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Date:	Sun, 18 Mar 2007 21:53:55 +0200
From:	"Michael S. Tsirkin" <mst@....mellanox.co.il>
To:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Cc:	"Michael S. Tsirkin" <mst@....mellanox.co.il>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org, general@...ts.openfabrics.org,
	Roland Dreier <rolandd@...co.com>
Subject: Re: dst_ifdown breaks infiniband?

Quoting Alexey Kuznetsov <kuznet@....inr.ac.ru>:
Subject: Re: dst_ifdown breaks infiniband?
> 
> Hello!
> 
> > This is not new code, and should have triggered long time ago,
> > so I am not sure how come we are triggering this only now,
> > but somehow this did not lead to crashes in 2.6.20
> 
> I see. I guess this was plain luck.
> 
> 
> > Why is neighbour->dev changed here?
> 
> It holds reference to device and prevents its destruction.
> If dst is held somewhere, we cannot destroy the device and deadlock
> while unregister.
> 
> We could not invalidate dst->neighbour but it looked safe to invalidate
> neigh->dev after quiescent state. Obviosuly, it is not and it never was safe.
> Was supposed to be repaired asap, but this did not happen. :-(
> 
> > Can dst->neighbour be changed to point to NULL instead, and the neighbour
> > released?
> 
> It should be cleared and we should be sure it will not be destroyed
> before quiescent state.

I'm confused. didn't you say dst_ifdown is called after quiescent state?

> Seems, this is the only correct solution, but to do this we have
> to audit all the places where dst->neighbour is dereferenced for
> RCU safety.
> 
> Actually, it is very good you caught this eventually, the bug was
> so _disgusting_ that it was "forgotten" all the time, waiting for
> someone who will point out that the king is naked. :-)
> 
> Alexey

This does not sound like something that's likely to be accepted in 2.6.21, right?

Any simpler ideas?

-- 
MST
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