netdev - Re: [IPV4 3/5] fib_trie: dump doesnt use RCU

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Message-Id: <20080123.232651.71974261.davem@davemloft.net>
Date:	Wed, 23 Jan 2008 23:26:51 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	kaber@...sh.net
Cc:	shemminger@...ux-foundation.org, netdev@...r.kernel.org
Subject: Re: [IPV4 3/5] fib_trie: dump doesnt use RCU

From: Patrick McHardy <kaber@...sh.net>
Date: Thu, 24 Jan 2008 07:47:39 +0100

> David Miller wrote:
> > From: Patrick McHardy <kaber@...sh.net>
> > Date: Thu, 24 Jan 2008 07:41:08 +0100
> >
> >> David Miller wrote:
> >>     
> >>> and nothing in that code path retakes the RTNL semaphore.
> >>>       
> >> Actually we're always holding the rtnl during dumps, nlk->cb_mutex points
> >> to rtnl_mutex in case of rtnetlink. It used to be held only during the first
> >> ->dump invocation and not on continuations, but I changed this a few 
> >> versions ago.
> >>     
> >
> > My bad.  Thanks for the correction Patrick.
> >
> > But continuations can occur on subsequent recvmsg() calls,
> > does it return to userspace with the mutex held?  If so
> > I'm pretty sure that's not allowed.
> 
> No, the mutex is dropped between different ->dump invocations.

Ok, great.

This does mean, however, that the RTNL semaphore is dropped between
->dump() invocations on the same nlk->cb[] instance.

And that has implications for the shortcut Stephen is taking.

Stephen's patch assumes that during a (top-level) dump the table
cannot change.  And by the above, we can only conclude that it can in
fact change between ->dump() calls for the same top-level dump run.

So the removal of the -EBUSY code block in his patch isn't valid.
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