new_count;
> > > +
> > > + new_count = nvme_rdma_init_sig_count(queue->queue_size);
> > > + old = atomic_cmpxchg(&queue->sig_count, v, new_count);
> > > + if (old == v)
> > > + return true;
> > > + }
> > > + v = old;
> > > + }
> > > +}
> > > +
> >
> > Ugh, no...
> >
> > How about just do:
> >
> > if (atomic_inc_return(queue->sig_count) % queue->sig_limit)
> > return true;
> > return false;
> >
> > where
> > queue->sig_limit = max(queue->queue_size / 2, 1);
>
> I tried to avoid that because this adds a division in the fast path Bart
> was unhappy about in v2.
>
> Unfortunately we do not have an atomic with on overflow operation like
> the one needed here.
Hello Marta,
The approach I proposed works well if sig_count is modified by a single thread
at a time. Seeing your code made me realize that it is nontrivial to implement
that approach if multiple threads can change sig_count concurrently. Since
atomic_cmpxchg() is relatively expensive, what Sagi proposed may be a better
alternative. Sorry that I sent you in the wrong direction.
Bart.--
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