Date: Wed, 19 Dec 2007 13:08:31 +0100 From: Maxime Henrion <mux@FreeBSD.org> To: net@FreeBSD.org Cc: Gleb Smirnoff <glebius@FreeBSD.org>, Julian Elischer <julian@elischer.org> Subject: Re: Deadlock in the routing code Message-ID: <20071219120831.GN71713@elvis.mu.org> In-Reply-To: <20071217101009.GL71713@elvis.mu.org> References: <20071213133817.GC71713@elvis.mu.org> <47617AF5.7070701@elischer.org> <20071214092539.GB14339@glebius.int.ru> <4762DD82.9070904@elischer.org> <20071217101009.GL71713@elvis.mu.org>
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Maxime Henrion wrote:
> 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.
It appears that this patch fixed the problem. My gateway server
now has a nearly two days uptime, whereas previously it would have
probably crashed already. I'm attaching the final version of the
patch here, since the last one had build-time errors. I'm going
to commit this in HEAD soon unless someone has an objection for it.
Cheers,
Maxime
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--- route.h.orig Tue Apr 4 22:07:23 2006
+++ route.h Mon Dec 17 13:11:44 2007
@@ -289,6 +289,7 @@
#define RT_LOCK_INIT(_rt) \
mtx_init(&(_rt)->rt_mtx, "rtentry", NULL, MTX_DEF | MTX_DUPOK)
#define RT_LOCK(_rt) mtx_lock(&(_rt)->rt_mtx)
+#define RT_TRYLOCK(_rt) mtx_trylock(&(_rt)->rt_mtx)
#define RT_UNLOCK(_rt) mtx_unlock(&(_rt)->rt_mtx)
#define RT_LOCK_DESTROY(_rt) mtx_destroy(&(_rt)->rt_mtx)
#define RT_LOCK_ASSERT(_rt) mtx_assert(&(_rt)->rt_mtx, MA_OWNED)
--- route.c.orig Tue Oct 30 19:07:54 2007
+++ route.c Mon Dec 17 15:13:20 2007
@@ -996,6 +996,7 @@
struct radix_node_head *rnh = rt_tables[dst->sa_family];
int dlen = SA_SIZE(dst), glen = SA_SIZE(gate);
+again:
RT_LOCK_ASSERT(rt);
/*
@@ -1029,7 +1030,15 @@
RT_REMREF(rt);
return (EADDRINUSE); /* failure */
}
- RT_LOCK(rt);
+ /*
+ * Try to reacquire the lock on rt, and if it fails,
+ * clean state and restart from scratch.
+ */
+ if (!RT_TRYLOCK(rt)) {
+ RTFREE_LOCKED(gwrt);
+ RT_LOCK(rt);
+ goto again;
+ }
/*
* If there is already a gwroute, then drop it. If we
* are asked to replace route with itself, then do
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