Deadlock in the routing code

Gleb Smirnoff glebius at
Fri Dec 14 01:50:19 PST 2007

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>  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?

Totus tuus, Glebius.

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