Date: Mon, 19 Sep 2011 14:46:37 +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_NbOGj4rEpHWBJooyrzYi2rehbxd5LChTga1DzWW6P44g@mail.gmail.com> In-Reply-To: <CACqU3MWMeAMcrDZ2NF_OytYgiAFxmHvYRKcCVk=-=_VVYAcExQ@mail.gmail.com> References: <CACqU3MXQ6tD804fKymeFeKDnHndSXVvHJwepYztB4DsnNmtMiw@mail.gmail.com> <CACqU3MWwOw_otd0sJ-c4OXedeeJtchwiX9Xpx7V0zNW%2BcNZ7Yw@mail.gmail.com> <CAHM0Q_NfoSoa52rAAF8iUPQoqardbgSsq0PDnfh%2BmUFN993ZVA@mail.gmail.com> <CACqU3MWMeAMcrDZ2NF_OytYgiAFxmHvYRKcCVk=-=_VVYAcExQ@mail.gmail.com>
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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@gmail.com> wrote:
> 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> wro=
te:
>>> Hi,
>>>
>>> On Wed, Sep 14, 2011 at 10:53 PM, Arnaud Lacombe <lacombar@gmail.com> w=
rote:
>>>> 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_siz=
e;
>>>> =A0 =A0 =A0 =A0int =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 br_prod_mas=
k;
>>>> =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,
> =A0- 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();
>>
>
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