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Subject: Re: [PATCH] IB/core: Make ib_mad_client_id atomic
From:   =?utf-8?Q?H=C3=A5kon_Bugge?= <haakon.bugge@oracle.com>
In-Reply-To: <20180423221618.0000795d@dev.mellanox.co.il>
Date:   Thu, 26 Apr 2018 18:06:10 +0200
Cc:     Don Hiatt <don.hiatt@intel.com>, Ira Weiny <ira.weiny@intel.com>,
        Sean Hefty <sean.hefty@intel.com>,
        OFED mailing list <linux-rdma@vger.kernel.org>,
        linux-kernel@vger.kernel.org, jackm <jackm@dev.mellanox.co.il>
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References: <20180418142450.15581-1-Haakon.Bugge@oracle.com>
 <1524196555.11756.30.camel@redhat.com> <20180420153451.GF30433@ziepe.ca>
 <50E3FE4A-2C6E-4493-92E1-383C349FAE3A@oracle.com>
 <20180423221618.0000795d@dev.mellanox.co.il>
To:     Jason Gunthorpe <jgg@ziepe.ca>, Doug Ledford <dledford@redhat.com>
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> On 23 Apr 2018, at 21:16, jackm <jackm@dev.mellanox.co.il> wrote:
>=20
> On Mon, 23 Apr 2018 16:19:57 +0200
> H=C3=A5kon Bugge <haakon.bugge@oracle.com> wrote:
>=20
>=20
>>>=20
>>> This actually looks like a genuine bug, why is it described only as
>>> 'confusing'? ib_register_mad_agent is callable from userspace, so at
>>> least two userspace agents can race and get the same TID=E2=80=99s. =20=

>>=20
>> My understanding is that every lookup is using the {port, TID} tuple.
>> As such, it is not a bug, but, very confusing.
> Haakon, you are correct (see snippet from the IB spec, below).
>=20
> We will NOT have a situation where there are 2 threads/apps
> with the same agent ID on the *same port* (accessing the agent ID
> allocator is protected by a per-port spinlock). Having the same agent =
ID
> on DIFFERENT ports is OK.
> Thus, there is NO bug here. (But as Haakon says, IMHO it is more =
robust
> to avoid having the same agent ID for 2 agents even if those agents =
are
> on different ports).
>=20
>>=20
>>> TIDs need to be globally unique on the entire machine. =20
>>=20
> Jason, that is not exactly correct.
>=20
> =46rom the IB Spec 1.3, C13-18.1.1 (in section 13.4.6.4 - =
TransactionID
> usage):
> "When initiating a new operation, MADHeader:TransactionID
> shall be set to such a value that within that MAD the combination of
> TID, SGID, and MgmtClass is different from that of any other currently
> executing operation. If the MAD does not have a GRH, its SLID is used
> in the combination in place of an SGID."
>=20
> Since the SGID/SLID is different for each port, the per-port guarantee
> of no 2 agents receiving the same agent-ID value is sufficient.
>=20
> -Jack

Shall I interpret this silence as my commit is good to go or that I =
should add Jack=E2=80=99s tangible information to the commit message?


Thxs, H=C3=A5kon