From mboxrd@z Thu Jan  1 00:00:00 1970
From: mrybczyn@kalray.eu (Marta Rybczynska)
Date: Wed, 3 May 2017 17:19:27 +0200 (CEST)
Subject: [PATCH v4, under testing] nvme-rdma: support devices with queue
 size < 32
In-Reply-To: <823aa3f0-685f-4569-11d6-238cc4f0b126@grimberg.me>
References: <79901165.5342369.1493805915415.JavaMail.zimbra@kalray.eu>
 <823aa3f0-685f-4569-11d6-238cc4f0b126@grimberg.me>
Message-ID: <780938034.8003164.1493824767084.JavaMail.zimbra@kalray.eu>

>> +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.

Marta