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

[Mika Westerberg]

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.