Deadlock in the routing code

Maxime Henrion mux at
Mon Dec 17 02:11:18 PST 2007

Julian Elischer wrote:
> Gleb Smirnoff wrote:
> >On Thu, Dec 13, 2007 at 10:33:25AM -0800, Julian Elischer wrote:
> >J>  Maxime Henrion wrote:
> >J> > Replying to myself on this one, sorry about that.
> >J> > I said in my previous mail that I didn't know yet what process was
> >J> > holding the lock of the rtentry that the routed process is dealing
> >J> > with in rt_setgate(), and I just could verify that it is held by
> >J> > the swi1: net thread.
> >J> > So, in a nutshell:
> >J> > - The routed process does its business on the routing socket, that 
> >ends up
> >J> >   calling rt_setgate().  While in rt_setgate() it drops the lock on 
> >its
> >J> >   rtentry in order to call rtalloc1().  At this point, the routed
> >J> >   process hold the gateway route (rtalloc1() returns it locked), and 
> >it
> >J> >   now tries to re-lock the original rtentry.
> >J> > - At the same time, the swi net thread calls arpresolve() which ends 
> >up
> >J> >   calling rt_check().  Then rt_check() locks the rtentry, and tries to
> >J> >   lock the gateway route.
> >J> > A classical case of deadlock with mutexes because of different locking
> >J> > order.  Now, it's not obvious to me how to fix it :-).
> >J> 
> >J>  On failure to re-lock, the routed call to rt_setgate should completely 
> >abort J>  and restart from scratch, releasing all locks it has on the way 
> >out.
> >
> >Do you suggest mtx_trylock?
> I think that would be the cleanest way..

So, here's what I've got.  I have yet to test it at all, I hope that
I'll be able to do so today, or tomorrow.  Any input appreciated.

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