netdev - Re: rtnl_lock deadlock on 3.10

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Message-ID: <20130906231901.GB10419@sbohrermbp13-local.rgmadvisors.com>
Date:	Fri, 6 Sep 2013 18:19:01 -0500
From:	Shawn Bohrer <shawn.bohrer@...il.com>
To:	Steve Wise <swise@...ngridcomputing.com>
Cc:	roland@...estorage.com, Bart Van Assche <bvanassche@....org>,
	Shawn Bohrer <sbohrer@...advisors.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org,
	linux-rdma@...r.kernel.org, swise@...lsio.com
Subject: Re: rtnl_lock deadlock on 3.10

On Thu, Sep 05, 2013 at 10:14:51AM -0500, Steve Wise wrote:
> Roland, what do you think?
> 
> As I've said, I think we should go ahead with using the rtnl lock in
> the core.  Is there a complete patch available for review?  looks
> like the original was a partial fix.

I guess I should realize that when no one jumps at fixing my issues
for me that they probably aren't simple to fix.  The solution that
Cong proposed was to acquire rtnl_lock() before acquiring the
infiniband device_mutex, and his partial patch did that in
ib_register_client().  The problem is that you would also need to do
that in ib_unregister_client(), ib_register_device(), and
ib_unregister_device(), and that brings us back to the original
problem which was that cxgb3 was holding the rtnl_lock() when it
called ib_register_device().  Thus with the proposed fix I believe
cxgb3 would already be holding the rtnl_lock() and then call
ib_register_device() which would try to acquire the rtnl_lock() again
and deadlock for a different reason.

Actually how does this currently work?  ib_register_device() calls
client->add() for each client in the list which should call
ipoib_add_one() which calls register_netdev().  Shouldn't that also
deadlock in the cxgb3 case?

Also while digging through this I think I see another bug which is
that ipoib_dev_cleanup() can be called from ipoib_add_port() but in
the current code ipoib_add_port() is not holding the rtnl_lock() which
appears to be a requirement of ipoib_dev_cleanup().

Sigh...  I'm going to stop looking at this for now and hopefully
someone can propose a better solution to this issue.

Thanks,
Shawn
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