buf_ring(9) API precisions

[K. Macy]

If the value lags next by one then it is ours. This rule applies to
all callers so the rule holds consistently.

On Mon, Sep 19, 2011 at 5:53 AM, Arnaud Lacombe <lacombar at gmail.com> wrote:
> 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();
>>
>