linux-kernel - Re: [PATCH] IB/core: Make ib_mad_client

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Date:   Thu, 26 Apr 2018 21:32:50 +0300
From:   jackm <jackm@....mellanox.co.il>
To:     Håkon Bugge <haakon.bugge@...cle.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Doug Ledford <dledford@...hat.com>,
        Don Hiatt <don.hiatt@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Sean Hefty <sean.hefty@...el.com>,
        OFED mailing list <linux-rdma@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] IB/core: Make ib_mad_client_id atomic

On Thu, 26 Apr 2018 18:06:10 +0200
Håkon Bugge <haakon.bugge@...cle.com> wrote:

> > On 23 Apr 2018, at 21:16, jackm <jackm@....mellanox.co.il> wrote:
> > 
> > On Mon, 23 Apr 2018 16:19:57 +0200
> > Håkon Bugge <haakon.bugge@...cle.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
>