buf_ring(9) API precisions

[K. Macy]

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.

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();