Date: Sun, 18 Sep 2011 23:53:07 -0400 From: Arnaud Lacombe <lacombar@gmail.com> To: "K. Macy" <kmacy@freebsd.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: buf_ring(9) API precisions Message-ID: <CACqU3MWMeAMcrDZ2NF_OytYgiAFxmHvYRKcCVk=-=_VVYAcExQ@mail.gmail.com> In-Reply-To: <CAHM0Q_NfoSoa52rAAF8iUPQoqardbgSsq0PDnfh%2BmUFN993ZVA@mail.gmail.com> References: <CACqU3MXQ6tD804fKymeFeKDnHndSXVvHJwepYztB4DsnNmtMiw@mail.gmail.com> <CACqU3MWwOw_otd0sJ-c4OXedeeJtchwiX9Xpx7V0zNW%2BcNZ7Yw@mail.gmail.com> <CAHM0Q_NfoSoa52rAAF8iUPQoqardbgSsq0PDnfh%2BmUFN993ZVA@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
Hi,
On Fri, Sep 16, 2011 at 10:41 AM, K. Macy <kmacy@freebsd.org> wrote:
> On Fri, Sep 16, 2011 at 3:02 AM, Arnaud Lacombe <lacombar@gmail.com> wrot=
e:
>> Hi,
>>
>> On Wed, Sep 14, 2011 at 10:53 PM, Arnaud Lacombe <lacombar@gmail.com> wr=
ote:
>>> 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.
>
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
>
> =A0 =A0 =A0 =A0/*
> =A0 =A0 =A0 =A0 * If there are other enqueues in progress
> =A0 =A0 =A0 =A0 * that preceeded us, we need to wait for them
> =A0 =A0 =A0 =A0 * to complete
> =A0 =A0 =A0 =A0 */
> =A0 =A0 =A0 =A0while (br->br_prod_tail !=3D prod_head)
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0cpu_spinwait();
> =A0 =A0 =A0 =A0br->br_prod_bufs++;
> =A0 =A0 =A0 =A0br->br_prod_bytes +=3D nbytes;
> =A0 =A0 =A0 =A0br->br_prod_tail =3D prod_next;
> =A0 =A0 =A0 =A0critical_exit();
>
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?CACqU3MWMeAMcrDZ2NF_OytYgiAFxmHvYRKcCVk=-=_VVYAcExQ>