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Date:	Tue, 12 Aug 2014 23:41:32 +0200
From:	Hannes Frederic Sowa <>
To:	Eric Dumazet <>
Cc:	Marcelo Ricardo Leitner <>,,
Subject: Re: [PATCH stable 3.4 1/2] ipv4: move route garbage collector to
 work queue

Hi Eric,

On Di, 2014-08-12 at 13:23 -0700, Eric Dumazet wrote:
> On Tue, 2014-08-12 at 20:50 +0200, Hannes Frederic Sowa wrote:
> > On Mo, 2014-08-11 at 19:41 -0300, Marcelo Ricardo Leitner wrote:
> > > Currently the route garbage collector gets called by dst_alloc() if it
> > > have more entries than the threshold. But it's an expensive call, that
> > > don't really need to be done by then.
> > > 
> > > Another issue with current way is that it allows running the garbage
> > > collector with the same start parameters on multiple CPUs at once, which
> > > is not optimal. A system may even soft lockup if the cache is big enough
> > > as the garbage collectors will be fighting over the hash lock entries.
> > > 
> > > This patch thus moves the garbage collector to run asynchronously on a
> > > work queue, much similar to how rt_expire_check runs.
> > > 
> > > There is one condition left that allows multiple executions, which is
> > > handled by the next patch.
> > > 
> > > Signed-off-by: Marcelo Ricardo Leitner <>
> > > Cc: Hannes Frederic Sowa <>
> > 
> > Acked-by: Hannes Frederic Sowa <>
> This does not look as stable material.

We hesitated at first, too, to send those out.

We had a machine being brought down by production traffic while using
TPROXY. The routing cache, while still having a relatively good hit
ratio, was filled with combinations of source and destination addresses.
Multiple GCs running and trying to grab the same per-chain spin_lock
caused a complete lockdown of the machine. That's why we submitted those
patches for review in the end.

> One can always disable route cache in 3.4 kernels

Sure, but we didn't like the fact that it is possible to bring down the
machine in the first place.


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