buf_ring(9) API precisions

[Arnaud Lacombe]

Hi,

On Fri, Sep 16, 2011 at 10:41 AM, K. Macy <kmacy at freebsd.org> wrote:
> On Fri, Sep 16, 2011 at 3:02 AM, Arnaud Lacombe <lacombar at gmail.com> wrote:
>> Hi,
>>
>> On Wed, Sep 14, 2011 at 10:53 PM, Arnaud Lacombe <lacombar at gmail.com> wrote:
>>> Hi Kip,
>>>
>>> I've got a few question about the buf_ring(9) API.
>>>
>>> 1) what means the 'drbr_' prefix. I can guess the two last letter, 'b'
>>> and 'r', for Buffer Ring, but what about 'd' and 'r' ?
>>>
>>> 2) in `sys/sys/buf_ring.h', you defined 'struct buf_ring' as:
>>>
>>> struct buf_ring {
>>>        volatile uint32_t       br_prod_head;
>>>        volatile uint32_t       br_prod_tail;
>>>        int                     br_prod_size;
>>>        int                     br_prod_mask;
>>>        uint64_t                br_drops;
>>>        uint64_t                br_prod_bufs;
>>>        uint64_t                br_prod_bytes;
>> shouldn't those 3 fields be updated atomically, especially on 32bits
>> platforms ? That might pose a problem as, AFAIK, FreeBSD do not have
>> MI 64bits atomics operations...
>
> Between the point at which br_prod_tail == prod_head and when we
> update br_prod_tail to point to prod_next we are the exclusive owners
> of the fields in buf_ring. That is why we wait for any other
> enqueueing threads to update br_prod_tail to point to prod_head before
> continuing.
>
How do you enforce ordering ? I do not see anything particular
forbidding the `br->br_prod_tail' to be committed first, leading other
thread to believe they have access to the statistics, while the other
thread has not yet committed its change.

Thanks,
 - Arnaud

> Cheers
>
>        /*
>         * If there are other enqueues in progress
>         * that preceeded us, we need to wait for them
>         * to complete
>         */
>        while (br->br_prod_tail != prod_head)
>                cpu_spinwait();
>        br->br_prod_bufs++;
>        br->br_prod_bytes += nbytes;
>        br->br_prod_tail = prod_next;
>        critical_exit();
>