linux-kernel - Re: [PATCH v2 12/28] thunderbolt: Add functions for allocating and releasing hop IDs

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Date:   Tue, 12 Feb 2019 15:02:21 +0200
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     linux-kernel@...r.kernel.org,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        Andreas Noever <andreas.noever@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v2 12/28] thunderbolt: Add functions for allocating and
 releasing hop IDs

On Tue, Feb 12, 2019 at 01:59:27PM +0100, Lukas Wunner wrote:
> On Tue, Feb 12, 2019 at 02:51:25PM +0200, Mika Westerberg wrote:
> > On Tue, Feb 12, 2019 at 01:43:33PM +0100, Lukas Wunner wrote:
> > > On Mon, Feb 11, 2019 at 10:30:43AM +0200, Mika Westerberg wrote:
> > > > On Sun, Feb 10, 2019 at 01:13:53PM +0100, Lukas Wunner wrote:
> > > > > If there are two Macs at the ends of the daisy-chain with Thunderbolt
> > > > > devices in-between, the other Mac may already have established tunnels
> > > > > to some of the devices and therefore has occupied hop entries in the
> > > > > devices' path config space.  How do you ensure that you don't allocate
> > > > > the same entries and overwrite the other Mac's hop entries, thereby
> > > > > breaking its tunnels?
> > > > 
> > > > If the other Mac has enumerated the device (set the upstream port,
> > > > route, depth) then the other Mac cannot access the device. You get an
> > > > error (we deal with that in the later patch in the series when we
> > > > identify XDomain connections). The Hop ID allocation is only relevant in
> > > > a single domain.  Crossing one needs to have protocol such as we have in
> > > > case of ThunderboltIP to negotiate Hop IDs used in the link between two
> > > > domains.
> > > 
> > > Understood now, thanks.  (Well, in part at least.)
> > > 
> > > It looks like there's a race condition currently in tb_switch_configure()
> > > wherein two machines on the daisy chain may write the config simultaneously
> > > and overwrite each other's changes.  Isn't there some kind of synchonization
> > > mechanism available to prevent such an outcome?
> > 
> > AFAICT that's expected. The host that first enumerated the device wins.
> 
> Yes but tb_switch_configure() goes on to blindly call
> tb_plug_events_active().  Does that or any other subsequently called
> function fail if another machine managed to overwrite the switch config?

Yes, once the switch is enumerated the other domain cannot access it
anymore but instead gets back errors.