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    SubjectRe: [PATCH] IB/core: Make ib_mad_client_id atomic
    On Thu, 26 Apr 2018 18:06:10 +0200
    Håkon Bugge <haakon.bugge@oracle.com> wrote:

    > > On 23 Apr 2018, at 21:16, jackm <jackm@dev.mellanox.co.il> wrote:
    > >
    > > On Mon, 23 Apr 2018 16:19:57 +0200
    > > Håkon Bugge <haakon.bugge@oracle.com> wrote:
    > >
    > >
    > >>>
    > >>> 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’s.
    > >>
    > >> 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).
    > >
    > > 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).
    > >
    > >>
    > >>> TIDs need to be globally unique on the entire machine.
    > >>
    > > Jason, that is not exactly correct.
    > >
    > > From 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."
    > >
    > > 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.
    > >
    > > -Jack
    >
    > Shall I interpret this silence as my commit is good to go or that I
    > should add Jack’s tangible information to the commit message?
    >
    >
    Haakon, I think Jason is on vacation this week. Best to wait for his
    response.

    -Jack

    > Thxs, Håkon
    >

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