Re: Freeing alive fib_info caused by ebc0ffae5

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 04 novembre 2010 à 11:30 +0100, Eric Dumazet a écrit :
> Le jeudi 04 novembre 2010 à 21:23 +1100, Michael Ellerman a écrit :
> > Hi all,
> > 
> > I'm running Linus' latest or thereabouts (ff8b16d), and I'm seeing
> > "Freeing alive fib_info" messages, from free_fib_info().
> > 
> > Actually I only get one per boot, when network interfaces come up.
> > Seemingly related I am getting refcount problems when I shutdown, ie.
> > unregister_netdevice() sees a usage count of 1, which never decrements.
> > 
> > Bisect says it's ebc0ffae5 which causes the problem, or makes it appear.
> > 
> >     fib: RCU conversion of fib_lookup()
> >     
> >     fib_lookup() converted to be called in RCU protected context, no
> >     reference taken and released on a contended cache line (fib_clntref)
> >     
> > 
> > Is this a bug in that commit, or a driver bug exposed?
> 
> Hi Michael, thanks for the report (and painful bisection I guess)
> 
> Thats hard to say... Is it reproductable on my machine ?
> 

Hmm, a review of the code spotted a bug in fib_result_assign()

Please try following patch :

Thanks again !

[PATCH] fib: fib_result_assign() should not change fib refcounts

After commit ebc0ffae5 (RCU conversion of fib_lookup()),
fib_result_assign()  should not change fib refcounts anymore.

Thanks to Michael who did the bisection and bug report.

Reported-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
---
 net/ipv4/fib_lookup.h |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/ipv4/fib_lookup.h b/net/ipv4/fib_lookup.h
index a29edf2..c079cc0 100644
--- a/net/ipv4/fib_lookup.h
+++ b/net/ipv4/fib_lookup.h
@@ -47,11 +47,8 @@ extern int fib_detect_death(struct fib_info *fi, int order,
 static inline void fib_result_assign(struct fib_result *res,
 				     struct fib_info *fi)
 {
-	if (res->fi != NULL)
-		fib_info_put(res->fi);
+	/* we used to play games with refcounts, but we now use RCU */
 	res->fi = fi;
-	if (fi != NULL)
-		atomic_inc(&fi->fib_clntref);
 }
 
 #endif /* _FIB_LOOKUP_H */