Date: Fri, 16 Sep 2011 16:41:26 +0200 From: "K. Macy" <kmacy@freebsd.org> To: Arnaud Lacombe <lacombar@gmail.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: buf_ring(9) API precisions Message-ID: <CAHM0Q_NfoSoa52rAAF8iUPQoqardbgSsq0PDnfh%2BmUFN993ZVA@mail.gmail.com> In-Reply-To: <CACqU3MWwOw_otd0sJ-c4OXedeeJtchwiX9Xpx7V0zNW%2BcNZ7Yw@mail.gmail.com> References: <CACqU3MXQ6tD804fKymeFeKDnHndSXVvHJwepYztB4DsnNmtMiw@mail.gmail.com> <CACqU3MWwOw_otd0sJ-c4OXedeeJtchwiX9Xpx7V0zNW%2BcNZ7Yw@mail.gmail.com>
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On Fri, Sep 16, 2011 at 3:02 AM, Arnaud Lacombe <lacombar@gmail.com> wrote:
> Hi,
>
> On Wed, Sep 14, 2011 at 10:53 PM, Arnaud Lacombe <lacombar@gmail.com> wro=
te:
>> 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 {
>> =A0 =A0 =A0 =A0volatile uint32_t =A0 =A0 =A0 br_prod_head;
>> =A0 =A0 =A0 =A0volatile uint32_t =A0 =A0 =A0 br_prod_tail;
>> =A0 =A0 =A0 =A0int =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 br_prod_size;
>> =A0 =A0 =A0 =A0int =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 br_prod_mask;
>> =A0 =A0 =A0 =A0uint64_t =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0br_drops;
>> =A0 =A0 =A0 =A0uint64_t =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0br_prod_bufs;
>> =A0 =A0 =A0 =A0uint64_t =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0br_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 =3D=3D 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 !=3D prod_head)
cpu_spinwait();
br->br_prod_bufs++;
br->br_prod_bytes +=3D nbytes;
br->br_prod_tail =3D prod_next;
critical_exit();
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