netdev - Re: [PATCH] netpoll: cleanup sparse warnings

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Date:	Thu, 7 Feb 2013 13:37:25 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, fengguang.wu@...el.com,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH] netpoll: cleanup sparse warnings

On Thu, Feb 07, 2013 at 07:52:56AM -0800, Eric Dumazet wrote:
> On Thu, 2013-02-07 at 09:56 -0500, Neil Horman wrote:
> > With my recent commit I introduced two sparse warnings.  Looking closer there
> > were a few more in the same file, so I fixed them all up.  Basic rcu pointer
> > dereferencing suff
> 
> > -	npinfo = np->dev->npinfo;
> > +	/* rtnl_dereference would be preferable here but
> > +	 * rcu_cleanup_netpoll path can put us in here safely without
> > +	 * holding the rtnl, so plain rcu_dereference it is
> > +	 */
> > +	npinfo = rcu_dereference(np->dev->npinfo);
> >  	if (!npinfo)
> >  		return;
> >  
> 
> Are you sure it wont trigger a LOCKDEP complain (CONFIG_PROVE_RCU=y) ?
> 
Hm, looking at it, you're probably right.  We're not holding the rcu_read_lock,
and I'd forgotten that rcu_dereference implicitly checks that rcu_read_lock is
held.  I guess, since the only paths that we get here on are in a bh rcu
quiescence point or with the rtnl held we should probably make this:

rcu_dereference_protected(np->dev->npinfo, rtnl_locked() || in_interrupt());

Although, thinking about this further somewhat begs the question as to how we
prevent one context from calling __netpoll_cleanup in a path holding rtnl, while
in parallel calling __netpoll_cleanup from the rcu callback.  That might not be
a huge deal as __netpoll_cleanup uses spinlocks to do list modification, and an
atomic_dec_and_test to gate the free, but it still seems ugly.

What do you think?
> 
> 
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